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Ask Jeff Yago
Solar & Energy-Related Issues

By Jeffrey Yago, P.E., CEM
Jeff Yago


Solar panel strength

Jeff:

I'm thinking about designing my new home to use solar energy to provide at least some of my family's power needs, but I have a question about the solar panels themselves. How well do they hold up against hail, wind and storm damage. I plan to use a metal roof with the flexible PV panels. All the others seem to be made of glass or plastic and look very fragile. What is your insight on their ruggedness? None of the websites I've visited seem to have any information as to whether they will hold up.

Carrie in Arkansas

Carrie:

Almost all solar pv modules made today that are UL rated are made from tempered glass and are guaranteed for 25 years by most manufacturers. I have taken them and supported each end and bounced up and down in the middle of a large pv module as then will "flex" even though they are glass. They are tested against hits from balls, hail, and the typical elements they would experience in the real world.

The only damaged modules I have ever seen are 2 or 3 out of about 1000 that failed in the first several months from initial thermal "shock". Since they are tempered glass bonded with vinyl material on the back, they tend to roll up in a curl, and the glass breaks like a car window in small "squares". I have seen a few that have been shop with a bullet, but about the only thing that will shatter them is being hit by a sharp point of a heavy metal object. I would not worry about damage from hail.

The non-glass modules that you refer to get their strength by being bonded to metal roofing after the roofing has been installed using factory trained installers and special bonding glues. I realize they are non-glass and will not break, but you may want to check their length of warranty as I think you will find they are not guaranteed as long as the glass type modules.

If you want this type of metal roof on your house then go for it, but I would not install a metal roof just to be able to attach non-glass solar modules. Having glass type modules mounted on aluminum rails raised several inches above your roof will reduce your attic temperatures, which would not be the case with the bonded flex modules.

Good luck,

Jeff Yago


Wood augmented domestic hot water

Jeff,

I found your email on Backwoods home.

Ware considering hooking our wood cook stove to an electric water heater. Yes we still use some electric. We have a flame view cook stove with a water coil in it. My question is have you ever heard of any one doing this? I have seen several converted electric water heater tanks being used this way. I just have not seen it where the electric has been used in conjunction with the cook stove. I have reasonable plumbing experience and have seen the others I previously mentioned. My concern is how to get it on parallel.

Thanks,

Mark

Mark:

Yes, a water coil in a wood cook stove was one of the first ways to heat bath water on the rural farm or homested, and many people still do. There are a few safety concerns you need to be aware of. I do not know if the coil you mentioned is something fairly new, was a stove add-on, or was some kind of kit that is sold for this purpose and already includes the needed valves and fittings. Regardless, be sure any piping you install to this stove coil has both a temperature relief valve and a pressure relief valve, or you may see parts of your stove heading through the roof from a steam explosion. The most important thing to consider, is always having the water in the coil moving when there is a fire, or it will quickly turn to steam and exit the nearest relief valve (or weak point in piping!).

If your coil is piped straight through from a cold water supply, then you want to adjust the flow for the hot water temperature desired, but never completely turned off while fire is burning. If your system will have a pump to re-circulate water from the coil to a hot water or storage tank, then you will probably extend the drain valve with a pipe tee, and connect the pump to this point, so the pump is pulling water from the lower and colder water in the tank. Be sure your pump is stainless steel as domestic water will rust out any cast iron pump. Most pumps for this service are either 1/12 or 1/20 HP, as there is very little flow or head pressure required. You will need to add a pipe tee to a fitting near the top of the tank and have the pump flow water through the stove coil and return the heated water near the top of the tank. Some people remove the top electric element and make this connection here since it is near the top. You can also connect into the hot water line leaving the top of the tank, but this will "mix" with the hottest water at the top of the tank and lower its temperature whenever the stove's coil is providing water at a lower temperature.

Be sure to located the relief valves as close to the coil discharge as possible. You should also add a check valve near the outlet side of the pump or coil to prevent reverse flow when you turn on a hot water faucet in the house and the pump is off. Without a check valve, cold water can flow around the hot water tank and through the stove heating coil backwards when there is no fire, resulting in cold water getting into your hot water lines.

The relief valves need to be selected to have setpoints lower than the pressure ratings of your piping and coil. Usually this is around 15 PSI for pressure and 200 degrees for temperature. Finally, if you plan to use the stove to really heat your water on a regular basis, remember, your electric tank has a thermostat that shuts the heating off when it gets to around 130 to 140 degrees, but your stove coil will keep heating the tank as hot as it can get it which could scald somebody. They make a "tempering" valve for this purpose which is located on the hot water pipe leaving the hot water tank. It also has a piping connection to the cold water line and will adjust the hot water going out to the hot water system to the preset temperature by mixing in cold water as needed.

Finally, you may want to turn the circuit breaker supplying the electric tank off when using your wood stove, as the water flow through the stove's coil at first may be colder than the setpoint of the heating element, and you end up heating the water in the loop with electricity until the stove gets really hot.

Good luck and by all means - call a plumber!

Jeff Yago


Solar cabin

Soon I will be moving to a 20 acre lot I've purchased in New Mexico and will be building a 400 square foot cabin (plus or minus not including the unheated basement) and would like to stay off the grid. Would 6 12v industrial batteries wired in series (3 sets of 2) provide enough power to feed an inverter for a 27" TV and satelite dish while still having power to directly run a 24v refrigerator?

I was also planning on a smaller 12 battery bank for things such as lights and charging my laptop. I would like to know what gauge wire would be best for running to these items. And lastly would 7 45watt solar panels be capable of charging all of them (such as those sold by Harbor Feight)?

J. Gill

J:

Because there are so many things that can affect the selection of specific component sizes, wire sizes, and battery sizes, I cannot give you a specific answer. However, here are a few things to consider based on the limited data you have provided.

First, do not try using 2 different arrays and 2 different battery banks. You will have many times when one battery bank is fully charged but not connected to needed loads powered from a different system having a low battery charge. Select one battery bank size and voltage, then select all loads to match that. For example, if you want a 48 volt battery bank to keep wire sizes smaller or to match a 48 volt inverter, then order your lights and appliances for 48 volt operation. There are many DC to DC converters that allow charging small loads like cell phones from a higher voltage battery bank.

Keep the battery strings to as few as possible. You will have far fewer battery maintenance and discharge issues with one battery series string of say 4 batteries at 6 volts to make 24 volts, than 4 strings in parallel of 2 batteries at 12 volts. Golf cart 6 volt batteries will give you far better service than a heavy duty 12 volt marine or truck battery, even though you will need twice as many.

Unless you have a long cable run (high voltage drop) from the solar array to the batteries and are planning to use a special solar charge controller that allows the array to be at a higher voltage than the batteries, you will have a hard time wiring the odd number of 7 solar modules. The modules you mentioned are nominal 12 volt units, so they are wired like 12 volt batteries. If you have an odd number, you will have no way to have equal "strings" of 24 or 48 volts.

Most of us are getting away from these small lower cost solar modules as they have less power and require more above roof wiring that is subject to the elements. For example, I could install two modules at 160 watts each and have 320 watts at 24 volts with much less wiring and mounting issues to deal with. These larger modules usually carry a 25 year warranty, which I think is much longer than offered with the modules you are considering.

Finally, as to how many of anything you need. I suggest buying the most efficient lights and appliances you can buy, a very high quality inverter having a low standby loss, buying very rugged golf cart batteries, and buy as many solar modules you can afford! Make sure your solar charge controller and array wiring can handle a much larger solar array than you first install. This makes it very easy to add more solar modules as your loads or run times increase. With over 25 years install experience behind me, I have had very few clients that installed as many modules as they really needed at first due to budget concerns, yet, most came back a few years later and added more.

You will always exceed your system capacity at some times of the year, so expect this and plan on what loads to keep off until the system can get back to full capacity, or plan to have a back-up generator. Finally, check out the back issues of Backwoods Home Magazine for several articles I did that address this type of solar installation: Issues #73, 83, 84, 87, 88, 89, 90, all in the 2003 to 2004 period.

Good luck,

Jeff Yago


Solar battery question

We are building a new home in the caribbean on the ocean front.....The PV array will be a little over 11 KW. Our question is where to put the battery bank of L-16 batteries. My architect wants to put it in the same large mechanical room as the generator (we are debating propane vs. diesel) The genny will not run often as we expect the solar array to provide most of our needs and we are also grid-tied. The generator is mainly for those times when a hurricane wipes out grid electric for a couple weeks. The problem is that while we can put louvred doors on both sides of the room, that will allow a steady flow of fresh salt-laden air that will increase corrosion on everything...we are open to the easterly trade winds, so the salt does a number on everything here.

We were thinking of perhaps putting the batteries in a vented cabinet.

Do we have to provide intake air to the cabinet? If we did that we could get rid of the louvred doors and solid doors would protect the generator and solar equipment from salt corrosion.

Larry Meyer

Larry,

Yes, you are correct. Anyone installing solar in an ocean and tropical enviroment will have an extreme corrosion problem as most solar modules, solar controls, inverters, and solar charge controllers have aluminum frames and cases which do not like salt air. Also, the higher temperatures make it important to provide good ventilation which just increases the problem.

First, many of the solar equipment manufacturers offer "optional" case materials or non-vented equipment enclosures designed to operate in salt air, as you are not the first to have this need. Be sure to specify this when you are ordering any solar hardware.

The salt air will do more harm to your electrical switchgear and electronic solar components than your batteries which do need ventilation, so I would put the batteries in their own room that is ventilated but does not contain anything else. A system this large will require many L-16 size batteries, so you may want to consider using larger "industrial" deep cycle 2 or 4 volt batteries which are larger and will require fewer separate battery strings. The more "strings" of batteries you have, the more problems you will have with un-balanced charging and dis-charging.

Use solid copper buss bars to inter-connect the batteries instead of cables which will corrode, and stainless steel terminal nuts and bolts. Provide a good floor drain so you can "wash down" the batteries every few months as you will get both a salt and acid "mist" build-up on the battery tops and connectors that will cause a voltage "leakage" between the terminals.

Since you will not use the generator very often, purchase a generator with weather-proof outdoor enclosure, and locate it on the side of the home that provides the most shielding from the salt breezes. If the generator uses a remote fuel supply like propane or diesel fuel, you may be able to cover the entire generator with a weatherproof cover or tarp, but remember most standby generators have a weekly exercise program that will start them without your intervention and you do not want this to happen when it is covered. Putting a generator inside any building requires lots of special design issues including fuel fumes, combustion air, cooling air, very hot exhaust gases, space temperature buildup, and regular access for service.

Finally, locate the inverter(s) and charge controller(s) in a protected room that is next to the battery room so cables to the batteries are less than 10 feet long. This room should be large enough that the equipment does not over-heat, but does not need to be vented to the outdoors. Keep in mind that some charge controllers and inverters do make noise, so you may want to locate in a laundry room or utility room on the opposite wall from the battery room, and not in your living room!

So, when do I get to visit?

Jeff Yago


Radiant floor heating system design

Hi Jeff,

Please excuse me if this subject has already been covered or if I seem I'm an idiot as I am a newbie to the subject. I was interested in installing a radiant floor heating system. I've done a little research and was wondering if the following design is viable.

My idea was to have a dedicated hot water heater and use it as a source for radiant floor heating. My plan is to use standard PEX type tubing and fittings. I would add an expansion tank and air bleeder valve for safety reasons. I planned to use an automotive water pump to circulate the water. I would simply have a pulley, belt and electric motor to drive the pump. The water would run in a continuous loop, returning to the water heater. The set up would be controlled by a thermostat which would cycle the electric motor on and off.

Has anyone made a system like the one I describe?

Thank you so much! And what a great website!

Sincerely,

Kevin Moelk

Kevin:

Most radiant floor systems in houses have the heat tubing embedded into a lightweight concrete floor slab or thinset concrete slab poured over top of an existing concrete floor. This gives the system lots of "mass" to hold the heat for longer periods and avoid rapid temperature swings.

As to your plans for heating the water from a hot water tank I am not sure why the question as this is done all of the time. You really need to stay with a tank type water heater as the instant one-pass type water heaters are not designed for water being returned from a closed heating loop. These instant type heaters and their heat exchangers are designed to be fed cold water entering to be heated and you can really screw them up if the cold supply line is connected to water that has already been heated.

As for pumps, I am not sure why you are trying to build a pump with belts and pulleys. Most heating circulating pumps are either 1/20 or 1/12 HP and are designed to be very quiet and use very little electrical power. Most also have the impeller running in ceramic bearings that are kept wet by the hot water and the pump section is inside a sealed stainless steel can. The "motor" section is actually an electric coil on the outside of this "can", so there is no motor to pump drive shaft and no shaft seal to leak or wear out. Since these pumps are also fairly cheap and are available in both AC and DC versions, I do not think it is worth the time and trouble to build your own.

Yes you need and expansion tank, check valve, temperature relief valve (usually 200 degrees) and a pressure relief valve (usually 15 or 30 PSI), but these safety valves need to be selected for your specific hot water tank and radiant tubing temperature and pressure ratings.

Most of these systems cycle the burner on and off based on the tank's discharge temperature setpoint and allow the water pump to stay on to circulate all the time (except during days or months heating not required). Cycling a pump on and off in this type system for temperature control can cause major temperature swings inside the water heater and cause erratic temperature sensing of the thermostat . Air bubbles will collect inside the pump when the flow stops and this can causes cavitation when it tries to start pumping again.

During water circulation, any air entrapped in the piping loop will pass by the system's air vent (yes, you need one of these also!) and be expelled to the outside. Its very hard to get air out of a closed loop system after you service or fill it, and any air always find its way into the pump and stops loop circulation.

Finally, you will need an auto-fill valve set to about 5 to 10 PSI pressure. Its job is to let in more make-up water if there is any small leak over time in the system, or when there is a discharge from one of the relief valves because the system over-heated (usually when the pump fails). You do not usually run these systems at city line pressure, and if you also plan to supply hot water to sinks or showers you will need to add a water tempering valve as the 160 to 180 degree loop temperature will be too hot. In addition, if the system stays "closed" and does not supply bathroom or kitchen fixtures, your circulating pump can be a cheaper cast iron model. If the system will also supply fixtures, this requires the pump to be higher cost stainless steel because the constant changing of hot water in a flow through system is very corrosive.

Lots of advice so I hope I answered all your questions.

Good Luck!

Jeff Yago


Radiant Floor Heating in an RV

Can radiant floor heating be used in a 5th wheel or any RV?

An enclosed system. Mabe on 12 volt?

In the winter it could keep us warm, and in the summer keep us cool. I think it can be done.

Thank you.

Karen Borghesi.

Karen,

Yes, you can have radiant floors in an RV and some high end models come that way from the manufacturer. However, to keep weight down and reduce chance of cracking, I believe most of these systems use the radiant tubing under a wood floor and have a metal "reflector" to direct the heat up. Of course this system would also require lots of batt or spray insulation below the heat tubing and reflectors to avoid major heat loss down.

Good luck!

Jeff Yago


Solar Package

Jeff,

I have a small cabin less than 500 sq ft with minimal energy used. It is in Southern Utah. Lots of sunlight.

I plan to have the following when I am all done.

-small TV
-radio
-Laptop
-(4) small fluorescent lights
-small fridge
-future freezer as well

Did I miss anything important?

How big of a system do I need? Is it better to buy a package and if so, would you recommend one?

Also, someone told me I could use golf cart batteries? Is that true?

Thanks a bunch for your time.

Brent Toft

Brent:

I have found after spending days calculating solar insolation data, estimating system loads, solar hours, operating hours, battery size, solar array tilt angles, to select the right solar array size, for almost every project I have designed the final system the client selected was based on what they could afford, not what I had designed!

I have come to the conclusion after many years of this to start with a budget range first and design the best system I can for that budget. Working backwards, for an offgrid system this means if they add too many loads or operate the loads for the array and battery they have, they will either need to start a generator from time to time or sit in the dark! Even when a client has followed my design with cost not an issue, they usually returned in a year asking me to increase their system capacity as they have decided to add that 60" home theater and martini blender they just have to have.

My point is, buy the most system you can afford, and keep your loads down to match what you have. Most of the loads you described can be found in 12 volt DC versions, which means you do not need an inverter, which can be expensive. However, if you later decide to add other household appliances or better lighting fixtures, you will find the choice in DC powered versions is very limited and an inverter would make things easier.

You asked if you forgot anything. The biggest instant load on any off grid solar system is a water or well pump which you did not mention. Refrigeration is the biggest "energy user", but most refrigerator loads are small - they just require more hours of this small load operating per day. Pumps are available in both DC and AC versions, but if AC, this will require a 3 or 4 kW inverter. If DC, it will cost about 4 times what an AC pump would cost. I am assuming you are using a hand pump, free flowing spring, or using bottled water.

If you stay with only the loads you mentioned, you may be able to get by with something in the 300 to 400 watt array size, which would be eight 50 watt modules or four 100 watt modules. However, I expect as soon as you see the price for these you will go out and buy four 50 watt modules and hope for the best!

Yes, golf cart batteries are an excellent choice for small remote offgrid systems because they are cheap, you can buy them almost anywhere, and after 4 years they can be recycled for new ones. Do not buy the newer 8 volt golf cart batteries. The older style 6 volt versions have big thick plates and will take the daily deep-charge and discharge cycling typical for solar applications. The more batteries you have, the longer you can go with no sun, but then, the longer it will take to re-charge, and the more chance there is that a small solar array will never is able to fully re-charge the batteries so they are always operating partially discharged. Bottom line, start with four 6-volt golf cart batteries with two "pairs" (two wired in series) wired in parallel to keep it at 12 volts. That is a good compromise between runtime and re-charge time for a small system. If you go with a smaller array you may need to drop down to only two batteries, but that will really limit how long the above listed loads can operate.

Buying a kit may make things easier, but I have found most kits always have something that you do not need or missing something you do need like special array mounts or a different battery size. You need solar modules, a good quality charge controller, a DC fuse center, and gold cart batteries.

Check some of the back issues as I have provided many photos and wiring examples of how to size and build a solar system for off-grid cabins.

Good Luck!

Jeff Yago


Outside wood boiler

I have a small new 500 sf sauna cabin on a nw aspect at 10,700 feet in sw Colorado. Heat will be a major problem. There is no grid power, so I anticipate 12v with solar (solar will work even with the nw aspect, located near top of ridge). The site is heavily wooded so firewood is abundent. There cannot be a well because access to the site is too steep for a drill rig to access. There is a seasonal stream on the property.

Does anybody manufacture an outside wood boiler that runs on 12v and doesnt need a water hookup, or even better if it didnt need any power?. Are you aware of any wood fired snowmelters for making water in the winter?

Mark:

Just about any inside or outside hot water boiler can have a 12 volt DC pump used to replace the 120 Volt AC pump, as long as you provide the flow rate for the given pumping head to prevent boiler over-heating due to low water flow. My question is why would you want to do this? You are just asking for pipe freezing problems when you are away even if you have anti-freeze mixture. The first time the system over-heats and discharges out the relief valve, make-up water enters which dilutes the anti-freeze mix and soon you well have little protection. Hydronic systems also require some effort to get the air out of the lines and do not like being drained down and re-filled each season. If you ever get the air out of one, you tend to leave them alone!

Keep it simple, you have lots of wood, heat with a good ol' pot belly stove or sealed wood stove having an outside air combustion air intake. No water to freeze, no zone control valves to stick or fail, no pumps to try and run when your battery is dead and you are cold.

As for water, check out issue #71 for Sept 2001. I describe a domestic water system I designed for an off-grid home they built where they could not drill a well. It pumps surface water from a nearby creek into a plastic holding tank with a slow DC pump. A second RV type DC pressure pump at the bottom outlet of the tank runs this tank water through a sand filter, carbon filter, and 12 volt DC ultra-violet water purifier to kill any remaining bacteria. This system piping can be drained to prevent freezing when you leave if you locate several easy to access drain valves.

Finally, if you want to melt snow, put a pan of snow on the top of a wood cook stove. We have one in our living room which we use on some cold winter days. This is a great way to add humidity to cold dry interior cabin air and get drinking water at the same time. Keep it simple!

Jeff Yago


Temperature Sensor Location

Jeff:

When installing a typical solar hot water heater, the location of the temperature sensors seems to me to make quite a bit of difference in operation. I understand that one sensor will be on the roof and the other will be at or near the tank. My question is in reference to the sensor on the roof. In a typical application, two panels could be used. These two panels will have to be connected somehow. In some of the systems I have seen this sensor is placed on this joint. I'm thinking that it would be better to have the sensor on the return side of the panels to be able to achieve a more accurate reading of the temperature differential between the tank and panels. What are your thoughts on this subject?

K. Foy

K. Foy:

You ask an age old question. Which is better, a low wing or high wing plane? Which gets better gas mileage, a 4 cylinder engine running high RPM or a 6 cylinder engine running low RPM? Drain-back, drain-down, or closed systems?

If you purchase a solar hot water module with an optional temperature sensor factory installed, it is usually mounted on the back of the absorber plate near the top center, with the wires out the back. If the sensor is installed in the field, most installers mount it on the top exit pipe from the array on closed loop systems, as this will be the first point in the piping to "sense" heated water as it starts to expand and shove heated water out the top end of the header even before the pump turns on. This is because there usually is a check valve at the pump keeping the flow from expanding back the inlet header.

On drain back systems when the solar panels are empty at first, air inside the empty array piping rises and heats the pipe sensor attached just at the exit header. Since the water entering at the bottom is heated before it reaches the top outlet where the sensor is located, hopefully it is hot enough to keep the pump from cycling off. You want the tank sensor near the lower part of the tank to "sense" the colder layer of water in the tank.

You want the differential temperature controller to compare the hottest water leaving the solar array, with the coldest layer of water it is going to heat. You do not want to keep the pump operating longer than required by "fooling" it with average temperatures. This can reduce system efficiency by actually cooling down the tank temperature. The controller has set-points that take the temperature delay and approach temperature (related to heat exchanger performance)issues into account.

On a side note, I was asked to check out a commercial solar hot water system on a government building that had over 5000 square feet of solar array. It had been in operation for about 3 years when I was asked to see how it was doing. I found out the installer had driven a retainer screw on the metal conduit that held the wires going to the 10,000 gallon tank temperature sensor. This shorted out the tank sensor so the differential controller "thought" the tank was at absolute zero, so no matter how hot or cold it was in the array, the tank always appeared to the controls to need heating.

The hot water heated during the day kept circulating all night and just cooled the tank down again since the pump never shut off. Nobody ever caught it which is the biggest problem I have with solar hot water systems. If you do not install easy to read temperature dial gauges on the supply and return piping to the array, and on the tank, the system can fail and the home-owner will never know because these systems always have a 100% capacity back-up gas or electric tank or in line heater. The water out the tap is always hot and the homeowner is happy knowing he has done his part to save the earth - even though their system may be operating full time on electric heat. Good luck and add some gauges!

Jeff Yago


Heat pump run with solar

Jeff,

I own land in northern Minnesota and am considering building a cabin (800 sq.ft.) there for retirement. I will build the cabin to be very well insulated (straw or conventional). Weather varies from -40 to over 100 degrees. The high water table is 5 feet below grade and the soil is pure beach sand from a glacial outwash. The sand goes down 45 ft. and sits on top of 135 ft. of clay. This seem like ideal conditions for an open loop heat pump except that the nearest electric line is 3/4 miles away.

Are there heat pumps and well pumps that are efficient enough to run off solar (maybe wind too) power without the cost of panels and inverters becoming exorbitant? It seems to me that the system would have to be sized to provide enough power to provide the surge needed to start both pumps simultaneously even though this would only happen occasionally? Is the real problem the size of the battery bank needed to run this for 3 days of cloudy weather?

Thanks,

Curt Decker

Curt:

To answer your first question, you can run anything electric from a solar power system. The problem is, very few non-commercial installations are large enough (expensive) to provide the power required. Unlike a regular well pump which still is a large motor load the size of many heat pump compressors, a well pump works well with a solar system because the well pump only operates a few minutes each day. However, it is not unusual for a heat pump compressor and the associated geo-thermal loop pump to operate 50 to 80% of every 24 hour day, depending on sizing and heat load. In other words, I doubt you want to spend over $100,000 on a solar power system and still may have a few winter days that you still need back-up power.

You may be able to do OK powering one or two small super-efficient split dx systems to cool bedrooms in the summer when you have longer sun-hours, but you will still need a fairly large solar power system. Also, unlike a heat pump which also runs at night which means powered from batteries, most cooling loads are in the late afternoon when you can power from the solar system and not the batteries. You would be amazed how fast a compressor, pump, or fan can drain down a battery bank at night.

For your winters I would definitely have a good wood stove and use a propane or an oil-fired hot water boiler, and use either radiant floor heating or baseboard radiation. These systems require no more electrical power than a 50 watt light bulb to run their small pump and controls and I have used many with off-grid solar homes. I have put hour meters on homes heated with heat pumps and homes with conventional gas furnaces which use ductwork and central circulating fans and I have found they both operate more than 12 hours of every 24 hour day when the temperature starts to drop. I do not recommend any heating or cooling system that requires ductwork and a large central for an off-grid home.

Good Luck,

Jeff Yago



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