Alternative energy advice from Ask Jeff Yago. Volume #11

Ask Jeff Yago
Solar & Energy-Related Issues

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

Sorry. Jeff no longer answers questions online

Corn Boiler


Do you have any plans for a corn boiler that I can use to heat my radiant floor heating system?




Funny you ask. You should have no problem finding plans on the Internet for any kind of corn stove and how to build one as I have already checked. The reason I am sure, is about 2 years ago I discussed in an article that an off-grid solar home should have some form of “wood” stove, even if just for back-up. I received a letter to the editor from a real corn stove crusader who was mad I did not say a “corn” stove. I pointed out that I also did not say a coal stove, an oil stove, a pellet stove, a kerosene stove, and was assuming readers would have the kind of stove that burned whatever fuel was the most common for their area. He then nailed me again about how a corn stove would work anywhere regardless of what fuel was the most “common”. I gave up after that debate, but know he is still lurking out there waiting for me to mention a corn stove!

Anyway, the last time I made a simple search on the Internet for corn stoves, not only did I find commercially made stoves, but I also found many how-to plans including some from universities doing research on the best designs.

Good luck, but don’t tell anyone you will be burning that 4 letter stuff “c—”

Jeff Yago

Homemade wood stove


We live in a old loggers cabin with little insulation, and we have gone through many wood stoves most 55 gallon drum types. I was given an old propane tank 300 gallon. This will allows for longer logs and more heat, but I want to build a secondary burn chamber to take advantage of the wood gas and more heat. I plan to run my stake out the middle back of my stove. Can you help and do you know of any books on building your own stove?



Glenda and Paul:

For safety reasons I am reluctant to tell you how to build a wood stove, but I will offer a few suggestions about what not to do. First, just because you have a free 300 gallon tank does not mean you have to use it for making a wood stove. This is extremely large and probably will not provide the efficiency you need unless you are planning to fire a steam train! The biggest problem with oil drum wood stoves is their very thin metal construction that burns out fairly quickly.

A much better choice which has much thicker steel, is also free, and a better size for heating a home is a 30 to 50 gallon discarded “electric” type water tank, with the outer insulation and shell removed. Most people cut out the end and weld on metal door hinges, pipe legs on the bottom, and flue out the back top. As for a secondary fire-box, I have seen many wood stoves with added sections of ductwork or secondary smaller “tanks” added to the exhaust flue of the main chamber. The will build up soot as the exhaust gasses slow down when entering from the smaller flue before speeding up again out the smaller exit flue, so this also needs a clean out.

Keep in mind, just because you have a big metal drum does not mean it will burn wood efficiently. There are 3 stages to wood burning. The first stage requires the un-burned wood to be “cooked” until it is completely dry and starts to give off gases. At around 500 degrees, these gases start to combust which provides the heat and drys out other un-burned wood. As the wood begins to form charcoal, the third state of combustion takes place and these hot coals can reach over 1000 degrees. As you can see, if you do not have the right combustion box size and arrangement, the wood can get too much air which cools the burn temperature and reduces combustion efficiency, or too little air which produces un-burned gases and not enough heat to dry out other wood. These cooler gasses can build up on the interior parts and flue which can cause a chimney fire later.

My advice is to locate a “free” electric hot water tank in the 30 to 50 gallon size range. These are also free and will accommodate very long logs. The hot water tank metal is many times thicker than an oil drum and will last much longer. There are many free designs on the Internet describing how to build one, including:

Good luck and stay warm!

Jeff Yago

12 volt thermoelectric refrigerators


I love your articles. Keep them coming. 12 volt thermoelectric refrigerators/coolers (as advertised) are limited to cooling about 40’F below ambient temperature. During a power outage during hurricane season, here in south Florida, a temperature of 55’F (95’F ambient – 40’F cooling ability) would probably not be safe for food storage. Can I put a smaller thermoelectric ‘cooler’ inside a larger thermoelectric ‘refrigerator’ for additive temperature cooling?

[For example:]

95’F Ambient temperature
– 40’F outer / bigger cooler #1
– 40’F inner / small cooler #2 (placed inside the bigger one)
= 80’F temperature reduction?

Will this provide for a final temperature safe for food storage, using less solar wattage than a standard compressor driven refrigerator that will bleed me dry of my limited solar power?



The answer is No No No!

If you place one refrigerator inside another, you are forgetting the exterior side of the solid-state thermal device is pumping out as much BTU’s of waste heat on the outside, as the interior side is putting out in cooling BTU’s. Lets say for example, the inside of the interior refrigerator was 45 degrees. However, to keep it at 45 degrees inside, it is ejecting very warm air being heated as it passes across the exterior fins of the thermo-electric device. Placed inside another refrigerator and you have a heater heating up the inside of the outer refrigerator as it is trying to cool its inside. In other words, if the efficiency of both refrigerators was 100% (which is not), The best the outer refrigerator could do is offset the heat being ejected from the inside refrigerator. When you toss in efficiency losses, the outer unit will never catch up with the heat being ejected by the small refrigerator inside.

If you refer back about 2 issues, I had a detailed article on many different 12 volt DC refrigerators and freezers I tested for possible solar application. These included many from the RV, trucking, and boating industries. I also noted in this article that I absolutely would not use a thermo-electric type solid-state refrigerator because they do not get cool enough, and they require too much electrical power over a given time period. I owned a small one for about 2 months for my truck which has a 100 watt solar module on the roof (about $950 worth of solar!) and it ran the battery down every day.

I would avoid any refrigerator planned for a solar power system that does not have lots of wall insulation, a really good door seal, and uses a small 12 volt DC compressor, not a solid state device.

Hope this helps, and keep us posted,

Jeff Yago

Bicycle-powered electricity generators


I’m trying to find a company that makes electricity generators that are bicycle-powered, but all my web searches are coming up with nothing–but my search brought me to your web site, so I’m hoping that you know what I’m talking about.

Basically I want to bring one of my bikes into the house and put it on a generator to power certain appliances or charge batteries. Any suggestions?



Buffalo, NY


What you are looking for was very popular in the late 1990’s, but not sure if they are still made. Below is an ad from a 2004 Real Goods Catalog for item #47-0103 “Bike Generator & Stand for $639. You use your bike, and place rear axle into this metal stand that holds the rear wheel off the ground and keeps a 12 volt DC generator turning by running against the rear tire.

This product is not in their 2006 catalog, but it is listed as “made in USA”, so they may be able to provide you with a contact number, by calling 800-919-2400 and giving the above part number.

Hope this helps,

Jeff Yago

Battery Bank For Mobile Lunchwagon’s Kitchen

Aloha Jeff:

I am operating a lunchwagon (mobile food concession) out of a P-30 Step Van (think Fed-Ex) on the Valley Isle of Maui. My truck contains a full kitchen: refrigerator, freezer, warmers, hot water heater, coffee maker, microwave… With every appliance running at once it requires ~9000 watts or 75 amps. My business operates for roughly 6 hours a day. At night I have access to grid supplied electricity. A 10,000 W generator is loud, cumbersome, and polluting, and I would very much like to explore the possibility of drawing the power I need from a bank of batteries. Ideally, at some point I would love to recharge the batteries with solar power, though I’d no doubt have to have twice as many batteries on hand to charge while at work. Could you please give me advice concerning what type of batteries could handle such a demand, any concerns I might face, and how much such a system might cost me. Thank you so much!

Byron Gardner


I think you better buy a trailer to haul all the batteries you would need to do this! A typical golf cart battery weighs 64 pounds and will store approx. 1 kWh of energy. You need 54 kWh’s of power for a typical day (9 kW X 6 hrs) and normally you should not discharge any battery bank more than 50% to 60% each day or they will not last very long. This means you will need 54 / 50% = 108 batteries which would weigh 6,912 pounds. This is also a VERY big electrical load for any solar system. Using a rough average of $9 per watt installed cost ( $8 to $12/watt is average installed system cost) A solar array that is to equal this 6 hour load would cost over $81,000.

My advice, buy a generator with a really quiet muffler!

Jeff Yago

Multi-zone Thermostats

Hi Jeff,

Do you have any recommendations for a good programmable thermostat to be used in a multi-zone hot water system (oil burning boiler)?

Dinos Hadjiloizou


Honeywell makes a nice model, but there are several other really good manufacturers. What is more important (as long as you stay with a name brand) is the features. For example, Some models offer four different set-points per 24 hour period. This allows setting a warmer temperature just before you wake up, then setting back to a lower temperature during the day when you are up and about (or gone!). Then around supper time (or when you get home from work) the temperature is set back up for the evening. Then at bedtime, the temperature is again dropped back to a much lower temperature since you will be under the covers! Some thermostats only allow 2 set-points per 24 hours, which limits you to just occupied and un-occupied program set-points.

Some of the better thermostats allow programming different occupied and un-occupied hours for each day of the week and week-end, while less expensive models only give you single weekday and weekend set-points. If you have a non-typical schedule, this may not offer enough options. Be sure the thermostat is easy to program and has good battery back-up. Look for a model that has a larger battery, but not some special order odd voltage type. If your power goes out for several days, this can really drain a small battery unless the display is a low energy liquid crystal type. Avoid models with display lights and complex program requirements – keep it simple!

Some models allow removal from their wall base plate without loss of programming. This allows programming while seated at your desk, instead of standing against the wall for a half hour!

Also be sure the thermostat is designed for your type system. More expensive thermostats have multi-stage heating controls for heat-pumps and dual heat systems that you will not need. Expect to pay at least $65, and better models can cost up to $95.

Good luck!

Jeff Yag

Solar Heat

Dear Jeff,

My house is a 3000 sq.ft. ranch with a detached car port. The house is well built and was heated with a boiler housed in the car port. The boiler (needing to be replaced) heated water and pumped it through copper piping under ground to my crawl space and up to the attic where it tapers down through smaller pipes spread through the thick plaster ceiling. All of this was thermostatic controlled with the cooled water returned to the boiler. The rear of my house faces south-south east. My question; is it possible with a new boiler to use a combination of solar collectors combined with the boiler to heat my home? If so how much of the cost could I recover with tax incentives? As you can imagine I have a lot of copper in my house. I live in central North Carolina.

My Regards,

Kevin Guthrie


The answer is – yes you can, but I am not sure I would do it. First, you have an un-usual type heating system. Although putting hot water coils (or electric heating elements) into the ceiling has been done for years, this is not as efficient as other heating systems because heat rises. Most of us put the heat supply device (radiators, floor registers, slab heat, baseboard heaters) at floor level or in the floor slab. Since the heat rises up, there is less heat loss down and more heat rising to heat all of the room from floor to ceiling. Without some form of air movement, the ceiling heat system tends to produce a heat layer that stays near the ceiling, not to mention the heat that is lost to the attic. You may have already noticed this with your system, not to mention the cold floors! Any type solar system is expensive, and should be designed to maximize the system efficiency. Piping solar heated water through heating tubes running through a well insulated slab floor offers a much better system heating performance. You must also consider existing system design. Most oil and gas fired boilers are designed to deliver 160 to 180 degree water. This higher temperature allows using smaller or fewer heating pipes, and spacing them further apart to deliver the desired BTU’s of heating capacity. Most solar hot water systems will have their winter hot water output in the 90 to 120 degree range (at best!) under a constant flow rate, which would have far less heat output when tied to a distribution system designed for much hotter water.

Hope this helps!

Jeff Yago

Radiant Floor Insulation

Dear Jeff,

I am adding a 490sq ft bedroom suite to my existing home. The construction is ICF; the floor system is conventional 2×10 joist floor 16″ on center with 3/4 T&G plywood with a center beam. I wood like to install radiant heat flooring on top of this. I have left 1&3/4″ to meet the floors in the existing house. I would like to put 1/2″ super Tuff-R then 1/2″ PEX imbedded in 7/8″ of concrete topped with 3/8″ engineered wood floors (glued down). I am some what concerned about the weight of the concrete. What do you think? What changes would you recomend?

Gef724 at

A 2 x 10 floor joist indicates to me you probably have a 12 foot span between supports. Longer spans use 2 x 12. Its not just the size of the joist, its also depends on the span. Without considering joist size and span you cannot determine load capacity. This means the joist size you have may require added center support if the span is boarder-line for the joist size. Also, you said you were adding concrete. Check to see if your piping system can work with “lightweight” concrete. This is a mix that includes lightweight materials mixed in which reduces the weight of your poured floor.

Good Luck,

Jeff Yago

Well system as double duty ground source cooling


My Parents have a 3500 sf country home, very well insulated, but lots of southern facing glass, and electric only (not too bad compared to NG prices).

They have a deep drilled domestic water well (about 650-700 feet) with crystal clear water at about 50-55 F.

My question is, could I simply tap off the existing well output to feed either a water based AC system or Heat pump?

If so, what is a rough approximation of the CFM/ GPH etc that such a system would run?

If the water usage is considerable I would like to recirculate the water in some way, though it occurrs to me that simply dumping it back in the top of well might cause considerable turbulence, and possibly cloudy water.

What do you think ?

Alex Brace


This can be done, but there are several design issues to consider. First, this is a really deep well which makes me think you may have a very low water table or slow re-fill, which means you could affect the domestic water by doing this. You are also right that there are “mixing” issues that could produce more sediment in the domestic water system. You also need to check local codes as some states or counties do not allow “dumping” water back down a drilled well which could affect the water table for others in the area with a well.

Yes, I have seen water source heat pumps with a domestic water connection that just runs through the unit and down a floor drain. Excellent cooling, but very wasteful! Some systems use two wells with one serving as a source and one as the “dump” , but again, if the water table is as low as your well depth suggests, you may spend more to operate a deep well pump than you get in cooling compressor energy savings.

Good luck!

Jeff Yago

Cookstove question


I have an older model wood cook stove in my shop. During normal times it does a pretty fair job of heating the area (20x40x12) but when it gets cold it doesn’t cut the mustard. It is designed as a cookstove, so the heat basically goes up the pipe unless I damp it down. Lately the cold has kept me from working out there and I wanted to ask if there is any way to make a cookstove radiate more heat?

I included a picture of a similar model but not exact. Mine is in great shape, has two “burner pads” on top and is covered with the “white” enameled metal. making the outside basically insulated from giving heat. Down low near the surface I’m using single wall pipe but up high it goes into thte double walled stuff.

Any help? Are there radiator fins for the stovepipe or ????


Tony May


We too have a cook stove in our living room. Its new but looks like one of the old types my grandmother cooked on as she hated her new electric range. We only use it to keep the coffee pot hot when we fire it up on a cold Saturday to read by. My stove pipe goes up about 10 feet before entering the stone chimney as we have a high ceiling, and this gives off lots of radiant heat.

There are many different types of “radiator” devices you can buy to capture stove pipe heat. I was in Lowes last week and noticed they had a black metal cube about 16″ on a side that had tubes running through it for air flow. One model also included a fan and cost around $100. The cube has a bottom flue fitting going to the stove, and a top flue fitting for the stack. The room air passing through the cross tubes is heated by the rising flue gases, with the small fan used to increase this airflow. It was well made and I was surprised to see it at Lowes. If you are near a Lowes they should have it in stock, and if not could order one for you. This is also a fairly common item in many wood stove stores, so shop around, but expect to pay at least $100 for a good one.

Good luck,

Jeff Yago

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