Can I get useful work out of a home furnace?

It’s heating season again and once again, I’ll be burning enough fuel every couple of months to push a large SUV across the country and back for no reason other than to generate heat. Seems sort of wasteful. Suppose I wanted to extract some useful work from the furnace before all that heat drifted up the chimney or through the ducts?

Suppose, for example, I ran an electric generator to charge a bunch of batteries. Some of the waste heat from the generator could be used to heat the house. If the rest of the energy is used to run lights and appliances, is the net result more efficient or less efficient than simply heating with the fuel and buying electricity? It seems as though the waste heat from the appliances and lights, being in the house, would accomplish pretty much what running the furnace would accomplish, but at least you’d get some intermediate use out of the energy first.

So…is this:

a. A good idea that everyone will be adopting once fuel costs hit a certain price point?

b. Totally retarded with some completely obvious logical flaw?

c. A reasonable idea that’s just too complicated to be cost effective?

This is probably a better question for Unapersson WRT can you generate power via a tiny steam turbine or something.

One thing that comes to mind to me is, can you cook on it? Just set a pot on top and let it heat things, that will save a few bucks off the bill.

A generator large enough to give off enough waste heat to heat your house would consume far more fuel than a furnace designed to heat your house.

I may be missing something here, but why do you consider the heat as being wasted? Are you not running the furnace to make heat , to make your house warm? Surely that is the point of the furnace.

Making heat, to drive an electric generator, to make heat is not a terribly efficient thing to do.

Is the heat that heads up through the ducts not making you house warm?
Probably the only heat lost is that which is heading up the chimney which is mixed with the fumes from combustion. I suppose you could wrap a heat exchanger around the vent and heat up water or air, but you have to be careful about cooling the exhaust too much other wise you won’t have a good draw to pull fresh air into the furnace.

Assuming you are the average user of electricity,you’re going to have great difficulty generating power any cheaper or more efficiently than your local utility.
Even if you have “free” hydro,wind or solar power,you’ll have start-up costs,and likely limited capacity,a direct effect being curtailment of much usage you take for granted.
Capturing the heat from a generator is possible,certainly,but any marketed for consumer product that uses an IC engine may suffer from such modification.
If you really want to cut down on heating bills the best bang for the buck would be improved insulation/thermal detailing of your house-the Feds were granting a tax break on that at one time.

Well, yes, but as long as I’m burning a thousand or so gallons of oil in the basement, why not do something useful with it before it gets turned into heat? Heat, after all, is pretty uninteresting. It’s essentially a waste product. And fuel, at nearly $3.00/gallon is an expensive commodity. It seems as though introducing some intermediate process between the burning of the fuel and the emitting of the waste heat would be more efficient.

What intermediate process do you have in mind? In a coal-fired power plant, or a nuclear one, the heat produced from a process is used to convert water to steam, which turns a generator.

It sounds like you’re thinking of the concept of an internal combustion engine, where the main power comes from expanding gases, and the energy used to heat the engine is wasted. In other processes, that’s not the case.

If you run the furnace to simply produce heat, you’re going to get a very high efficiency, unless you really have a poor furnace. Converting fuel to simple heat is generally a very efficient process overall.

I’m not sure how you will be getting more useful energy from an economic standpoint out of doing something like, say, running an IC engine off of the oil and trying to capture the waste heat from it. The capex and opex of such a system seems like it would far outweigh any actual efficiency gain by trying to do what’s in effect a cogeneration or CHP system at home.

In other words, I believe on paper you can design a system where you have a very small net gain in efficiency, maybe, if you are able to recover all the heat from the IC engine (cooling water, oil, radiation/convection, as well as the flue gas). On a small scale that’s probably iffy; I’m a little busy right now to be running some calcs here at work. But you might be much better off economically just trying to do something like adding an additional heat exchanger for your furnace flue, super-insulating, or getting a more efficient furnace (if applicable).

I’ve seen complicated systems where folks cool their furnace flue to almost ambient, along the lines of how folks put heat exchangers on their tumble-dryer flues, but I don’t know if it’s economical from an energy standpoint.

If you’ve got a gas furnace in a cold climate you can increase the efficiency by retrofitting it to use outside air for combustion and dilution instead of the heated air in your house. This link says it may be as high as six percent, but that the cost effectiveness will vary widely with building codes and the location of the furnace.

Either I’m totally misunderstanding you, or you’ve got one heckuva ginormous logic problem.

If we are talking about incandescent lightbulbs, then yes, heat is a waste product, because the desired product is the bulb’s light.

But we’re not talking about light bulbs. We’re talking about a furnace. For a furnace, heat is the primary product.

What the heck do you mean by “why not do something useful with it before it gets turned into heat?Before it gets turned into heat?

Why on earth would you want to interfere with the furnace’s production of heat from oil? That what’s keeping your house warm!!!

The ideal situation would be for all the heat created by the furnace to be used to heat the house. But alas, some of the heat does escape up the chimney. I think what you might really want to do is put some kind of device in the chimney, which can convert that heat to electricity, to help run your home’s devices. Or maybe something which can reclaim that waste heat and use it for cooking.

Yes, indeed. In fact, such an “intermediate process” already exists. Between the burning of the fuel and the emitting of the waste heat, the house gets heated!!!

One caution…

I’d be careful to add a DIY heat exchanger to a residential furnace designed for human comfort.

Almost all newer furnaces have small draft inducer fans. Those fans are engineered to draw the flue gases thrue the heat exchanger and to/through the flue/chimney. Those fans are designed to overcome the resistance of the heat exchanger and [ a typical] flue. Too much air and you’ll affect the gas/air mixture and pull the flames off the burners. Not enough air and the flue gases will not be pushed through the flue. (read: carbon monoxide in the structure)

If you’re adding a heat exchanger to already engineered system, you best be careful that the new heat exchanger doesn’t provide more resistance than the draft fan can handle. I would strongly suggest that a draft report/analyses and combustion analyses be done before that system is put to use. (and the necessary changes made)

If a heat exchanger is added without regard to venting, you may wake up dead. :frowning:

I think what the OP is suggesting goes by the name of cogeneration. Here’s a quote from the linked Wiki article:

This could make sense in an area where electricity is relatively expensive.

I think the OP is finding a tortured way of essentially saying, “How can I make my furnace more efficient?”

Finnagle, is this right?

Perhaps you could consider a ground source heat pump? Then at least you get absolutely free hot water during cooling season, and virtually free hot water during heating season? Plus, you won’t have to burn oil. And, the carbon offset is enough to make a hippie drive a Hummer.

I’m not sure why so many people are having such a problem understanding the basic question. The OP wants to get some useful work out of the fuel he is already burning for heat. This will not majorly impact the efficiency of his furnace, that I can see.

If he took his existing heater and put a Stirling engine on top of it, the only inefficiency that would added would be the extra energy required to heat the Stirling engine. The engine, though, would be able to use the difference in temp between the furnace and his house to do some meaningful work. I’m not sure if the work you could do is worth the expense, but that’s the question being asked.

The work you can get out of an engine (in this case, generator) depends on the difference between the input and output temperature. Heat, after all, is one form of energy and if you turn some of it into electricity, it is no longer heat. Of course some of it goes up the flu and you could, in principle, recapture some of that. But that leads to another problem. If the flu gasses are too cool, you get tars building on the chimney, which could eventually ignite unless the chimney is swept regularly.

There are two better ways. One is to replace the furnace with a heat pump, which helps cool the outside air in the winter and heat it in the summer. Even better is pump heat from underground. I can hardly believe this but I recently read (where? I cannot remember) that if you go as little as ten feet underground, you will find essentially constant temperatures and you can pump heat from it all winter and coolth from it all summer (while replenishing the heat for the following winter). I guess it makes sense and makes heat and air conditioning very cheap both in money and CO2 generation. I imagine the capital cost is high, but it seems like an ideal solution. Only you have to find contractors who can do it.

Try looking at for their contractors. There are other brands as well. Complete systems are said to cost double what conventional systems are, and sometimes it’s said that they go for $4000 to $6000 per ton of capacity (heating and cooling are both rated in tons or MBTUH’s). In my area (SE Michigan), the depth is about 6-feet for a closed loop system, if you go horizontal. You can also go vertical if you have a tiny lot, and which case it could be 300-feet for more. If you have a well, you can go open loop. If you have a lake or pond, you can go open or closed loop, depending on the temperature. Efficiencies vary. In my area one could expect to get four to five units of heat for every unit of energy (a COP of 4 to 5) for heat, and about 7 (yeah!) for cooling, using a horizontal, closed loop system. I figure that for my area, payback would be about four years (versus installed cost of a new, conventional, high-efficiency gas system with A/C), and about eight years for the entire system (versus keeping the existing, 80% efficient gas, 90% efficient electric A/C). At that point, it’s just cheap climate control and wearing sweaters in the house in winter be damned.

As an aside, waste heat from electical applainces and lights does offset the requirement for heat from the furnace during the winter. The economics of this depends on the cost of electricity vs. the cost of the furnace fuel. Here is an article that questions the overall energy savings of compact fluorescent lamps during winter months.


Consider the following dumb example. If I put 15 gallons of gas in my car, I can drive 400 or so miles. Because energy is conserved, eventually all that work done by moving the car about is going to get converted into heat as the car brakes or bangs into air molecules. Now, pretend for the sake of example that I had a racetrack in my basement and was driving the car on that race track. For the investment of those 15 gallons, I’d have 400 miles of (boring) driving * plus all that heat would remain inside the house for heating purposes *.

This the heart of my argument. If you have a fuel that’s capable of producing so much work and all you do is produce the end product of heat, it seems terribly wasteful. Particularly because almost every household north of the Mason Dixon line has a furnace which adds up to tens of billions of dollars of fuel per year going up in smoke.

If you think merely heating up your house in the winter is boring, then simply turn the furnace off. You’ll save all that fuel and have hours of stimulating mental activity frantically trying to think of ways to keep warm.

In other words: :smack: