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Old 02-06-12, 09:09 AM   #1
abogart
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Lightbulb Forced air heat buffer?

I have been wondering if there would be any advantage to adding an air/water HX with an insulated tank and pump to a forced air furnace. One of the benefits of hot water heat is the ability to add heat storage capacity in the form of accumulator and buffer tanks. Why not do the same with forced air? When the furnace is running, the water is heated by the furnace, storing some of the heat in the water. When the furnace finishes its cycle, the pump and furnace blower remain on, continuing to heat the house with the stored heat. The pump could be set to turn off at a set low temperature, and always on when the furnace is running.

Would there be any benefit to this setup? I can see reduced cycling as a potential gain, increasing furnace efficiency with longer run times. DHW could also be preheated in this manner, as well as adding the capability to grab excess heat from other sources, such as solar, wood, or even a DIY heat pump.

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Old 02-06-12, 01:23 PM   #2
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I have been wondering if there would be any advantage to...
Interesting idea.

Some things to keep in mind, when your furnace fires, the fuel can reach very high temperatures (400F to 600F and higher), so the area of the furnace's HX does not need to be very large to supply the amount of heat that is required to the house.

With fossil fueled hot water heating, temperatures of the water can run to about 160F to 180F, so the area of the HX would need to be larger to transfer heat at the rate you want.

If you are going for lower temperature heat, such as what solar or maybe even a heat pump can provide (90F to 140F), your HX area needs to be larger still.

One of the ways to get a really large area HX that will work for lower temperature heat sources is to use the whole floor as a heat source!

Most of the radiant floor design work that has happened to date in the US assumes fossil fuel heated water, and the water pipe that carries heat to the floor is assumed to carry water in the range of 140F to 180F, so spacings of 12 to 18 inches work out pretty well.

If you want to take advantage of lower temperature water (90F to 140F), you would be disappointed by the performance of a floor that was designed for higher temperature water.

So you would want to increase the 'heat density' of the floor's hydronic system. In other words, the spacing of the tubes that carry the water would need to be closer together.

European hydronic designers seem to be more aware of this than designers in the US. One reason is that energy in Europe costs about twice what it does in the US.

So, long story short is that your idea will work, but you will need a large energy-dense radiating surface area to make it work the way you envision.

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Old 02-07-12, 09:01 AM   #3
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... OR I could just put a big block of cement inside the supply plenum.
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Old 02-07-12, 04:36 PM   #4
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Or for tempering incoming air for the furnace?
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Old 02-07-12, 05:59 PM   #5
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Or for tempering incoming air for the furnace?
Do you mean combustion air? I thought about running a shell tube heat exchanger (a pipe-in-a-pipe) over the vent pipe to remove SOME heat from it to temper the incoming air, maybe just up to room temp or so. In order to do that I'd have to air seal the burner compartment (not very difficult) and make sure that the exhaust gas temp doesn't drop too low because it's an 80% AFUE non-condensing furnace. I feel better about using waste heat for things like that.
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Old 02-07-12, 06:05 PM   #6
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In order to do that I'd have to air seal the burner compartment (not very difficult) and make sure that the exhaust gas temp doesn't drop too low because it's an 80% AFUE non-condensing furnace. I feel better about using waste heat for things like that.
I had a thread about doing that and it turned out to be unsafe. If you want direct vent you need to get a direct vent unit. Sorry. But, you can add a duct to bring fresh air right to the boiler (I also have a thread about this too) and you could temper that air. Not sure what it would gain you though.
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Old 02-07-12, 08:01 PM   #7
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I'm not sure if tempering the incoming air is a good idea, I have often thought that the cooler the incoming air the more heat you can pull off the heat exchanger resulting in less heat that is vented out with the exhaust.
If your furnace is anything like mine, the fan draws 100's of watts, mine is around 600 watts, furnace fans tend to be one of the single largest electrical draws in the house, turning the thermostat down and using a 600 watt heat lamp would keep you more comfortable.
Adding mass to your duct work, like a cement block really could help, putting that mass in the room on or near the heat register would increase the thermal mass of the room and help even out the fluctuation.
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Old 02-08-12, 09:10 AM   #8
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I had a thread about doing that and it turned out to be unsafe. If you want direct vent you need to get a direct vent unit. Sorry.
Yeah, I'm beginning to realize that. It's just that every time I feel the heat radiating from that vent pipe, a little piece of me dies inside.

I plan on running a fresh air pipe for the furnace and water heater. Not sure yet if I'm going to seal them up, but I don't like the outside air mixing with the basement air.
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Old 02-08-12, 09:14 AM   #9
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Yeah, I'm beginning to realize that. It's just that every time I feel the heat radiating from that vent pipe, a little piece of me dies inside.

I plan on running a fresh air pipe for the furnace and water heater. Not sure yet if I'm going to seal them up, but I don't like the outside air mixing with the basement air.
Did you find my threads on this stuff? Or maybe it was only one thread. There are motorized baffles that go at the end of an insulated duct that only open then the burner fires up. Ironically, about a week after I went through that whole process I visited my grandmother and she had recently gotten one of these things installed. She called it a "fan in a can".
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Old 02-08-12, 09:30 AM   #10
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Did you find my threads on this stuff? Or maybe it was only one thread. There are motorized baffles that go at the end of an insulated duct that only open then the burner fires up. Ironically, about a week after I went through that whole process I visited my grandmother and she had recently gotten one of these things installed. She called it a "fan in a can".
Yes, I read that thread when I was thinking about doing that to my furnace too. I think the main problem is that these non-condensing furnaces are not designed to handle condensation, and the vent has to maintain a high enough temperature to get the water vapor out of the building without condensing in the vent itself.

Were you successful in installing the motorized damper and fresh air vent? I'm not about to unload that much money on a motorized vent, but I did find these manual backdraft dampers at amazon for a good price.

Fantech RSK 4 Backdraft Damper 4" Duct

They are basically just a check-valve that only lets air flow in one direction. I might use one of these in my install. But if I'm sealing both the furnace and water heater burner compartments, it won't really matter if there is a little bit of convective flow when the units are off.

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