Heat pump gurus ?'s

tdatk · 05:40 PM

I've been thinking, I know ,BAD idea, but in the warm months the condenser coil uses the available "cool" air for doing it's work. In the winter the same unit reverses and uses available "heat in the surrounding air. When the air dips below 20 or so, the heat pump is ineffective. My question is , why wouldn;t you build a box around the unit during the winter to capture the heat from the mechnicals and just recirculate the warmer air surrounding the condenser coil?

turbo6bar · 02-13-2007, 06:20 PM

The only heat to pick up is the waste energy used to operate the compressor. This would be like electric heat but with potentially much worse efficiency.

tdatk · 02-13-2007, 06:44 PM

Quote:

Originally posted by turbo6bar
The only heat to pick up is the waste energy used to operate the compressor. This would be like electric heat but with potentially much worse efficiency.

I guess I must be missing something here. If average outdoor temp is 25 and you insulate the coil from that and introduce an elevated ambient around the coil which should be compounded, you should see greater heat pump efficiency. No?

Flatbutt1 · 02-13-2007, 06:56 PM

isn't that kinda like "perpetual moton"?

turbo6bar · 02-13-2007, 07:03 PM

Sure, you're definitely better than pulling 25 degree air across the coil, but you're still limited to the heat generated by the compressor. The efficiency is still going to be terribly low--low enough that you'll be aching for a supplemental heat source.

I will run your "BAD idea" by my HVAC tutor and let you know what he says.

tdatk · 02-13-2007, 07:04 PM

Quote:

Originally posted by Flatbutt1
isn't that kinda like "perpetual moton"?

Yes and no....
A/C is far more efficient in cooler months by natural cooler ambient. I am just trying to raise the ambient around the condenser coil in winter. The energy and heat created by the compressor is just being wasted in the current configuration so why not capture that heat and use it.? A heat pump captures heat from the ambient air. The system does not care if it is fresh or recycled air.

tdatk · 02-13-2007, 07:09 PM

Quote:

Originally posted by turbo6bar
Sure, you're definitely better than pulling 25 degree air across the coil, but you're still limited to the heat generated by the compressor. The efficiency is still going to be terribly low--low enough that you'll be aching for a supplemental heat source.

I will run your "BAD idea" by my HVAC tutor and let you know what he says.

I am currently running my heat pump during the winter anyway, has to, as that is the primary system on my house. Even in sub 20 degree weather it still creates heat, although the emergency heat strip comes on as needed. If I can eliminate the e-heat strip from coming on, thats a bonus.

Let me know what your HVAC guru thinks.

turbo6bar · 02-13-2007, 07:11 PM

OK, now I understand where you're heading with this. I was under the impression you're looking for substantial gains. That simply isn't going to happen.

tdatk · 02-13-2007, 07:22 PM

Naw I wouldn't expect to see substantial gains....unless.... I insulated the "box" and ran a sub duct to it.

Nostril Cheese · 02-13-2007, 07:24 PM

heh, I got a heat pump for ya...

tdatk · 02-13-2007, 07:34 PM

Quote:

Originally posted by Nostril Cheese
heh, I got a heat pump for ya...

Brilliant!

928ram · 02-13-2007, 09:04 PM

You actually answered your own question; when heat your house the heat pump is cooling the outside air, making far more "cold" than the compressor is making residual "warm". Boxing it in would give you a large ice cube in a short amount of time.

rick-l · 02-13-2007, 09:30 PM

I don't do this very often so see if this reasoning/conversion is right or if I even understand the question.

So if you had a 3 ton heat pump, the box you built around the unit outside would have 36000 BTUs being taken out of it. (I think a heat pump in heat mode the box outside is the evaporator).

The energy added to the box would be what you put in to run the compressor with say 5 hp max (3750 watts) or about 13,000 BTUs.

You would take 3 times the heat out of the box as what you add so the box would get mighty cold.

turbo6bar · 02-14-2007, 05:21 AM

I think 928ram and rick-l are hitting on this idea's fault. The BTUs added by the wasted heat are less than the heat added by 25 degree air going across the coil.

I'm not sure the coil would freeze up, because you have limited water vapor in the air.

rick-l, when it's really cold outside, are you getting full capacity out of the system (ie. 3 ton unit tranferring 36000 BTUs)?

nineoneone · 02-14-2007, 08:16 AM

Key to remember....

Air conditioning is the REMOVAL of heat. . .;
You never have a 20' indoor air temp, so I think if you covered your a/c unit it would build up the pressure and add to the heat inside.
Heat pumps were designed by Canadians, as are good ice machine , go figure. I think the real question is will it produce more heat on a warm day (or covered box) answer is ......... it will only produce what has been installed at start-up, meaning it's all designed on refridgerant pressures. I have a 3 ton 14 seer unit on my house and it seems to run to much for my liken,but it's pretty good heat source in Georgia. You need a coil temp at least 20' above room temp wanted. so, you want 70 rm temp, coil temp needs to be reaching 90..

tdatk · 11:34 AM

Quote:

Originally posted by rick-l
I don't do this very often so see if this reasoning/conversion is right or if I even understand the question.

So if you had a 3 ton heat pump, the box you built around the unit outside would have 36000 BTUs being taken out of it. (I think a heat pump in heat mode the box outside is the evaporator).

The energy added to the box would be what you put in to run the compressor with say 5 hp max (3750 watts) or about 13,000 BTUs.

You would take 3 times the heat out of the box as what you add so the box would get mighty cold.

Agreed,
So the question would be, how might you isolate the now evaporator to "pull" more heat from the surrounding air? The fan never changes direction, it still pulls air through the coils and exausts through the top, what if you boxed only the coil area drawing makeup air from say my crawl space (constant 50 degrees below grade) and left the fan open.?

Wait... that would work in the summer too.... wheres my screw gun and duct tape?

turbo6bar · 02-14-2007, 11:45 AM

tdatk, the HVAC mentor tells me it's a real fight when temps get down to 25 degrees. Shield the condenser from strong winds. Try to get any sunlight on the unit. When mercury drops below, turn on electric heat. The is only so much you can do before thermodynamic forces win.

I think the crawlspace make-up air would certainly help, but at some point you may start pulling heat through the floor of the house. After all, if you reduce the temp in the crawlspace, you increase heat transfer through floor.

What is the temp of the coil pre and post compressor?

tdatk · 02-14-2007, 12:23 PM

Quote:

Originally posted by turbo6bar
tdatk, the HVAC mentor tells me it's a real fight when temps get down to 25 degrees. Shield the condenser from strong winds. Try to get any sunlight on the unit. When mercury drops below, turn on electric heat. The is only so much you can do before thermodynamic forces win.

I think the crawlspace make-up air would certainly help, but at some point you may start pulling heat through the floor of the house. After all, if you reduce the temp in the crawlspace, you increase heat transfer through floor.

What is the temp of the coil pre and post compressor?

Too cold out today to get any temps off the units. My crawl space is ~900sq ft by 4 feet high with 4 10"x 20" vents. The vents are already open year round but it still maintains 50 degrees under there. I have 10" insulation under the floor. I wonder how quick it might drop to ambient under there if I tried that. Or would the thermals of the ground (dirt) keep up? It's looking more like no free lunch.

Rob Channell · 02-14-2007, 06:34 PM

Try a heat pump that gets its heat from the ground. The ground temperature is relatively constant once you get down deep enough. I'd personally love to try one of these, but the entry cost is pretty high.

http://en.wikipedia.org/wiki/Geothermal_exchange_heat_pump

SEER 24 anyone??

tdatk · 02-14-2007, 07:29 PM

Quote:

Originally posted by Rob Channell
Try a heat pump that gets its heat from the ground. The ground temperature is relatively constant once you get down deep enough. I'd personally love to try one of these, but the entry cost is pretty high.

http://en.wikipedia.org/wiki/Geothermal_exchange_heat_pump

SEER 24 anyone??

Ahh Yes...So basically the coil is in the ground....great idea....constant year round temp a few meters down. Sound like what I was trying to think of by pulling the air from my crawl space. Thanks.