HVAC water to air heat pump question

Ok, I have these two 5000 btu water to air heat pumps. They were originally intended to be used to heat a hot water tank and blew out cold air each has it's own small pump and they are hooked up in series.

I run water from our pool to them and then duct the air out of them to the furnace returns, running about 18 hours a day would keep the pool at temp and greatly offsets the AC running time. I only run them off peak (when electricity is ½ price).

My first question is I have calculated it dumps the water back at a 10 degree F rise (85F to 95F) at 2 gallons per minute. I know you can somehow convert that to BTU's but don't remember how.

Secondly. Right now the intake and return lines are 1/2 inch and about 40 feet total. I was told making the pipes larger, say 5/8 or even 3/4 wouldn't make any more heat, just a faster water flow and lower overall temp and the air output temp would also be the same. Is this true or would larger lines help?

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Brock said:
Ok, I have these two 5000 btu water to air heat pumps. They were originally intended to be used to heat a hot water tank and blew out cold air each has it's own small pump and they are hooked up in series.

I run water from our pool to them and then duct the air out of them to the furnace returns, running about 18 hours a day would keep the pool at temp and greatly offsets the AC running time. I only run them off peak (when electricity is ½ price).

My first question is I have calculated it dumps the water back at a 10 degree F rise (85F to 95F) at 2 gallons per minute. I know you can somehow convert that to BTU's but don't remember how.

Secondly. Right now the intake and return lines are 1/2 inch and about 40 feet total. I was told making the pipes larger, say 5/8 or even 3/4 wouldn't make any more heat, just a faster water flow and lower overall temp and the air output temp would also be the same. Is this true or would larger lines help?

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Well, 2 gallons at 10F rise:
2 gallons is 7.57L
7.57L is 7,570 cc

Specific heat of water is ~1 cal/cc/C

And, 10F=5.55C

So, 7,570cc * 5.55C = 42,000 cal/min

And 1 calorie = 4.19 joules

So, 42,000 * 4.19 = 175,980 J/min

Joules/seconds=watts

So, 175,980/60 = 2933 watts

And, 1 kwh = 3412 btu

(pauses and scratches head)... gotta keep these units straight

So, 2933 watts is 2.933 kwh if ran for 1 hour continuously.

So, 2.933 * 3412 = 10,007 BTU

(Or I guess I could have said, you've got 2 5000-btu units.... how many btus do YOU think you're moving? /ubbthreads/images/graemlins/tongue.gif )

Always hated thermodynamics. Tough class. Hated it almost as much as fluid mechanics. Yuck.

Larger pipes (to a point) will help you efficiency some since the water flow will be slightly higher. But really, you're only pushing 2 gpm through a pipe. You can do that with a 1/4 line with no problem. I'd leave the pipes alone.

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But really, you're only pushing 2 gpm through a pipe. You can do that with a 1/4 line with no problem. I'd leave the pipes alone.

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That part sounds OK to me, but why are they in series? You're piping the half way heated water from the first into the second meaning that it is not going to get as much heat from the coil, which if they are the same type of unit would be about the same temp as the first one.

Wouldn't it make more sense to run them in parallel?

You should get some temperature measurements both for the coil entrance and exit, as well as the water entrance and exit temperatures. It may be that you can fine tune the flow rate to get the best efficiency and transfer.

are these really a water source heat pump? Or do they have an extra coil on hot liquid line for the water and a regular condenser/fan arrangement? I think those are called "de-super-heaters" or something like that are are commonly used for regular hot water heaters and such. but you wouldn't use a regular hot water tank as the only source for a heat pump would you?

Thanks Turbo. I thought a btu was 1 pound of water 1 degree F. So in my thinking I had
2gallons about 16 lbs so 16 btu per degree
10 degrees at 16 btu = 160 btu per minute
160 btu times 60 minutes = 9600 btu
I have no idea if I remembered that correctly

James I put them in series so if I get an air bubble in the line it would have to be large enough to be in both pumps at the same time to stop the flow. The kids often pull the intake line out of the water to see if it shoots warm water out also /ubbthreads/images/graemlins/wink.gif

Each pump has the same 5 degree temp rise on the water side and both put out about 52F depending on the room temp.

I think they do have a regular compressor and exchange that heat to the water. I just had one of the units charged and they kept referring to the unit as a dehumidifier.

I guess the military used these in the south as the only source for their water heaters. My brother in law had one in Hawaii at Pearl. It was plumbed to the drain line and either the hot or cold line and ran for about 3 hours after a shower. It had a hard time keeping up with 4 people when we were there, but if you closed all the windows it did cool the house down a bit.

I wouldn't use it on a hot water heater in my case since I can use the pool with almost limitless heat sink ability with the 16k gallons. Basically I get my pool heating for free while paying for the AC or I get free AC while heating the pool.

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Brock said:
Thanks Turbo. I thought a btu was 1 pound of water 1 degree F. So in my thinking I had
2gallons about 16 lbs so 16 btu per degree
10 degrees at 16 btu = 160 btu per minute
160 btu times 60 minutes = 9600 btu
I have no idea if I remembered that correctly



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That's "sounds" like how btu would be measured.

1 gal water is about 8.4 pounds in weight.

Ok, then I suppose they are pretty much right on.