View Full Version : BTU/hour and heat pump size?
07-03-2006, 05:47 PM
House is a new construction ( 2389 SQ. FT) located in South Central Kentucky on the Tennessee border . Our local electric company performed a Right J load calculation based on a 70 degree F temp difference during the heating season and a 20 degree F temperature difference in the cooling season, 4,760 heating degree days and 1,300 cooling degree days. Load sensible heat ratio is 86 (whatever that means). Heating BTU per hour is 47,654 and cooling BTU per hour is 23,547. Adding 15 % to each of those numbers for incorrect factors such as duct sizing, refrigerant charge, dirty coils, air infiltration or leaky ducts we get: 54,802 BTU per hour for heat and 27,079 BTU per hour for cooling. The air-to-air heat pump recommendation is a 3 ton unit. Does this look correct? Would a 3 ton unit not supply approximately 36,000 BTU of heating or cooling under the best conditions? More than enough for the cooling, but falling sort for the heating…
Would someone please explain to me what size heat pump I need.
07-03-2006, 06:09 PM
Adding another source of heating, like an electric or gas furnace as the air handler isn't a very expensive option either to install or operate, but too much AC and it cools the air quickly, then stops and humidity rises, so you end up needing a lower more expensive temperature for the same comfort level.
You local contractor should be able to take those numbers and make sense of them and explain all your options to you.
07-03-2006, 06:52 PM
If I translate this correctly: the HP is sized for the a/c portion and a supplemental heat source (electric strip or gas furnace,…) will make up the difference in what the HP can not produce from an air to air transfer. If this is a correct interpretation, would the heat strip not bring the efficiency of the entire system down.
07-03-2006, 11:43 PM
Consider a 2 stage 3 ton heat pump. But, make sure your contractor verifies that your ductwork will handle the airflow for the full 3 ton capacity.
Even if your ductwork limited you to 2 or 2.5 ton total heat pump, you will still probably save money instead of a furnace only setup.
Pro's tell me if I am correct on this, but a 3 ton (36,000 btu) heat pump requires more airflow (CFM) than a 40,000 btu furnace ???
07-04-2006, 08:16 AM
You need supplemental heating coil.
Just curious. Why add more safety factor to Manual J? It has already included some safety factors into it. You're oversizing it, resulting in unnecessary electricity bill.
How was the duct sized? It should be sized using Manual D.
07-04-2006, 08:27 AM
Electric aux heat is 100% efficent, so the efficency won't drop like you would first thing.
Check the COP of the unit you are considering, the higher the better. The same as the HSPF, the higher the better.
And yes, XV, a 3 ton HP does need to move more air then a 40,000 btu gas furnace.
07-04-2006, 10:19 AM
The load calculation that was performed (Right J) had absolutely NO safety factor built into it. I ask the person performing the calculation and this is what I was told “It gives the actual value, no safety factor, no accounting for air leaks,…) Is 15% a suitable safety factor? We do live on a very windy hill.
07-04-2006, 10:54 AM
I'm hoping he meant air leaks in the duct work.
It would help if you posted the sensible and latent loads.
The Affinity 2 ton with VS blower at 800 CFM only has a total capacity of 22,000 btu's at 95 OD, and 75ID.
The 3 ton has a total capacity of 31,500, at 1100 CFM.
As the guy that did the load calc said, it only tells you what capacity you need, not what size equipment.
Thats what manual S is for.
The ratings I posted are for the standard matching coils on VS blowers.
If you go with a 3 ton, I would recommend the G2FD046(s,h)17 coil, and the N1VSB12 blower.
That will give you better humidity control, and still provide you with good heating performance. 21,000 btu's heating at 17° OD, and 8.55 HSPF.
You actually lose 200 BTU's heating ability with the 048 coil, on the N1VSC16 blower.
Although you don't need to add a saftey factor to the calc.
Not knowing what the leakage rate is of your duct system, 15% is not out of line. the 3 ton Affinity YZE, with the 12 VS blower, and 046 coil will provide you with control of humidity, and good heating performance.
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