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vdu 09-01-2009 10:41 AM

Installing a New Central Air System
In my first step of installing a central air system in my house, I performed a load calculation using the ACCA spreadsheet and a manual J book.
This is in northern Virginia area. The cooling load is 21315 BTU and the heat load is 37190 BTU. Does this sound reasonable for a 2000 Sqrt feet house with 600 sqrt feet basement? The house is 2 levels and a basement. Relative tight (new windows installed). It's 1960 house, a bit old.

I initially thought about install an AC/heat pump system. However, as I read further, it says that for selecting a system above, a 21315 BTU heat pump will be used. Therefore, to make up for the difference between heating load and cooling load (even in the ideal temperature for heat pump, above 47 degree F for example) is made up by a heating coil.
The system can only have that much of the BTU (21315), not 37190 BTU. The rest will be handled via a straight heating coil, which is not that good, or economical.

Is this a correct assumption? Is there a heat pump system that can handle different heating load and cooling load depending on winter and summer? Initially, I thought with heat pump, the heating coil only kicks in if the temperature is colder than optimal heat pump operating temperature. Here, since the system is smaller than the heating load (it's for cooling load), the coil will work a lot of the time.

Is this a correct understanding? Also, did any of you who live in a similar weather region use heat pump system? How do they perform with respect to cost?

This is the design condition for the house:

Indoor Design Heating db: 70
Outdoor 99% db: 21
HTD: 49
Indoor Design Cooling db: 75
Outdoor 1% db/: 92
CTD 17
Indoor Design Cooling RH: 50%
Grains Difference: 48
Daily Range: Medium


dac122 09-01-2009 12:32 PM

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It is standard practice to "round up" with heat pumps. I have heard folks successfully oversizing their heat pumps but you must be careful or the cooling side could be grossly oversized.

With regards to your heat loss number, understand the Manual J is sizing those based on your design temps to meet your needs 97.5% of the time in an average winter. Know that your actual heat needs most of the time will be much lower, well within the capacity of a heat pump without running an auxiliary. It is only when your heat loss becomes greater than your heat pumps BTUH output do you need some supplementary auxiliary, but in many cases that is in the 20-30 range, and sometimes lower. Heat loss and HP output are classical supply-demand curves. See the below graph borrowed from here, and note the region where it says Resist. Heat Required for when your backup aux heat strips will be needed. Even past the intersection of those graphs your heat pump is producing economical if insufficient heat. It is only when it reach a COP of 1 should it be locked out, and you run solely on aux heat.

Even though you will rarely need it, it is common to size your aux heat to handle your entire heat load acting as a full backup if your heat pump breaks down.

You bring up a common issues about heat loss and heat gain disparity. One way overcome or lessen that disparity you can look into two stage compressors as they have different capacities based on the low or high speed.

vdu 09-01-2009 12:50 PM

Thanks for the explanation. My concern is probably answered by your last paragraph. To summarize it, I need to use a 2 stage compressor type.

From reading the first part of you reply, it appears to base the sizing of equipment on the heating load. Since heating load is larger than the cooling load, the equipment will be able to handle the load. However, it will have a short life, and in-efficiency due to short cycle when it's doing air conditioning. If I understand it correctly, a 2 stage compressor would solve this issue.

Because I read at this page:

which advice to size the equipment based on the cooling load, and then use the heating coil for the difference, I have the concern about the big inefficiency.

beenthere 09-01-2009 01:08 PM

Even using a 2 stage heat pump, you can have humidity problems during the milder temp days of summer when you size to the heating load.

What was the sensible load for cooling?

vdu 09-01-2009 02:26 PM


Originally Posted by beenthere (Post 322373)
Even using a 2 stage heat pump, you can have humidity problems during the milder temp days of summer when you size to the heating load.

Could you please explain?


What was the sensible load for cooling?
Actually that was the sensible loss/gain ( The cooling load is 21315 BTU and the heat load is 37190 BTU)

The latent gain is 4722 BTU

beenthere 09-01-2009 03:13 PM

A 2 stage unit, doesn't always have good ability to remove moisture in first stage. It relies on a long run time.
So if you install an oversized unit. That its first stage is the size the house needs for cooling. It does poorly at removing moisture. And causes high humidity problems.

EG: A 3 ton 2 stage heat pump.
Can have a first stage sensible capacity of 17,8000 BTUs at 95 OD temp and 75 ID temp.
But, at 75 OD temp and 75 ID temp, it can have a sensible capacity of 18,700.
Meaning it would do a great job at keeping the humidity low when its 95 outside.
But could cool too quick when its only 75 outside. Unless the blower is slowed down.

And in the winter, at 30 degrees OD temp, it still only provides 25,100 BTUs of heat.

Your posted sensible put you between a 2 and a 2.5 ton unit unit.

I believe only Trane has a 2.5 ton 2 stage unit.

vdu 09-01-2009 03:31 PM

Is it true that with your estimate of 2 to 2.5 ton heat pump, that is based on the cooling load. Which is smaller than the heating load (about 3.5 tons)?

From my understanding of 2 stage unit, it runs under 2 different loads. So if the temperature is not very hot, it would run at smaller setting. Assuming that the smaller setting matches the cooling load, then it would run long enough to remove the moister.

However, if I base the system on the heating load, for example, and the lower setting stage is still too large for the cooling load, then short cycle will happen, and all sort of issues occur.

Ideally, based on the equipment datasheet, a correct equipment must have at 1 of their stages to meet the manual J's calculation.

I found an example datasheet here:

Would someone please tell me what would be a good match for me from this data sheet?
I can see this:
Outdoor unit RPRL would be: 036JEC
ID Coil: RCFM-H*3617A*
ID Air Mower: RGFD-06?MCK?
Stage: 2
It's a bit low for the winter. Maybe something a little bigger.
Also, assuming that the control is good enough to automatically do this so the users don't have to set the stage manually.

Since my problem is not new, and I don't see anything unique about it (just calculate heat gain/loss, and find a heat pump unit for it), would the pros who work on this often please tell me if it's easy to get a system to satisfy my need?

beenthere 09-01-2009 03:45 PM

Size to the cooling load.

If the first stage of a 2 stage is close to the capacit you need for cooling at design. It won't dehumidify enough at lower summer temps.

Seen it happen.

2 stage units have a high SHR in first stage, they need long run times. First stage on an oversized unit will be too high of a sensible capacity at mild summer temps.

If you insist on oversizing it/sizing it to the heating load.
Then also invest in a whole house dehumidifier.

A 4 ton 2 stage heat pump probably won't deliver 31,000 BTUs of heat at 21 degreees outdoor temp.
Or just barely provide it.

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