Heat pump sizing, ducts and Lots of questions
Hello,
I am replacing my existing air handler and condensing unit with a heat pump. My house has two stories and I am considering only replacing the top story unit. I have some questions regarding this:
1. I used a program to calculate heat loss and gain and came up with 14,000 BTUH gain and 16,300 BTUH loss. When sizing a heat pump how do you determine the tonnage? Is it just the Max(gain, loss) / 12,000 BTUH? The Rheem 1.5-ton unit I’m looking at has options for a 10 kW heat strip to supplement. Since I’m on baseboard electric heat, is this sufficient?
2. The current outlets are in the ceiling. I was reading some textbooks and they say that for heating ceiling outlets are not a good idea. Is this in practice a bad idea? Is it worth putting some vents in the floor or wall?
3. Is it better to run flex duct to each outlet directly, or better to run a trunk line with flex duct off to each outlet?
4. I’m reading on Breaktime that installation is the key. What constitutes installation? I’m getting estimates of $4K – $6K to replace the system which is way out of my budget. If I put the units in place, replace coolant lines, run electrical, do the ducts, and let a HVAC technician hook up the wiring and coolant lines and charge the system, is that fairly straightforward?
5. Say I did 3 tons for the whole house, can I run ducts vertically from the 2nd floor through 2×4 walls to the crawlspace and then to floor outlets one the 1st floor? I’m concerned about losses in the long duct runs.
Thanks much for any help. Attached is the heat gain/loss calculations.
Nils
Replies
COP of 5.6
How did you measure this?
measure the vacuum in microns? With an electronic vacuum gauge, bought mine off ebay for about $50.
Or measure something else?
the COP
I was looking at a test setup where someone was getting COP readings. They had anemometers set up to measure airflow, temp probes to get rise, and wire probes to measure current usage by the heat pump, all fed to a datalogger that gave the data in almost real time.
You stated COP = 5.6, which makes me think you measured it in operation.
Dumb me, didn't read the title of your original question.
No anemometer or anything fancy. Never have taken the time to hook up everything to a computer at home, the setup you saw sounds similar to some $30K setups we have use at work.
Here is a hand calculation method:
For a HP with a heat strip, figuring out the airflow is fairly straightforward. Just turn on only the heat strip and fan, and measure inlet and outlet temperatures. Or put a big electric heater in the duct with the fan on. Using that temp rise an psychometric chart, the cfm airflow is calculated. CFM = kWinput * (5.687 BTU-min/kW) / 0.24 BTU/#air *15.14cuft/# / temp diff F.
I have an electronic kW-hr meter on the input to the HP, which also reads out kW being drawn. Any wattmeter will also work, or just a voltmeter and current meter (but then you need to guess at power factor) can also work.
So now you know the airflow and the power input. Since COP = BTU out/BTUin, just crank the numbers.
My numbers based on airflow were 58,080BTU/hr output and 3.04 kW input , from wattmeter or from 239V*14.9A @ 0.85 PF).
58,080BTU/hr / (239V*14.9A*.85*3.412btu-hr/w) = 5.6
58,080 was gotten from: 70F to 112F = 42F. 1# air is 15.14 cuft at 112F, 1# air needs 0.24 BTU to heat it 1F. Air flow using reverse of this calculation as described above for a 10kW heater plus the fan power gave me an airflow of 1454CFM.
1454 CFM at 112F carries (112-70)*(1454/15.14)*0.24 BTU/# = 968 BTU/minute = 58,080 BTU/hr
The only way I got that high COP was using a 7-1/2 ton condensor on a 4T scroll compressor, no reversing valve (HP operation only), absolute minimal refrigerant line lengths (<4 ft total), and a shop built tube in tube water source evaporator, and a txv operating at only 4F superheat.
Excellent and
Art, thanks for taking the time to write that up. I printed it out and put it my binder of Technical Writing by Persons Smarter Than Myself.
I had not considered the method of using the temperature split in the airstream given a known BTU input as a method of determining CFM. HVAC techs usually seem to use either an anemometer to traverse the duct, or a flow plate to measure delta P at the filter slot. Hey, I could show you a new geothermal radiant system that has a COP of 1.0, and it only cost about 50 large! Not a mature industry, at least on the residential side....
Thanks for the helpful advice. I think I will try what you say.