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RJ_4
Posts: **484**Member

Here are some formulas that are helpful in figuring out suppl. heat on air to air heat pumps.

KW required when CFM and desired temp. rise are known ( rule of thumb 1 ton = 400-500 cfm ) KW = CFM times T.R.(temp rise) divided by 3150.

Temp. rise = KW times 3150 divided by CFM

Amps = KW times 1000 divided by volts

example A 5 ton heat pump would need a 7.9 KW heater for a 10 degree T.R. This would require a 40 amp service at 208 volts just for the supplemental heater.

KW required when CFM and desired temp. rise are known ( rule of thumb 1 ton = 400-500 cfm ) KW = CFM times T.R.(temp rise) divided by 3150.

Temp. rise = KW times 3150 divided by CFM

Amps = KW times 1000 divided by volts

example A 5 ton heat pump would need a 7.9 KW heater for a 10 degree T.R. This would require a 40 amp service at 208 volts just for the supplemental heater.

RJ

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## Comments

2,343MemberUntil I can spend more time on this, your figures....... something does not add up. 10* Delta t is useless. 5 ton or 60k BTU hp = 60,000. 60k TO kw hr =17.584kw/h. If You get a chance, check this or I can when I get home. Your 7.9KW will convert to 24885btu/hr. It is expressed in KW/H. As a result, you amp draw is incredibly hi for 7.9 KW or my amp probe is broken...lol. We know or agree that 1 KW= 3150 btu/hr

Peace

Mike T.

484Memberthe 10 degrees is the supply air temp. rise I would like to see during ht pump defrost mode considering the supply air temp. could be 50 deg. actually 15 -20 degees would be better. using formula for single phase, 3 phase will differ. Amps = watts divided by volts 7.5 kw (7,500 watts ) divided by 208 volts = 36 amps, or a 40 amp service. Let me Know what you think I dont want to give out wrong info

Thanks Rick

484MemberI used 7.5 kw instead of 7.9 kw formula stays the same my mistake

2,144MemberI kinda remember a 5t H.P. having 15 kw in 3 -5kw stages and in the old days using o.d. sensors set at each "balance point".And a 40a breaker is only good 32a or 80% of the breaker (40a ) . and the supp heat is supposed to handle the total heat load incase the compressor craps out. I think?!?!

2,343MemberSince My Ugly's reference book has grown legs, I cannot give any rebuttal. My only concern is that at a 10 * Delta T you are going to get complaint of cold air every time. Your supplemental heat during Defrost should be 1 stage of multi elements on your emergency heat. I'm not sure if your using strictly Electric for sup, but think about the worse case scenario if for some reason defrost fails or takes too long. Can you shoot for 40* rise?.

It drives me crazy when I lend out literature of someone who asks and they don't return it. I had 2 books and now I have None! I can't remember who takes what, but it's going to end/.....lol

Peace

Mike T.

2,144MemberI agree David , but the load calc in the given example is 5tons. It wasn't me Mike T. I have 2 Ugly's ,one I lent out and it came back ALL coffee stain colored so I got a new one just for me.LOL!!!!!!!!!!

484Member400 cfm for cooling and 500 cfm for heat pumps is data used in Carrier ht pump and air balance manuals, as well as many other unit manufacts. the other data came from SMACMA. I agree a 10 degree temp rise is'nt much but some of the jobs I looked at in northern CA had no suppl. heat and barely had an electrical service to the bldg big enough to support 40 amps to each unit, so a 10 degree rise was better than nothing. I do know about all the stages needed in colder climates as i started my career in Denver in 1974 and also worked in SF for 20 yrs.

2,343MemberTerry you have 2 books and I am missing 2....??? Very suspicious....lol

2,144MemberGee Mike, I did go to Buffalo a few times cause I heard books were easy to come by up there but the guy I got the books from was not a handsome guy so it couldn't have been you! GOL -Giggling out loud!!!!!!!!!!!

349MemberAs David mentioned, let's do it right so the "rules of thumb" are not needed. The rules of thumb that are used by many today are the same ones that my father used over 30 years ago. And that is very scary. In my father's day, he used R-12 to fill the service van tires when they got low and we were nowhere near a service station. My father used R-11 as a solvent to clean the shop floor.

See how far we've come since then? But why are we still using rules of thumb to size and evaluate brand new, high efficiency equipment?

How many times have you had someone ask you what size air conditioner they need for their 2,000 square foot house? How many times do they look at you like you are crazy when you mention "Heat gain/heat loss calculations"?

Using a 400-500 cfm per ton "rule of thumb" is nearly as ancient as roll down windows on your car. If you are looking for a ballpark number, in a kind of back-of-the-napkin calculation, go for it. But if you are looking to properly size a particular piece of equipment, we need to do it right. In an age where efficiency is (or should be) a major concern, we can't randomly select equipment. Oh, I forget that we are so busy and we just don't have the time. Think about this:

If we can't find the time to do it right, how will we ever find the time to do it over?

By the way....

1 Kw = 3,413 btu

The sensible heat formula is Qs = 1.08 x cfm x DeltaT

The 3150 number that so many use is the result when you divide the 1.08 in the sensible heat formula by the 3413.

Have a great holiday all!!!!!

484MemberAs I stated carrier and other manufacts. use this cfm data. I Installed , started up and serviced hvacr euip. for 38 yrs I agree for system design do a heat load, but in alot of cases when your on the job not in the office this rule of thumb gets you to a starting point. I came from your fathers day when a service man was expected to know all phases of this trade, Lets appreciate what we have taught the younger guys, without them this trade would be in trouble

349MemberWe are definitely on the same page, RJ.

When we are in the field, it is always good practice to get a feel for the equipment so that you can better interpret the readings you get from the system.

As I instruct my students, I tell them to write down what they would expect the system to do under the conditions present BEFORE diving in. For example, given a specific outdoor ambient temperature and the condition of the equipment, what do you think your condenser saturation temperature SHOULD BE? Of course they would need to make "estimations".

As I mentioned, for back of the napkin calculations, these rules of thumb are okay, but they should still be used as, as you mentioned, a starting point.