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Tex
Premium
join:2012-10-20
kudos:2
reply to pende_tim

Re: [HVAC] Manual J and heat pump sizing

You took the words right out of my mouth. Everything you've posted is dead on.

The only thing I would add to your most excellent post, without getting into the nuts and bolts of psychrometrics (wet bulb, dry bulb, latent loads, sensible loads, grains of moisture, etc.), the OP is sizing the AC part of the system based on an outside design temperature of 89 degrees and, I assume, indoor 75 degrees and 50% relative humidity. At those designs, he's still oversized ~10,000 to ~20,000 BTUs for cooling, depending on the system. Imagine how oversized (both for the sensible & the latent load) he will be under part load conditions. Most likely, the AC wouldn't run more than a few minutes at a time, which isn't close to enough time, as you mentioned, for lowering the humidity in the living space. Cold is good. Cold and damp is not. If the OP does go this route, I highly suggest a whole house dehumidifier. And, lookie here, Climate Master sells one. Imagine that!



Ken
Premium,MVM
join:2003-06-16
Markle, IN

1 edit

said by Tex:

OP is sizing the AC part of the system based on an outside design temperature of 89 degrees and, I assume, indoor 75 degrees and 50% relative humidity. At those designs, he's still oversized ~10,000 to ~20,000 BTUs for cooling, depending on the system. Imagine how oversized (both for the sensible & the latent load) he will be under part load conditions.

I guess it just bothers me to think about spending $6,000 on a geothermal heat pump and have to use electric heat strips all the time. I just assumed after I got my manual J results it would be obvious what size unit I needed, but the 3, 4, & 5 ton units all have their pro's and con's. I understand many people just size for cooling and rely on heat strips to make up the difference, but during my research I have found many people who don't even have heat strips in their unit. They somehow have sized a unit to work for heating and cooling, which is what I would like to do.

I've also just become aware of variable speed compressors. While more expensive they would seem to solve all of my problems. Another thing I have seen is the ability of some geothermal units to dehumidify by cooling the air and then reheating it as it leaves. I also have started to consider putting in 2 units, a 2 ton for the 2nd floor, and 3 ton for 1st floor and basement.


pende_tim
Premium
join:2004-01-04
Andover, NJ
kudos:1
Reviews:
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1 edit

said by Ken:

thing I have seen is the ability of some geothermal units to dehumidify by cooling the air and then reheating it as it leaves. I also have started to consider putting in 2 units, a 2 ton for the 2nd floor, and 3 ton for 1st floor and basement.

If you put in 2 units, it may be possible to install dampers and use one to do the entire house for cooling and then use both for heating on 2 zones.

As far as sizing the units based on future "improvements", be cautious here. I do not doubt that you have every intention of doing the work, but it has been said many times that you should not size HAVC based on future work.

Suppose you spend $20,000 on a new HVAC system(s) based on the future state of the basement. Now lets suppose that something happens that you don't do this project. What do you now do with the undersized unit you just installed?

--
The difference between genius and stupidity is that genius has its limits.


Ken
Premium,MVM
join:2003-06-16
Markle, IN

said by pende_tim:

If you put in 2 units, it may be possible to install dampers and use one to do the entire house for cooling and then use both for heating on 2 zones.

That's what I was thinking. Just one unit would do a/c all summer. Then in the winter both could run to provide heat. I'll have to change a bunch of ductwork in the basement, but it's all totally exposed right now.