Hello and thanks for offering so much valuable info.

I am getting prepared to replace a split system heat pump in my home and need some info/advice. I have done a heat gain/loss calculation using computer software based on Manual J. Results are 25k btuh total gain and 45k btuh loss. Design conditions were adjusted for my area(Central Alabama). Considering that this is around 2-2.5 ton load, my current unit is a 3.5 ton. This is a 2400 sq ft single level brick home about 30 years old and insulated fairly well in walls and attic. This load calculation seems very conservative for a home this size. Insulate ducts are located in half unconditioned basement and half crawlspace.

It is shaded well, but will a heat pump this size cool a home this size when it is regularly 95-100 degrees in the south?? And will the backup heat engage regularly in winter virtually cancelling my attempts to get the most efficiency out of my new system?

Also, assuming that I go with a heat pump of around 30,000 btu, should my backup heat make up only the difference between my unit heating capacity(30,000) and actual heat loss of my home(45,000) plus some margin, or should my backup heat be sized to carry the full load if needed?

I have read that 90% of older homes have oversized equipment. Is my home one of them?

 

A good heat lose and load should tell you just what you need for sure. But I still like to look at a home and find out just how the people in it live . It all helps
Id say a 3 ton for sure for 2400 sq ft + a basement there.How does you 3.5 ton unit do with the home does it like have a long run or just seem to turn on and right off all the time. If on and off its to big. You want to try and get the humidity out with a longer run and you will feel cooler this way.
Crawl way do you have poly down in it and the walls have insulation on them?.All vents closed? Also the strip heaters should cover you if the heat pump goes out so you do want them for the full heat loss .
Just my .02 cents

ED

 

load calc does sound low for your sq. footage, you may want to do it again to be sure. not doubting your calculations, just sounds low.

i am with ed, 3 tons sounds like what is needed........... but....

if your calc is correct, i would go with the 2.5 ton. i know you are closer to 2 tons in the calc, but undersizing the equipment would be a mistake in your area.

the 2 ton would run more, thereby reducing humidity better, but it may run itself to death on those 95 plus degree days and not pull the temp down for your liking

be sure that the ductwork is sized for this new system

10 KW heat strips should do what you need. generally 15 KW (next step up) go in 3.5 ton and larger units

blowing more insulation after install (or before) into attic will reduce heat loss thereby reducing run time on the strips.

 

I certainly don't want to undersize. But I want to be as efficient as possible. When cooling, my unit cycles 4-5 times an hour and out door temp is around 90 degrees. I have 54 degree air from the registers and 70 degrees at the return grill. However, humidity stays close to 50% which is ideal? It seems to me that my unit cycles too often indicating that it is oversized a little. What do you think?

My half crawl space has poly down and foundation is block- so I guess some insulation. My floors are not insulated. My basement is only slightly underground, but temp doesn't vary alot. Ceiling joists are filled with insulation already.

 

Then Id say a 3 ton sounds good.It would be better on the heat side there. Even down here they all go for 12" insulation or better for R-38 in the attic.


ED

 

If I increase to R-38 for my ceiling insulation, my heat loss falls inline with a 3-3.5 ton unit. But now my heat gain is around 2-ton. Shouldn't I size more for cooling capacity than heating? Trust me, I am not questioning your advice because I see that you are in the southeast like me. I'm just trying to get a better understanding. I want to learn.

 

Go the 3 ton . For that home at 2400sq ft on one floor you need a large blower and duct work to do a good job there.


ED

 

looking at all info posted here, i am inclined to go with the 3 ton my self. count your registers. 4X10 would usually be a 6 inch pipe, 4X12 is a 7, 4X14 is an 8. how many returns, how big and where?

this will help us figure out what capacity your existing duct system is

 

I have 11 supply ducts. All but one are 4x12. The one is 6x12.

There is only on return in my hallway about center of the house. The grill is 25Wx32L and located directly above my air handler which is in the basement. The return duct is about 25x20.It is approx 9' tall and enters the bottom of my airhandler for an upflow arrangement. One tapered trunk line down center of house. All runners have exterior insulation and trunk line has interior insulation-at least my return does.

 

I agree with all of the above. In your climate, I would size the unit on the cooling side for maximum comfort and efficiency and supplement with strip heat in the winter. The key in your case is the airflow.

 

fwiw, my new house (2300 sf) is getting a 4 ton heat pump. R38 in the ceiling, 22 floor and 15 in the walls. I have double pane windows and doors, jump ducts for the rooms and two returns (one each on the end thirds of the house).

42 kbtu for cooling, 39 kbtu for heating.

Sounds to me like your 3 ton would be way undersized for a home that age (and presumably not as well insulated).

 

You said it right there . Over there in alabama go the 3 ton and make the unit run. It costs less for it to run than to do all that start and stop. It will also get more humidity out of the home for you.

ED

 

I have a heat pump that the 3 way valve switch,the lines connected to the valve have come apart can I braze them back on or should I replace the 3 way valve.What I mean is the switch has this 4 copper lines coming out of it and 2 of them have come apart the unit is only 14 months since the day it was install.

 

I call the company that made it and the guy that put it in. FOR SURE HERE. You have to be EPA to work on it. then you wont have the tool to pump it down or to vacuum it out when you done. You have to use like a silfos15 to solder it back in. you also want to look out for the phosgene gas that can come off of it when the flame is down there.

ED

 

reversing valves are difficult to replace, the high heat will burn it up if not careful. almost all units have 5 years on parts these days, what is the make/model?