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What are options when there's oversized hvac?

1777 Views 5 Replies 4 Participants Last post by  beenthere
I don't think my hvac installer ever did a manual j, they just switched out the same size furnace and central ac. I have roughly 800 square feet of conditioned space with an unfinished and uninsulated basement. Both the furnace and ac short cycle and I'm constantly battling with the thermostat. It's most uncomfortable during summer evenings when the temp is dropping but the humidity is still the same.

Obviously I can just find a new hvac company to do a manual j and have them install the appropriate sized furnace and ac but the units are only 4 years old and it will cost a lot of money.

Can I make some adjustments to the equipment so they run longer and at lower capacity? The furnace is a 2 stage. I don't know much about the ac except it is a 2 ton 15 seer.

For cooling, I hardly need anything because I'm completely shaded in. I live in climate zone 5 so there's a lot of humidity. Would it be wise to talk to someone about installing a whole house dehumidifier?

The house is a hundred years old but has had a lot of remodeling done and it is pretty well insulated (batts in the walls, lots of insulation in the attic, rim joists foamed).

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If a manual J calls for less then 18,000 BTU's (1.5 ton) then you can install a mini-split A/C in the correct size to match your heat load as an option.

As for your existing system there are a few things that can be checked before throwing it all out. Have your contractor check to make sure the blower is set to match the tonnage of the condenser outside (furnaces come with the blower set for a 3-5 ton condenser depending on the furnace size) If the CFM's are set too high that will cause high humidity/clamminess.

Once the blower is made certain that it's set for the correct CFM for the size condenser you have (2 ton needs 800 CFM at 400 CFM/ton or 700 CFM at 350 CFM/ton) then it can be lowered from 400 CFM/ton to 350 CFM/ton which will slow the airflow across the evaporator and help some with keeping humidity down. (you shouldn't be at risk of a freeze up any more then normal unless you have some terrible air flow issues to begin with)

You say your furnace is a two stage, if it's also got a variable drive blower you can install a thermostat like the Honeywell Vision Pro IAQ and set it up to dehumidify by slowing the blower down (all variable speed 2 stage furnaces usually have a terminal on the board to slow the blower down on a call for it) This is standard practice and can help alot in reducing humidity.

and last but not least make sure the thermostat is set for a larger swing/CPH. If it's a Honeywell you'll wanna set it to 2 CPH (cycles per hour) or any other brand usually more then .5 degree swing. (this adjustment varies on brand and model)

It is also possible that your condenser was never charged right from the beginning which could cause or add to the humidity issues. If it's slightly off it would still cool fine, and even short cycle if it's oversized but may not pull the evap temp down enough to remove humidity like it should.
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