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Old 02-19-2013, 01:17 PM   #1
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What are options when there's oversized hvac?


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).

Thanks!

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Old 02-19-2013, 02:46 PM   #2
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What are options when there's oversized hvac?


Ouch, you really need a 1.5 ton unit and unfortunately there is nothing you can do to get rid of the extra 1/2 ton capacity you got. Slowing down the furnace fan will just cause it to freeze up. A whole house dehumidifier is quite expensive, uses a lot of electricity and is basically a bar fridge with a fan attached to it. It needs to work off your ductwork as a bypass unit and I doubt you can get one small enough for your house. Good for Florida etc but they are pricey specialty units. Best bet is to get the proper sized AC and maybe you can sell your existing one with Craigs list etc.

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Old 02-19-2013, 03:35 PM   #3
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What are options when there's oversized hvac?


I talked with a guy today who said he would come out and look at my ductwork and give me his opinion. He said most people don't want to pay for a manual j so they are not generally done. He said he would be able to do one for me.

I'm still afraid that a one and a half ton unit is going to be big. There are many days when all I need is dehumidification.

Would it really be cost prohibitive to buy a whole house dehumidifier myself that is designed to be hooked up to my plenum? I see some online by aprilaire and sanidry that look like they would work. Price is probably high but what other choices do I have? I think the smallest heat pump size I could get is a 1 ton right?
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Old 02-19-2013, 05:37 PM   #4
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What are options when there's oversized hvac?


if you can get one and DIY then go 4 it. you need to know the specs in airflow thru it and how it works with your furnace and interlocks with it etc. not sure about heat pump sizes as I don't sell them. I believe the aprilaire unit is the same one Lennox sells as they just rebrand it for Lennox so it should be good. I just looked up the install info for the Lennox and if you absolutely have to you can just park the unit in the basement and take one vent/return from upstairs and shoot another supply one upstairs w/o having to use the furnace. if you use the furnace you need 6" between where it takes off on the reutrn duct to where it reattaches to the same duct which may or may not be possible 4 U. you should download the install manual for it B4 buying it as it is not as simple as it looks.

DIY load calc

http://www.hvaccomputer.com/gtarget1...Fe4-MgodPF4AoQ
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Last edited by yuri; 02-19-2013 at 06:14 PM.
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Old 02-21-2013, 02:38 AM   #5
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What are options when there's oversized hvac?


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.

Last edited by gusherb94; 02-21-2013 at 02:41 AM.
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Old 02-21-2013, 05:18 AM   #6
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What are options when there's oversized hvac?


If your 2 stage furnace has a VS blower. Then it can probably be se to either 350, or 320 CFM per ton, to help keep the humidity a bit lower. Next, is the furnace wired to use both stages, or did the installer lock it into second stage. Also, what brand and model number thermostat do you have. Also post full model number of furnace. the model number will be inside the panel where the gas valve is.

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