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Question DetailsAsked on 7/20/2015

Is it better to add a small HVAC system and have zone control or replace undersized unit with larger one outright?

I have a 2400 sq. ft. single story house with a 1 ton unit for the room over the garage and a 3.5 ton unit for the rest of the house. There are 9 ft ceilings everywhere except the kitchen, dining, and den which are 20 ft ceilings. Both are 16 year old Goodman HVAC systems. The 1 ton handles its space just fine and shuts off periodically throughout the day. The 3.5 ton struggles to maintain temp below 80 and runs continuously until after dark. Outside of struggling to keep up, it seems to run fine. Recently had R22 added to both units to make sure everything was topped off and then rechecked pressure a month and a half later. R22 pressure in the 3.5 ton is 70 psig and the 1 ton is a little over pressure at 82-83 psig.

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Afraid you need an A/C (Heating and A/C is your Search the List category) - because different brands and models run at different pressures, and the operating pressure can run from typically about 40-50 psi at cold tempeatures to around 100 psi at high ambient temps, and refrigerant temsp are also affected by the indoor temperature of the air blowing over the evaporator - without that data and the operating temperature curve tables for the particular unit, one cannot tell if the pressure is normal or not.


Generally, one central unit will be more efficient that two separate ones - but in cases where one area is significantly different operating conditions like an over-garage room, separate unit can often be more comfortable and efficient that a multi-zone single unit, because the single hotter room may make the larger central unit run a lot to handle the higher cooling load, but does not need all the cooling capacity ofthe larger unit. This can result in short-cycling of the main unit, and also mold issues due to overcooling of the ducts. So - it may well be a 1 ton unit over the garage and then an adjustment to the main house unit is the best solution. I sopecifically did not say replacement of the house unit - would take a diagnosis of operating conditions and a Manual J calculation to determine if a 3.5 ton unit is just too small, or if you just have a distribution and delivery problem. Also, if the existing 3.5 unit is too small, with the higher operating efficiencies of some newer units it may turn out that a new 3.5 would be OK, even if the nominal capacity is the same. However, if the current unit is running continuously that is VERY bad for it and will wear it out quickly, so it is likely a larger unit will be called for - a 2400 SF single story house would commonly use a 4-5 ton unit in heavy cooling country, especially if a strung-out configuration.


I would recommend have a good, experienced and well-reviewed Heating and A/C contractor or two look at your system and run the Manual J, S and D calculations to determine where your issue is and just how much cooling capacity you need.


You might also, or then, want to look at an Energy Audit followed by insulation and weatherstripping too - because it may well be, especially if an older house, that improvments in that area would improve your thermal picture enough the 3.5 ton unit would be OK, as well as saving you more money for less dollars.

Answered 4 years ago by LCD




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