HVAC in high-efficiency homes under heavy load
In planning for our upcoming high efficiency home remodel, I had a concern about our HVAC system when under temporary heavy load. I am in the chicago area. Last weekend our neighbor who owns a brand new well-constructed (though not necc highly efficient) home had a party on a hot humid day. The doors were often open as people were coming and going. There were about 10 people in the kitchen/great room area over a couple of hours. Initially, they left the AC off due to the open doors but it quickly became uncomfortable so they turned the AC on. Our neighbor said that she had both upper and lower AC units set to 68 but the house was still near 80 degrees!
Since we are planning a single moderately powered HVAC unit in our project, are we going to experience this all the time where the house becomes uncomfortable under heavy load? Do we need to have a secondary system for times of heavy load?
I appreciate any comments or suggestions.
thanks.
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Replies
Alok,
When the party started at your neighbors' house, the AC was off (so the house started out warm and humid). Doors were often open, and 10 guests showed up. When the house because uncomfortable, the hosts decided to turn on the AC. (This was about 6 hours too late, by the way.)
Under these circumstances, of course it will take a few hours for the AC to lower the temperature indoors. The house is humid. The house is hot. The house is full of visitors. The lights are on. The doors are open because people are inebriated.
If you want your house to be cool for an upcoming party, turn on the AC at noon.
"The doors are open because people are inebriated." - funny
ok so if i understand your point... you need to precondition the air for upcoming heavy loads?
what about unexpected loads? is a high efficiency home with a modestly powered hvac any worse than a built-to-code home with a standard hvac?
it just seemed to me that the HVAC in a high efficiency home is not not designed to address heavy demand. I was concerned that if the house did get hot or humid for whatever reason, the amount of time to get back to normal might be intolerable,
Alok,
A heating system or a cooling system performs best when it is operated continuously, and when doors and windows are closed.
During the summer, once the weather warms up, close your windows and turn on the AC. It will work fine.
If you leave the AC off for most of the summer, and open your windows, you can't expect the equipment to cool off the house in 20 minutes if you start it up after a hot, humid day. Everything in the house will be humid when the AC is turned on -- wringing out all that moisture takes a while.
Alok. One of the worst things you can do is install an oversized HVAC system. It is critical to have a Manual J calculation done by a competent installer or HVAC designer.
My house is pretty efficient (1.1 ACH/50), and I am able to heat and cool 3,200 square feet in a warm humid Zone 3 climate with a 2-tonf air-source heat pump. I run the system year round. The few weeks of nice weather we have when it might be possible to open the windows invariably coincide with pollen season.
BTW. I never understand why people think setting a thermostat to a low temperature will accelerate the cooling process. A system will certainly run longer, but it will not cool any faster.
Sure, as houses get more efficient and the AC is downsized accordingly, the unusual loads (like 10 people = 1/2 ton) go up as a percentage. Inverter AC units allow a greater capacity range. Pre-chilling/drying will help.