It has the word "heat" in its name, so.
November 10, 2014 7:18 AM   Subscribe

What am I not understanding about my HVAC system?

Back in August, work was finally completed on a new 20 SEER dual-zone central air system in our house. It replaces window units in the summer and oil heat in the winter. It made all the difference in the last couple months of summer in our 1925 house.

Now that it's colder, the oil furnace is kicking in to bring the house up to temperature in the mornings. My understanding was that the furnace would only be used as supplemental or emergency heat. I thought we had spent all that money so we wouldn't need the furnace nearly as much.

Any tips for getting better performance out of our HVAC? This is an American Standard programmable thermostat, and we've programmed it with Home, Sleep and Away schedules.
posted by emelenjr to Home & Garden (10 answers total)
This sounds like an excellent question to ask the company that installed your new system. If it's supposed to be heating the house and isn't, then there may be something wrong that they can diagnose while it's still under warranty. If there's something misconfigured, they should fix that (for free) as well.
posted by erst at 7:23 AM on November 10, 2014 [7 favorites]

What do you have the thermostat set to for Home and Sleep?

It looks like you are getting lows around 40° in Virginia right now, so a heat pump should still be sufficient to bring the house to 65°-68° unless you're living in a drafty barn or something.

I would call the installer and have them come out and look. You should still be on parts and labor warranty.
posted by muddgirl at 7:25 AM on November 10, 2014

You've probably already checked this, but just in case: make sure it's not set to "emergency heat" - I think that's the setting that tells it to use the backup furnace heat all the time. But yes, ask the installer to come look at it. 40 degrees seems much too high to be requiring the backup furnace.
posted by aka burlap at 7:28 AM on November 10, 2014 [2 favorites]

work was finally completed on a new 20 SEER dual-zone central air system in our house

Just to clarify: you got a heat pump and not just central air, right?
posted by jon1270 at 8:07 AM on November 10, 2014 [1 favorite]

Can I jump in and ask a related question - In general at what outside temperature does a "modern" heat pump become inefficient and back-up heat should kick in?
posted by achrise at 8:26 AM on November 10, 2014

I'm in North Carolina and have been told my electric heat pump can manage a 30 degree temperature differential but starts to rely some on emergency heat after that (i.e., if it's set at 63 it will be okay as long as it doesn't dip much below freezing).

I just want to encourage you, as others have done, to call your HVAC installers and ask questions NOW. My heat pump is 3 years old and having problems and the labor costs to diagnose and fix it make the 5-year parts warranty seem like a joke. You want to find out if there's a problem and fix it while you are still under full warranty.
posted by something something at 8:57 AM on November 10, 2014

Response by poster: Thanks for the answers. I talked to our HVAC company and found out that freezing or near-freezing temperatures are enough to make the system kick over to "supplemental heat. " Also, we're apparently setting our "Sleep" temperature low enough that the differential between Sleep and Home is enough to require the furnace's help in the morning.

It's all pretty surprising because I thought the giant piece of machinery installed in our backyard would be able to do more work, but I'm off to tinker with our scheduled temperature adjustments.
posted by emelenjr at 9:21 AM on November 10, 2014

I don't consider 40° to be "near freezing" but perhaps it was colder last week? 35° is near freezing and about where our HVAC stops working as effectively.

Some systems are designed to gradually use more furnace heat as difference between ambient temperature and desired temperature increases, but this should be adjustable by their technicians if you are willing to wait for your heat pump to warm the house in the morning. It may actually be cheaper to let your oil furnace do this depending on your local electricity prices.
posted by muddgirl at 9:33 AM on November 10, 2014

In my experience, heat pumps aren't amazing at bringing the temp back up quickly. They do great at maintaining, or with smaller changes in the temperature. As muddgirl says, it might be worth investigating using the oil furnace to bring the heat back up in the morning, then let the heat pump maintain throughout the day.

Also - expect it to get "slower" as the temperature drops outside. There is still heat differential available so it will work, but it will take longer as there isn't as much heat available to pull out of the air.
posted by ish__ at 10:48 AM on November 10, 2014

Also, we're apparently setting our "Sleep" temperature low enough that the differential between Sleep and Home is enough to require the furnace's help in the morning.

Any heat-pump-compatible programmable thermostat should have a fuzzy logic mode where it can figure out how far ahead to start the heating up process in order to have things warm by the scheduled time without kicking on the backup furnace (except of course in weather cold enough that the heat pump isn't efficient). Either you are trying to have huge temperature swings (much more than the six or eight degree setbacks that are common in the US) or there is something not right with either your system or the thermostat.
posted by Dip Flash at 3:11 PM on November 10, 2014

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