I have a heatpump in my home and want to understand more about how it operates? I understand that the heatpump is like an A/C unit but runs in reverse. I live in Texas and have winter temps. between 20 and 50 degrees. When the temp is in the 50’s the heatpump works fine, and when the temp drops to the 30’s then my natural Gas kicks in to heat the house. I feel that I have a good system, but my question is the way my heating system chooses between the two heat sources. Sometime when the heater comes on the systems uses gas, then later at night, when the temp is colder, the heater tries the heat the house with the heatpump. The heatpump starts up and run for about five minutes or so. During this time the air coming out of the vent is cold. Then the system stops and starts up again using gas this time. Even when the temp. is in the 20’s the heatpump tries to heat first. And all this time blowing cold air in the house, and wasting electricity. I understand that the heatpump is one of the more efficient ways the heat a home, but for the heatpump to come on when the outside temp to too cold, then that is just a waste of my money and then when the gas comes on more gas is need to makeup for the additional cold air that the heatpump blew in. I have asked my homebuilder about this, but they don’t care and just ignored my question. Is this normal on how heatpumps works? Why is there not a preset temp to make the change over from heatpump to gas? Can someone who knows dual heating systems help me?
Thanks,
CWalker
Replies
Although I'm not a HVAC contractor, and will gladly defer to anyone who is, I do have two heat pumps and they are coupled with a boiler for heat, and some experience in their operation. But to add a disclaimer, my system and its configuration might be different than yours, and therefore my explanation not be applicable. You should post this on a HVAC board where there may be more knowledgable people to help.
Your heating controls should be wired such that there are two stages of heat (at least, that is how my Trane units are configured), the first stage is when the thermostat first recognizes that indoor temps have fallen below a preset temperature. At this point, the HP initially starts. If the room thermostat continues to fall, the second stage (your gas unit, my boiler, quite often the backup is resistance coils in the air handler) might kick on, depending on how the controls are wired.
There are several different ways of wiring the controls; these are called restrictive and nonrestrictive modes of operation.
In the restrictive mode the heat pump energizes when first stage heat is called for. The Gas unit kicks on when there is a defrost demand from the heat pump, or second stage heat is called for. There should be an Outdoor Temperature Switch set near 32 degrees. When outdoor temps fall below the ODTS, the switch locks out the HP and the gas unit provides the sole source of heat.
In the nonrestrictive mode, the HP will operate whenever the thermostat calls for first stage heat, regardless the outdoor temps. The gas unit will not run when second stage heat is called for unless below the ODTS setting, or when the HP needs to defrost.
The nonrestrictive mode is the least efficient (IMO) as it allows the HP to operate below freezing. As temps drop below freezing, so does the HP efficiency.
Reading your post, it sounds as though you have a version of the nonrestricve mode. As the temps fall, your thermostat calls for first stage heat from the HP regardless of the outside temps. As indoor temps continue to fall, second stage heat is called for, locking out the heat pump. Between the first and second stage operations, the system should shutdown for a few minutes to realign. Also, there should be time-delay (or aquastat if you had a hydronic backup) to allow duct (or hydronic) temps to rise to a certain temp before kicking the air handler back on, to minimize blowing cold air into the house.
To be honest, for someone to fully understand your system, they would need to review the schematics that came with your gas unit, air handler, and HP. And then to be sure I understood it, I would want to cycle it through several times to ensure I undersood the actual or real operating sequence. Only thru understanding the actual sequence could you begn to diagnose if there is a problem. And I can't do that from here.
Sorry for the length, and to reinterate, what I've tried to explain is based on Trane configurations, and a few basic HP/resistance heater setups I've done work on for friends. You might want to call the installer and ask a service tech or installation tech to draft you a short synopsis of the sequence of operations.
Good luck.
"I am the master of my fate, I am the captain of my soul." Invictus, by Henley.
Nick,
I am finding out that I have a "dumb" system that is not top of the line. I have also learned that the lifespan for the HP that I have is about 10 years. Most of my neighborhood have the same HP, and they have all died at or near ten years of use. Yea, I must be next. My HP has never put out heat at 30 or lower deg. Time to start looking for new and BETTER HP.
Thanks for the help,
Cary
Heat pumps are sized for the air conditioning load.Sizing for the AC load is very important, specially in a hot humid climate. Otherwise you get poor tempature or humidity control.Then the amount of heat that you get is what you get in HP mode.As the amount of heat that is can produce is limited by the outside temp at some point it can't produce enough heat to keep the house warm.How junkhound in the NW might not really need AC and have a unit that is sized for heating and cooling be dammed.Now although the amount of heat that it the HP produces is limited in colder weather it still produces some heat.However, it is my understanding that with gas furnace the air out of the furnace is too hot for the heat pump and thus when it gets too cold the HP is suppose to turn off and the gas on.One basic way of doing that is with an outside thermostat.Another way is to use a multiple stage thermostat. When the inside temp starts dropping because the hp can't keep up then aux heat is turned on. With resistive heat as backup then it will come on in stages. I don't know if that system can be used with a gas backup or not, but I think that for gas the use of an outside thermostat would be the best bet.
Main item to look for is to get one with a Copeland Scroll compressor, 2nd is finding a a competent installer (or DIY once you know enough to get the EPA license and repair your present system at least once for practice. )
There are so many variations on systems, you would need to describe the make and model of dual heat and the thermostat model. If you never get below 20F, you should never need the gas.
Sons' system in Seattle area (same winter temps, did get down to 23F last week, rare) has never had the gas come on in the last 3 years (well , turned it off is why it never comes on, HP is fine down to about 10F here). Cold air - well, a HP only puts out about 80F air when it is < 20F outside, you may be interpreting that as cold compared to 150F with gas.
Due to the many different systems, try HVAC-talk with your query - what they will say is generally to have a competent tech check it out - obviously whoever set your system up originally was a hack. ?
Just checked your profile, Welcome, see it is your first post. Let us (or me via e- mail) some more details, if you are a complete DIY and willing to try alternatives, I can help, otherwise do the HVAC-talk advice thing.