Heat pump best schedule?

[Aaron Rosenthal]

So, my non-profit had a catastrophic failure of a heating system which was not repairable. Our only option was to install a heat pump, thanks to an edict from the Province of B.C. regarding emissions controls.
The space is controlled by a programmable set back thermostat, with a remote sensor to average out the space's monitoring. The pump's heating capabilities are augmented with electric heat module.
I've been searching on line for the best, energy (cost) efficient method of using the heat pump, and, naturally, no two authorities agree. Some say, set a range and leave it on all the time 24/7, others say to minimize running costs, set it back within a near-to-optimal temperature; others, use it as a regular furnace and shut it off when the space is not in use.
I'm totally confused.
I know on this forum thee are experts in these systems. Please guide me, or ask the questions of me which will help you channel me in the most efficient use of the unit.

[Michael Schuch]

To me it makes sense if you are heating during the day and the air outside is warmer during the day the heat pump should run more efficiently. So shut it off at night when no one is there.

Likewise if you are cooling and the air is colder at night the heat pump will be more efficient if run during the night, so I would set the AC to come on in the early morning during the summer. Whether this will save money will have a lot to do with how well insulated your building is.

Central heat pumps are starting to catch up with mini-splits where the speed of the compressor matches the load at any particular time. My mom had one of these that died after 7 years and had to be completely replaced because it was out of warranty and a replacement control board was no longer available.

Maybe try one of the smart thermostats that learn your occupancy patterns and adjusts itself?

[Bill George]

Having serviced Heat pumps in the past in my career 30 plus years commercial HVAC, never set a HP on heating colder at night it takes many hours for it recover and usually in the coldest part of the day... the morning. Sometimes forcing it to bring on the AUX more expensive electric heat! Yes this is from experience and customer feed back!!

[Tom M King]

It's all guesswork until you try it both ways. If it's occupied half the time, my money would be on letting it run.

[Jim Becker]

There are a lot of variables here, including "what kind of heat pump design", outside temperature factors, size of the structure, insulation, use pattern of the structure, etc. Personally, I set a temperature and leave it there. That works for me in the house (heat pump with oil augmentation) and in the shop (mini split). The latter, however, is a lot more intelligent as it has multi-speed features on both ends and the smarts to be able to work with acceptable ranges so it can "loaf" a little when it feels it can which reduces energy consumption.

In your situation, I'm pretty much with Tom...more than half-time use, let it be set at a constant temperature and see how that works. If you do choose to do setbacks, make them very mild...a few degrees...so that there is less work to bring the space back to occupied temperature.

[Allan Dozier]

This is an age old question that really needs a good, controlled scientific experiment. I had this discussion with a physics professor once. He said as long as the resistance heat strips don't come on then every minute that you don't run the system when not needed saves you energy. Now there are some qualifiers I think. This was back in Florida so we were mainly dealing with the air conditioning function and this was before variable speed compressors were around. He said "catching up" in the morning when the system runs longer still uses less energy than keeping the building conditioned all night. I guess it would also depend a whole lot on the level of insulation and air infiltration.
I will admit that in my home I set it and leave it but in the farm building I program it. Now that building has gas furnace for heat so a different situation but I also do it for A/C in summer.

[Lee Schierer]

I've heated my home with a geothermal heat pump for over 40 years. Outside temperature has little effect on the efficiency. I've tried setting the thermostat back at night and listened to the system run steady for several hours to adjust the temp back to the normal setting. Most thermostats for heat pumps usually have a 1 degree operating on/off band. If the new temp setting is more than 2 degrees of change the heat pump will go into high capacity mode and possibly even turn on the back up electric heat strips on to make up the difference. I only change the temp if we are going to be gone for several days. My current smart thermostat will will recover in steps if I use the set back feature or vacation mode so it doesn't run constantly when recovering. I can also turn off automatic use of the backup heat strips.

[Bill Dufour]

Any chance of indoor pipes freezing with no heat?
Bill D.

[Bill George]

I'm not a college professor but I worked in the field for over 30 years and then taught the subject for 12 more years before retirement. Folks with Heat Pumps would let you know right away when they when got those high electric bills and were cold all the time. Industry advice was No Setback at night and show the customer how to switch off that expensive back up electric heat if he insisted on doing so. My mini has no electric back up and when its below zero and windy it can not keep up. Going to install a natural gas backup heater this summer.

Sure turn it off at night and when the power is off because of an Ice storm tell me how long you can go without freezing pipes!

[Bruce Wrenn]

In 45 years, we have NEVER done a night time set back, even when we had a gas furnace for back up heat. We run our units (two) at 68, and 67 degrees respectively.

[Allan Dozier]

It would seem the only valid reason to not have a setback is that the unit will work harder to catch up the next morning. I could see the possibility in the winter with an air to air heat pump working to transfer heat in the morning when outside temps are the lowest. But with a gas or oil furnace or a geothermal heat pump I don't see the reasoning. They put out a constant amount of heat. They don't have an accelerator pedal like a car. Not talking about modern mini splits that can run at varying speeds. There has to be a time period when everyone would agree that turning the temp back is economical. You wouldn't leave the thermostat up if you were going to be gone on vacation for two weeks. So that time frame would have to take into account all those variables like insulation and infiltration. Would it be 6 or 19 or 78 or 121 hours for your situation?

Not talking about turning it down so low in the winter that pipes freeze or so high in the summer that houseplants die. And leaving resistance heat strips out of it. On a side note back in the 70s with the basic crude thermostat we had I soldered in a small rocker switch to be able to turn off the heat strips (in Florida).

The argument is reversed in the summer. If you have a single speed air to air heat pump why would you keep your downstairs cooler all night when the next morning the outside temps are the lowest and the pump run more efficiently?

[Bill George]

The Big issue is with Heat Pumps here in North country and its been this way since day one. You folks in the South don't need to deal with this but up here the Heat Loss or load is greater than the Cooling. Therefore IF you size the HP for the max heating load and to Not need backup or Aux heat which in most cases is electric which costs on an average 2 or 3 times what it costs to run a HP then your way over sized for Cooling. Look up heat pump COP.

The newer Mini's mostly solve that by varying speed compressors and fan motors. They are still or should be sized for the normal design heating load in your area, zero extra for morning catchup. So do as you feel best if you have a newer split. I feel running the compressors and unit at Max for hours on end will cause it to wear faster, but that is again up to the user to decide.

[Lee Schierer]

But with a gas or oil furnace or a geothermal heat pump I don't see the reasoning. They put out a constant amount of heat. They don't have an accelerator pedal like a car. Not talking about modern mini splits that can run at varying speeds. There has to be a time period when everyone would agree that turning the temp back is economical. You wouldn't leave the thermostat up if you were going to be gone on vacation for two weeks. So that time frame would have to take into account all those variables like insulation and infiltration. Would it be 6 or 19 or 78 or 121 hours for your situation?

My geothermal heat pump system does not run continuously. It cycles when needed according to the thermostat. If for example, the power goes off for a long enough period of time for the thermostat to see more than a 2 degree temperature change or if I were to raise the temperature more than 2 degrees, the system would come on and shift automatically into high mode. Once it gets inside the 2 degree window, it automatically shifts back to normal mode. The new thermostat I have, in vacation mode, is supposed to gradually return the temperature to the "normal" setting so as not to use the third stage heating or second stage when in cooling mode.

There are a lot of variables you would need to account for if you were to try to determine the ideal number of hours to save costs by lowering (or raising the temp in cooling mode) in heat mode. Every conditioned space is different so there is not one standard answer. For example: The out side temperature, the wind speed to account for air infiltration, the rate of heat loss/gain for your conditioned space, the amount of time your system has to run to achieve the new temperature, whether or not your electric service charges more for usage during certain periods of time and whether they are limiting your use during peak demand times.

You could start by determining how many cycles per hour your system runs to maintain the current setting. Then set the temperature back the amount you would plan to use. Wait for the system to cycle again and shut off/on a few times. Then change the the temperature back to your normal setting and record how long the system runs before it shuts down and returns to the normal cycles per hour.

Let us know what you determine is the optimum length of time for a setback to create a savings.

[Malcolm McLeod]

... Every conditioned space is different so there is not one standard answer. For example: The out side temperature, the wind speed to account for air infiltration, the rate of heat loss/gain for your conditioned space, the amount of time your system has to run to achieve the new temperature, whether or not your electric service charges more for usage during certain periods of time and whether they are limiting your use during peak demand times.

...

A significant factor is the thermal mass contained in a conditioned space: how much 'stuff' do you have; how many BTUs does it contain; and how fast does it absorb or emit those BTUs.

[Allan Dozier]

My geothermal heat pump system does not run continuously. It cycles when needed according to the thermostat. If for example, the power goes off for a long enough period of time for the thermostat to see more than a 2 degree temperature change or if I were to raise the temperature more than 2 degrees, the system would come on and shift automatically into high mode. Once it gets inside the 2 degree window, it automatically shifts back to normal mode. The new thermostat I have, in vacation mode, is supposed to gradually return the temperature to the "normal" setting so as not to use the third stage heating or second stage when in cooling mode.

There are a lot of variables you would need to account for if you were to try to determine the ideal number of hours to save costs by lowering (or raising the temp in cooling mode) in heat mode. Every conditioned space is different so there is not one standard answer. For example: The out side temperature, the wind speed to account for air infiltration, the rate of heat loss/gain for your conditioned space, the amount of time your system has to run to achieve the new temperature, whether or not your electric service charges more for usage during certain periods of time and whether they are limiting your use during peak demand times.

You could start by determining how many cycles per hour your system runs to maintain the current setting. Then set the temperature back the amount you would plan to use. Wait for the system to cycle again and shut off/on a few times. Then change the the temperature back to your normal setting and record how long the system runs before it shuts down and returns to the normal cycles per hour.

Let us know what you determine is the optimum length of time for a setback to create a savings.

Thanks. So many variables. My position is that any amount of setback will save energy as long as we are talking about single speed heat pumps with heat strips turned off or fossil fuel furnaces. There is no penalty for "catching up" in the morning for those units, they don't work any harder just longer. It takes more energy to keep the downstairs warmer through the night than it takes to warm it back up in the morning. The leakier the house and the less insulation the more this is true. I have seen studies mentioned online that show this. The only exception may involve the morning temperature outside in the winter for air to air heat pumps like George mentioned and that would be difficult to figure out. If that is true and significant then it is reversed for summer A/C with no doubt a setback is cheaper.

Glad you like the geothermal units. My home has had 3 geothermal units since new in 1988. They worked great and I still have one of them operating. Two of them finally bit the dust in the last couple years and I replaced them with modern air to air A/C and got natural gas furnaces.
As far as run time I would submit that the compressor running for an hour to catch up in the morning would be less damaging than turning on and off more repeatedly during the night.