Heat Pumps and Programmable Thermostats

TenMile

Hey all,

I've been doing some tweaking to our home energy consumption over the past several months. I made the investment in a TED5000 so that I can watch how electricity is being used in our house. Some quick background -- our home is a split level with a walkout basement. The main and upper floor are approximately 2500sqft total and are heated via an air-source Heat Pump. The lower level is only used from time to time and we heat it primarily via electric baseboard heater only when we need them (i.e. they are generally left off). Basement does get some residual heat from the Heat Pump as well. We are located in the Pacific NorthWest were the temperature in winter generally sits about 0-10C in the winter most days. Nights are marginally cooler and will hover around 0-3C.

In any event, now that the cooler weather is upon us I've noticed some unusual "sawtoothed consumption" readings from the TED and I believe that it might be related to our Heat Pump and Programmable Thermostat. Our Heat Pump is old ~25yrs, but still works and with a replacement cost of roughly $12-15K there is currently no cost justification to swapping it out. We have the unit connected to a Honeywell Programmable thermostat that is "supposed" to be designed for a 2-stage Heat Pump. It appears however from graphs provided by the TED that the Thermostat immediately kicks in the "emergency heat source" stage of the Heat Pump rather than let it recover from the outside air and that my energy consumption goes from a typical .5kW to 2.5kW as soon as the Heat Pump kicks in.

Attached is a graph from the TED during the day today. The thermostat is set during the day to 19.5C (67F). Take a look at the time roughly between 10am and 3pm (before 10am, things are more active with showers, some washing/drying etc...). There are a few lights, PC etc on in the house that consume about .5kW and that's it. Note how the energy consumption sawtooths between about 10am and 3pm as the Heat Pump kicks on/off... Any thoughts? Is this system working correctly or should I take a closer look at the Thermostat or heat pump configuration?

Attachment not found.

westbranch

Re: Heat Pumps and Programmable Thermostats

Hmmmm, pattern looks like post #10 here
http://forum.solar-electric.com/showthread.php?14136-Solar-Harvest

I assume you have also waded through the never ending Sanyo Mini-split thread?
http://forum.solar-electric.com/showthread.php?5104-Sanyo-mini-split-AC-%28inverter-variable-speed%29

rgk1

Re: Heat Pumps and Programmable Thermostats

I dont know if it will help but my heat pump thermostat has a "recovery mode" setting. It is a programmable. I dont remember if its on or off, but one way the heat pump kicks on prior to the the time/temp setpoint so that the heat pump only will supposedly will have the home up to temperature at a given time without the emergency heat (in my case 10kw heat strips). The other setting kicks in the heat pump and the heat strips together to immediatly warm the house, then only uses the heat pump to maintain temp, as long as the temp. doesnt drop more than 3 degrees below the thermostat setpoint.

LucMan

Re: Heat Pumps and Programmable Thermostats

The second stage on a heatpump is usually 2 degrees below the first stage setpoint. If the first stage (heatpump) can't maintain the setpoint then the second stage (electric strip heat) will kick in to maintain setpoint. The strip heaters will also maintain temperature when the heat pump goes into defrost mode (cooling cycle). The colder the outdoor temps get the more defrost cycles are required to keep the condenser free of ice.
It is never a good idea to setback a heat pump, because when the thermostat is set to a higher temperature again the electric strip heaters will be energized when both stages call for heat. Check the electric heat sequencers to make sure they are operating properly, any strip heaters over 5 kw usually have 2 stages.
You may want to look into purchasing a newer electronic Honeywell T8000 pro thermostat as they are fully adjustable and can be used with an outdoor sensor to lock out the second stage.

rgk1

Re: Heat Pumps and Programmable Thermostats

I haven't had a chance to look at the Honeywell mentioned yet but I will. I don't know a whole lot about the one my system uses now, but it is a two stage. As you mentioned the strips do come on in the defrost cycle. There is some kind of outdoor adjustable themostat inside the compressor unit. (compressor outside/air handler and heat stips unit inside the house) The installers said the outdoor thermostat just keeps the heat strips from being activated unless the outside air is "xx" degrees or lower. That mechanical thermostat quit working (or became highly innacurate) a 2 winters ago. Last winter I removed it and rewired the heat strip portion to the emergency heat part of the indoor thermostat. Now I use the heat pump 99% of the time, the heat strips do come on in defrost mode. Whenever the forcast is for about 13 degrees or lower I switch the thermostat to emergency mode. I don't remember to where, but I had to put a jumper wire so it cycled on and off like normal. Otherwise it just ran and got hotter and hotter, never shutting off (in emergency mode). Not my favorite way to do it, but It works. So far the biggest power bill..electricity portion I have had was about $125. Not because there wasnt a lot of usage, as there are 4 people in my household and everything is electric, but electricity is relatively cheap. I will check into the Honeywell and maybe it will help me out. Thanks.

LucMan

Re: Heat Pumps and Programmable Thermostats

The thermostat in the outdoor unit is a balance point thermostat, they are set to the temp. where the heat pump can no longer keep up with the heatloss of the house. They keep the strip heaters off until this temp is reached and then allow the strip heaters to be energized. The honeywell will allow you to do the same thing but from the indoor thermostat, just make sure you purchase the outdoor sensor for it.
Emergency mode operates the strip heaters only and disables the compressor. It is more efficient to leave the compressor running as it has a COP of 2 down to about 5 degrees F, electric COP is 1.

TenMile

Re: Heat Pumps and Programmable Thermostats

I've got an update to this thread. As it turned out we did have some issues with the Heat Pump itself. It ended up having a coolant leak and so the heat pump portion of the unit was only providing a small portion of the heat to our home. We evaluated the age of the unit (30 years) and decided to replace the entire system. We ended up with a Rheem 2-stage heat pump with variable speed air handler that has a 15kw backup heat strip option. The new unit has the ability to heat our home without kicking in the heat strips well down below -10C so in our case, the heat strips will almost never be used. The old unit kicked in the aux heating strips below +5C. The new units are also practically silent where the old machine sounded like a jet aircraft on afterburners. New programmable thermostat also has some intelligence as it measures the heat and humidity inside and out and optimizes itself for efficiency. Also has the ability to provide textual readouts of any faults, change filter warnings etc... Pretty nice machine.

solar_dave

Re: Heat Pumps and Programmable Thermostats

TenMile wrote: »

I've got an update to this thread. As it turned out we did have some issues with the Heat Pump itself. It ended up having a coolant leak and so the heat pump portion of the unit was only providing a small portion of the heat to our home. We evaluated the age of the unit (30 years) and decided to replace the entire system. We ended up with a Rheem 2-stage heat pump with variable speed air handler that has a 15kw backup heat strip option. The new unit has the ability to heat our home without kicking in the heat strips well down below -10C so in our case, the heat strips will almost never be used. The old unit kicked in the aux heating strips below +5C. The new units are also practically silent where the old machine sounded like a jet aircraft on afterburners. New programmable thermostat also has some intelligence as it measures the heat and humidity inside and out and optimizes itself for efficiency. Also has the ability to provide textual readouts of any faults, change filter warnings etc... Pretty nice machine.

Show us what the TED looks like now.

I bet your consumption is way down, I know it is the wrong time of year to really tell.

TenMile

Re: Heat Pumps and Programmable Thermostats

Doing the comparison over a 1 year time window will be better for sure -- here's what PlotWatt is saying about our electricity use -- I have my TED feeding their service. Heat pump was installed about the 15th of Feb. Beginning of January we had an obvious cold snap where the heat to our house was certainly being fed by the 15kw Aux heat strips.

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techntrek

Re: Heat Pumps and Programmable Thermostats

Looks like your old heat pump wasn't doing too bad as long as there wasn't a cold snap. Comparing the average post-snap to mid-Febrary, then mid-February to mid-March, the average looks about the same (eyeballing it).