Heat pump operation as I know it.

I have always been a gadgeteer and interested in heat pump technology. Maybe there are others on this site that are also interested in heat pumps.

We have had 4 heat pumps. Our first 3 heat pumps were piston pump types. They are not as efficient as the scroll compressor designs. Scroll compressors are sort of like a turbine equivalent with no reciprocating parts that are used in newer models. The area of the condenser (outdoor unit) and evaporator (indoor unit) have also gotten progressively larger which increases the efficiency of the heat transfer to/from those units.

Our current system is a 16 SEER, two speed, scroll compressor design with a matching 2 speed air handler. In heating mode, the coil in the air handler raises the temperature of the return air by 10 to 25 degrees. The discharge temperature on high speed isn't much different than low speed because both the air handler fan (air volume) and compressor speed are reduced at the same time. Our older heat pumps were simply off/on. The current heat pump has multiple steps. Off/low/high/high with strip heater supplement. Those 4 steps make for more even heating. Many newer heat pumps are variable speed.

Heat pumps are not as effective at lower temperatures. If you remember Charles Law from high school physics, as you lower the temperature of a closed system the pressure will drop. The freon gas charged heat pump is one of those systems. Lower outdoor temperatures results in a lowering of heat pump pressure. That causes a reduced capacity and lower compressor motor current. Heat pump size is reduced as the temperature drops. At the same time, the heat losses from a home go up as the temperature drops. At some point, the heat pump can't keep up. Some systems are dual fuel and at that point, they would switch over to natural gas or propane. Our heat pump is not dual fuel. It has 2 banks of electrical strip heaters in the air handler (furnace).

Our programmable Honeywell thermostat has a user selectable lockout temperature. Lockout temperature is the point where a heat pump is switched off and natural gas, propane or electrical heaters take over. The disk in the electric meter really spins once our heat pump drops below that lockout temperature and all heat is provided by strip heaters.

I recorded all the parameters in the thermostat when the system was first installed. The technician set the lockout temperature to 25 degrees. I lowered it to 5 degrees. At 5 degrees the heat pump provides heat for 99% of our heating season here in Ohio. The technician cautioned me that lowering that lockout point will have the heat pump running almost continuously at lower temperatures and just wear it out. I believe that starts and stops are harder on compressor life than run time. I base that on the fact that my previous 3 heat pumps didn't even have a lockout setting and in 35 years we have never had a compressor failure.

Since heat pumps extract heat from the outside air in cold temperatures, the outside coil (condenser) will frost up. There is a defrost cycle that switches off the outdoor fan, reverses the operation of the heat pump to cool the inside coil while heating the outside coil. This melts off any ice on that outside coil. In our case, the electrical strip heaters come on during this process so that you don't get a blast of cold air from the registers during defrost mode. Older heat pumps did this on a timer and once in a defrost cycle, ran for a fixed amount of time. Newer models sense when a defrost is necessary and the defrost cycle only runs as long as required. Heat pumps are installed on a pedestal that allows the water running off the coil during defrost a place to go.

I have attached a portion of the manual that came with our heat pump. 12,000 BTU is considered a ton of energy. Our heat pump is a 3 ton (36,000 btu) unit. From the chart, you will see that rated capacity (36) is based on output at 50 degrees. Above 50 degrees it has a higher capacity/motor current and below 50 deg it has a lower capacity/motor current.



The bottom row of the chart COP (coefficient of performance) shows the thermal energy produced (BTU's) and electrical energy consumed. Even at 5 degrees, the heat pump produces more than 2 units of heat for every unit of electricity. Strip heaters have a COP of 1. The problem is that at 5 degrees the heat pump doesn't produce enough BTU's to keep up with losses from our home and the electrical strip heaters periodically kick in to supplement the heat pump.

Below is a few rows of a spreadsheet that I have maintained for the past 15 years. It is of kilowatts used by our all electric home each month. Obviously Winter months have higher usage. The last column is total cost/kilowatt of electricity from AEP. This is the monthly kilowatts used divided by monthly charges. We get offers from other providers for 5 or 6 cents/kw but those don't include the transmission costs, riders and other fees that make up the total cost per kilowatt on your bill.



We are on a $117/month budget but the last chart is of actual cost per month and year for the last couple years.



Hope this has been helpful.

 

We purchased our previous home in 1985. It was built in 1976. In 1976, there was a moratorium on new natural gas hookups so every home in our neighborhood started out with a heat pump. We replaced the 15 year old, 8 SEER, Rheem in about 1992. At that time the HVAC guy said that it needed work and he would think twice about putting money into a 15 year old heat pump so we replaced it. The second pump, a 10 SEER Rheem, was working fine when we sold the home in 2000.

We were both driving about 45 minutes to work. In 2000, we found a fixer-upper about 5 minutes from both of our jobs. It was built in 1927. We bought it, gutted it, added energy efficient windows and doors to replace single pane windows. We stripped it down to the studs and added insulation in the walls and ceilings. We added house wrap before adding new siding on the outside. We replaced the single pane basement windows with glass block. We removed the gas furnace and the chimney.

We purchased our 3rd heat pump which was our last piston compressor pump, a 12 SEER Rheem unit for this home. We had that unit about 10 years when our electric utility offered a $1500 rebate to replace our current heating system with a higher efficiency one. In 2010 we took advantage of that rebate and bought our 4th and current heat pump. It is a 3 ton Westinghouse 16 SEER, 2 speed, scroll compressor unit. We replaced the air handler with a 2 speed unit with strip heaters for backup/emergency heat at the same time.

Life expectancy of these units is typically about 15 to 20 years. We have had this pump for 11 years and I fully expect that this current unit will last at least 15 years. My reasoning is that it always starts on low speed and lets the system pressures normalize before it switches over to high speed after a 30 second delay. That puts much less strain on everything.

This may be our final heat pump as we are in our 70's now and are considering a move to Florida to be near our daughter. We can take the Florida heat better than the bleak Ohio winters.

 

The weather today is 17 degrees and calm. It looks like we are at the crossover point where my heat pump output just matches the losses from our home. The thermostat setpoint is 68 degrees and the temperature in the house has been hovering around 67/68 degrees all day. Our heat pump has run continuously on high all day long.

A digital thermometer revealed that the cold air return temperature is 64 degrees and the air handler discharge temperature is 77 degrees for a 13 degree temperature rise (delta).

From the heat pump chart in my original post, at 15 degrees it produces about 20,000 btu.

I am sure that there must be some HVAC calculations where you enter the square footage of the home, the form factor (ranches lose more compared to 2 story), slab or basement, the window size and type of glass, and the amount of insulation. With that you could size the heating equipment. It would confirm those calculations if they knew that at 17 degrees, the 20,000 btu would maintain the home at 68 degrees.

 

Our electric utility, AEP, had a program where they offered to replace our water heater with their rental water heater. Their tank was larger. Our original was a 52 gallon and this tank is 120 gallon. It is not a glass lined metal tank like most water heaters. The installer said that the walls are 4" thick concrete substance which is supposed to be very low heat loss. It weighs about 400 lbs. The installer had a nifty battery operated dolly that lowered it down our basement steps to allow for a one man install.

The rental fee is $18.95 a month but they give us a special discounted electric rate for being in the rental heater program. The discount is from about $3 to $10 and varies from month to month It is shown on our statement but the discount always makes the rental cost less than $18.95. The tank has a controller that I believe receives commands from the electrical grid. It sheds the heating elements during times of high demand but allows them to operate in the evenings and on weekends. They claim that it helps them avoid building more generating capacity just to meet peak demands.

We had a lightning strike about 7 years ago and it blew out everything that was hooked to our water system. The garbage disposer we realized that day. The refrigerator ice maker took a day or two to discover. My wife commented that the ice maker wasn't working and when I looked in, there was a black charred stain around the motor. We didn't discover the sump pump until the first heavy rain. It took 4 days before my wife noticed that the water wasn't as hot as it normally was. We called AEP and a technician came out and replaced the thermostats and heating elements at no charge.

I considered one of the heat pump water heaters but was concerned that the heat loss in the basement in winter would be a problem.

My daughter lives in Florida and her water heater is in her garage. The heat in that garage is unbearable for about 9 months of the year. It would be a perfect location for a heat pump water heater. I believe that the high ambient temperature would be an excellent heat source plus it would cool the garage while heating her water.