Heat Pump Defrost

Steve@Advance

Well-known Member
I'm sure several of you have heat pump systems in your home. I just installed mine last fall.

Now that the temps are falling, I noticed it was going into defrost cycle quite often. Any time it goes into defrost, the system reverses, it takes heat from the house, sends it to the outdoor coil to melt the ice, and turns on the emergency heat to compensate for the lost heat. A very wasteful situation!

I got to wondering why it was going into defrost so often, though there appeared to be no sign of ice on the outdoor coil. Did a little Youtube research, made a discovery.

There is a defrost board in the outdoor unit. It registers run time when the outdoor thermostat is below freezing. There is a set of jumpers (some have dip switches) that can be set for 30, 60, or 90 minutes. This setting determines how often it goes into defrost. I checked mine, sure enough it was set for 30 minutes!

The 30 minute setting is the factory default "worse case" for extreme cold, damp weather. Since I live in a mild climate, it should be set for 90 minutes. So if you have a heat pump, you'll want to have that timer set for your climate area, the longer you can get by between defrosts, the more efficient the system will be!

Here is the Youtube video that explains it, look at 4:50.

https://www.youtube.com/watch?v=0Y_-pb-WU3o
 
Every minute a HP is in defrost you are air conditioning inside. Then it takes same time to replace energy HP removed, so during defrost you p pay twice and get nothing. I'll Never own a HP.
 
Not sure what you mean, M2Allis- a heat pump is also an air conditioner in summer, which is one of its main attractions. They don't work too well below 20?, so not much advantage over plain old furnace in the cold winter climates. But defrost cycle or no, we save over $1,000 a year on power with ours.
 
Ours is a ground source heat pump, and has no outside part at all, other than the underground piping. Indoor temperature whether in heat or cool mode is simply set by one thermostat. It also has an auxiliary electrical heater for extreme cold, buy in 6 years, has not turned on yet. Sounds sorta like the milk cooler in the barn when it starts!

Ben
 
(quoted from post at 07:15:25 01/13/16) Not sure what you mean, M2Allis- a heat pump is also an air conditioner in summer, which is one of its main attractions. They don't work too well below 20?, so not much advantage over plain old furnace in the cold winter climates. But defrost cycle or no, we save over $1,000 a year on power with ours.
Sure, I think people buy heat pumps for the air conditioning value. If you don't care about being cool (which I completely understand, I grew up without air conditioning and was used to it) and you live in an area where it's cold more often a heat pump is probably not for you. I have (2) HPs, a conventional unit and geothermal for the main house. My wife is from the south and she is used to being cool in the summer. We're in Western Maryland so it can be warm here in summer. HPs do well in thus area.
 
What have you got for a backup,Steve?Heat strips?Gas?OIL?.If you have Fossil fuel.then you should be using that in the 25-30F and down range and not your Heat Pump.If using Electric Strips then it depends on your house needs how you do it.I have had a HP system since 1980,but with fossil backup and shut the pump off when the weather gets down below 35-40.Thanksgiving to Easter.I believe in them,but there are people like George who don't,but that is only YOUR business to desire.I have 5 in 4 houses now.
 

So yours is now set for 90 minutes, that is certainly a good deal better than 30 minutes - good job. I have a question: does it defrost every 90 minutes regardless, even if no ice has formed? And does it defrost for a set time or just until the ice is melted?
 
In these discussions we tend to lump ground source and air source geothermal in the same bag and they are very different animals. I have used ground source geothermal heating and cooling in North Central Minnesota for the past 16 years and would never use anything else. My average monthly energy cost in any year is always between $40 and $50 and my back up systems are always off. I do get a discounted electrical rate for having geothermal heating and cooling. Defrosting is not an issue with ground source.

Air source is a different situation. I have two friends here that have air source systems and they are happy with them, but they are not used during extreme cold weather. As the temperature drops, they become less efficient and it is more cost effective to use a backup system which in our case is propane. They shut their systems down at about 17 degrees. There are also some air source designs on the market that I have looked into that I would not ever use. They tend to be called ?cold temp? air source and employ a variety of techniques to allow them to operate at lower temps. I am not an expert on this, but the few I have looked into concerned me. Frost buildup always is a concern and their publications even warn of frost buildup. I would avoid them. My opinion.

In my humble opinion, a well-designed air source is an excellent and cost effective source for air conditioning and heating early and late in the heating season and on warm winter days, but one needs a backup for the coldest days. Here in Minnesota our power company gives generous electricity discounts for geothermal with an approved backup.
 
welled, I should have mentioned this is an air source system. Where I live the coldest average temp is a bone chilling 33*! LOL

It will on rare occasion get down to the teens, and single digits are once in 10 year events, and typically don't last long. It hasn't been much below freezing since I installed it, but from looking at the electric bills, I'm seeing quite an improvement.

Thanks!
 
Indiana Ken, it now defrosts every 90 minutes of "cumulative below freezing run time". I haven't timed the length of the defrost cycle, but I'm guessing about 2 minutes. That part is not adjustable that I have found.

No, it has no way of actually detecting ice, wish it did...

Thanks!
 
massey333, I have heat 30 kw strip backup, so the meter really spins when it's on! I turned one breaker off, dropped it to 20 kw, don't know if that makes much difference, just an experiment. So far this year it's only been down to 25*, and that was just for an hour or so, warmed back to mid 40's that day, so it has a pretty easy life.

Thanks!
 
Glad that setting works for you but you're not understanding the defrost system. At the end of the 30/60/90 run time cycle it does NOT automatically defrost. What it does is check the defrost thermostat near the bottom of the coil for ice. When there is none it reset's the timer without going in to defrost. If there is ice it go's in to defrost to clear it. Your bulb is mounted improperly or it's bad. I set them all at 30 minutes for our area. If the system is healthy they won't defrost without ice so it makes minimal difference.
 
I have a 2000 ft total electric home, use the least efficient heat source, electric baseboard heat. Installed electric heat 15 years ago, not the first service call or repair. During the 4 month of heat season, my total electric bill isn't $1000. Before I went to fixed billing my largest winter bill was $275/mo. I have a 12 seer rudd AC. My fixed electric bill is only $144/mo, total for year $1728. I keep house at 65, basement stays at 60 without heat. Garage stays a 55 no heat. I also run a workshop off same service.

If a heat pump saved me $1000 a year I would only pay $728. Don't think that will happen.
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Probably true, Indiana may be a bit far north for them to be efficient. My biggest problem with heat pumps is the cost of replacing the equipment and the service calls to keep it running. You may be getting efficient performance, but forget about actually saving money when it comes time to pay for a new system every 10 years. The HVAC contractors and the manufacturers tend to push new equipment. If I had to do over again like massey says, I would run a HP in summer and switch to oil or gas backup in the winter. I'll bet oil/gas furnaces outlast heat pump units by many years.
 
son installed and air source system around 10 years ago the air conditioning part works good, the heat is only good for around 25 degrees,then it's the heat strips or other means of heating if you intend to stay warm, simply because the outside air temperature is the same or lower than the refrigerant temperature of the outside coil,and therefore there is little or no heat exchange to warm the outside coil it freezes over thereby rendering a reversible air conditioner useless as a heater
 
(quoted from post at 19:01:15 01/13/16) son installed and air source system around 10 years ago the air conditioning part works good, the heat is only good for around 25 degrees,then it's the heat strips or other means of heating if you intend to stay warm, simply because the outside air temperature is the same or lower than the refrigerant temperature of the outside coil,and therefore there is little or no heat exchange to warm the outside coil it freezes over thereby rendering a reversible air conditioner useless as a heater
ne of the sticking points about a heat pump is the loss of usable capacity in cold weather. If I recall correctly they lose right at one third of there heating capacity at around 25 to 28 degrees. Based on that a three ton unit is really only producing about what a 2 ton at say 50 degrees would. These # are all estimates so don't hold me to them. I have wondered why in the colder areas they did not advocate going to a two stage unit in a large enough capacity to handle the extreme high as well as low temps. Many co offer a unit that can operate a different capacity as needed, 4 ton/2 ton, 3 ton/1&1/2 ton. If the set temp is not maintained they switch to the higher capacity.
 
We ran a ground source HP for 15 yrs.The dealer undersized the underground piping(6T unit),first yr.was fine,but second yr.temps.got down into the 0 to -20 range for most of a month and we froze the ground.Dealer had went belly up(salesman placed orders,but kept deposit money),but I had some good Univ.ties and I became a guinea pig to new ideas for a couple yrs.At 9-10yrs the water coil leaked flooding the Freon,but the company(Factory)rebuilt it under warranty.At 15yrs.(flooded again)we junked it for a 5T air source+oil backup.At 5 yrs.,the out door unit locked up and they put a whole new unit in which has been in 16 yrs.now with NO PROBLEMS.I still believe in the ground source,but90-95% of dealers do not know how to work on them.The first or second line in our unit manual was DO NOT LET A HEATING COOLING PERSON WORK ON THIS UNIT.ONLY A COMMERCIAL REFRIGERATION PERSON.In 2003 we remodeled and added 1200 more feet and put a 2THP+ a gas unit in it and the HP has never been touched.but the Gas unit has been rebuilt 8 or 10 or more times.We are Heating-Cooling a little over 5000Sq.Ft.
 
We put a ground source heat pump in in 2000, never got around to installing any heat strips, but the heat pump has always kept us comfortable from -20 F to 110 F outside.
 

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