Why Are Heat Pumps So Dumb About Frost?
Heat pumps can get frosty when they run in heating mode. It doesn't happen all that often, but it's a fact of life when you're trying to extract heat from cold, outdoor air. Sometimes the outdoor coil goes below the dew point, and frost results. There's nothing wrong with that. Every heat pump comes with a built-in defrost system so it will keep running smoothly. Unfortunately, the defrost system in most heat pumps is just plain dumb. And fortunately, I have a smart friend named Mike MacFarland who's come up with a way to overcome that liability.
What's so dumb about heat pump defrost systems?
Let me ask you a question. If you were to design a defrost system with sensors and controls to determine when the defrost cycle ran, how would you design it? Would your heat pump defrost system activate when it detects there's a small chance of frost forming on the coil? Or would it instead turn on the defrost cycle when it actually measures that frost has formed?
As it turns out, the defrost system on most heat pumps does the former. The image below shows the text from the product literature for a common heat pump, and it explains what happens. The outdoor coil has a temperature sensor mounted on it. When the temperature it reads hits 31° F, it starts a timer, which on this piece of equipment will be set to 30, 60, or 90 minutes. If the temperature is still 31° F or below at the end of the timer cycle, the defrost cycle begins.
Maybe there's frost. Maybe not. This system is based only on temperature and time. Apparently, heat pump manufacturers forgot that frost forming on a surface also depends on the amount of moisture in the air and this thing called the dew point.
OK. We both know they didn't forget that. They made a choice to reduce their manufacturing cost at the expense of the homeowner's comfort, peace of mind, and energy costs. And the equipment may not last as long either. But hey, who's gonna notice?
What's wrong with a dumb defrost system?
When a heat pump goes into the defrost mode, the reversing valve activates and runs the refrigerant backwards. Instead of extracting heat from the outdoor air and dumping it into the house, it extracts heat from the house and dumps it into the cold outdoor air. That heats up the outdoor coil and melts any frost that might be on it.
What do you think happens to the indoors when the heat pump is in defrost mode? You guessed it. The heat pump, absent the effects of supplemental heat, would be cooling the house. In winter. On a very cold night.
For most homes with heat pumps, that's when the supplemental heat kicks in. You can't have the system blowing cold air inside when people want heat, right? That just wouldn't do. So, the supplemental heat fires up, and for most people, that means electric resistance (a.k.a. strip) heat. It's 100% efficient...but that's only about a third as efficient as your heat pump.
If you have a dual fuel system, the back up heat is usually a furnace. There will often be a delay before it comes on, though, so you might get a blast of cold air from your heat pump running in air conditioning mode before the furnace gets going. (Did that just explain that mystery for you?)
So, either you'll be paying for expensive strip heat for a while or you'll get a chill in the house. Neither is ideal. If there's no actual frost on the outdoor unit, that's just a slap in the face.
Oh, and that defrost cycle puts a lot of extra wear and tear on the equipment. As my friend Mike MacFarland explained it, going into defrost cycle is like driving down the freeway at full speed and every once in a while suddenly having to drop it into reverse.
Another problem is noise. When the heat pump changes the direction of refrigerant flow, it's something you'll notice if you happen to be in earshot of the outdoor unit. If you don't know what's going on, you might think something is wrong and put in a service call.
Mike MacFarland makes heat pumps intelligent!
So how do you design a defrost system that operates only when the outdoor unit actually has frost on it? Well, you don't have to because Mike MacFarland, owner of Energy Docs in Redding, California, has already done it. He calls it the Energy Docs Intelligent Defrost System (EDIDS). His system uses the temperature sensor and timer but adds another key component: a pressure sensor.
In the photo of his kit below, the pressure sensor is the part with Dwyer on the label. That gets mounted under the service panel and measures the pressure difference between the inside and outside of the unit. With a clean coil that has no frost on it, that pressure difference won't be much. As the coil loads up with frost, the pressure difference increases.
MacFarland has configured the kit so that the defrost cycle doesn't start until the pressure difference climbs to about 0.35 inches of water column. With frost on the unit creating that much pressure drop, he's measured a performance drop of about 24%. He's OK taking that much of a hit from frost, because sometimes the ambient conditions change, and there's no need to go into defrost mode. If not, the heat pump switches into reverse, melts the frost, and then goes back into heating mode.
A typical heat pump will go into defrost every 30, 60, or 90 minutes if the outdoor coil temperature stays below setpoint (~31° F). MacFarland's system goes into defrost mode only when necessary. The graph below shows how the pressure builds during a long runtime, eventually kicking off the defrost cycle when it hits 0.35 i.w.c.
The results are in
MacFarland recently put his heat pump with the Energy Docs Intelligent Defrost System to the test, logging the data of its performance over a 62 day period in December 2013 and January 2014. The graph above is one small part of what he's learned. Here's what's even better:
Actual # of defrost cycles
# of defrost cycles at 2 hr interval
# of defrost cycles at 1/2 hr interval
Energy saved per heating season compared to 2 hr interval
Energy saved per heating season compared to 1/2 hr interval
As you can see, the EDIDS cut the number of defrost cycles way down. He got rid of 76% of the unnecessary defrost cycles that would have occurred without the EDIDS and with the timer set to the 2 hour interval and eliminated 94% of the unnecessary defrost cycles with the timer set to 1/2 hour intervals.
The energy savings aren't huge, but as he told me recently, his primary reasons for doing this were comfort, noise, and wear-and-tear on the equipment. Also, he doesn't have supplemental heat set up on his heat pump. If you factor in the energy used by strip heat on a typical heat pump, the savings are greater than shown above.
The takeaway here is that most heat pumps implement defrost in a really dumb way. Now you have a choice, though. An intelligent defrost system exists - the Energy Docs Intelligent Defrost System. If you're interested, you can find MacFarland's contact info on the Energy Docs website.
In case you're wondering, no, I don't make anything by promoting his product here. I just like good stuff that makes sense. Mike is a shining star in the home performance/HVAC industry. You could probably learn a lot by hanging out with him for a while. If you can't do that in person, you should definitely go hit the Energy Docs Like button if you're on Facebook. How else are going to find out that this guy is so nutty with his single-minded focus on measuring things that he's got a datalogger hanging in his shower?! Just go do it.
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