Help keep me warm, mkay?

January 26, 2013 4:59 AM

My electric heat pump is icing over something fierce, and the technicians don't really have a clue what's going on. There is one theory I'd like you to comment on, and if it makes no sense, I'd appreciate alternate theories.

Background: this has been going on for a few years, where the outside of the unit ices up and just won't defrost. Currently there's about an inch of ice, which built up overnight. I've had different companies out several times, and it never seems to get resolved. Last week they replaced the defrost board and sensor, which clearly didn't solve the problem.

The guy last week proposed that maybe it's actually due to my gutters overflowing onto the unit when it rains. This is because last week when I called him out we had just had a lot of rain. While there is some detritus in the gutters, the pattern of freezing up doesn't make sense to me. Why would it have that pattern that you can see in the photo? Why wouldn't the other parts of the casing be covered with ice? Also, it was freezing rain last night and i went out and it didn't look like anything was overflowing. Yet this morning when I went to check, it had gone from no ice to what you see in the picture.

More info: this only happens when it's below freezing. I live in North Carolina, US.

So: Could it be the gutters leaking like the HVAC guy suggests?
Or: Is it something else? Ideas?

posted by Stewriffic to home & garden (13 answers total)

Oh, and the freon level is fine.
posted by Stewriffic at 5:34 AM on January 26

It would have to be something like that, because there really isn't enough water vapor in the air when it is so cold out. It's also possible that the ground near the unit is saturated, which then evaporates and is sucked into the coils and freezes up.
posted by gjc at 5:53 AM on January 26

I wonder what would change if the pvc drain pipe was re-routed to drain back away from the face of the unit?
posted by Thorzdad at 6:11 AM on January 26

It's true that constant dripping water on an external unit of a heat pump can promote icing.

But, I'm going to go with "something else" as the main cause of your issues. Specifically, that the photo shows your grass is still pretty green, that your post says you live in NC, and that your unit appears to be 10+ years old. You live in a pretty temperate climate, where the relative humidity appears to be pretty high on average, and where cold days generally follow a pattern of daytime temps a little above freezing, with night time temps dropping slowly into the below freezing temperatures (in other words, no so different than the climate of Northeast Florida, where I live). And the bit that gjc offers above, that water from the unit, and the dripping gutters above it, maybe saturating the ground around the machine and raising the local humidity to 100% much of the time it operates is spot on, in terms of creating icing problems.

Units of your vintage and earlier have a pretty simple "defrost" cycle for the outer coils, that simply senses reduced heat transfer efficiency on the outside coil (based on air temp), and either reverses heat pump flow for a few minutes, while blocking internal air circulation (to raise the outside coil temperature above the frost point long enough for ice to melt and water to evaporate) or may even turn on a simple band heater on the outside coil. But the effectiveness of this system is limited, because it never really measures whether water has truly evaporated entirely from the outside coils, before going back into heat pump mode. Your unit has probably always been prone to ice up, and may have even operated satisfactorily with partial ice up for quite a while, but now is actually visibly icing up more frequently to the point it is affecting your comfort. So, you've begun to notice, and to seek a solution. What's changed recently is probably very little due to a new problem in your unit, but certainly, your notice of the problem.

I suspect that your outside coil and fins have also finally degraded their finish, via micro-corrosion, to the point that there are now thousands of nucleation sites on which water droplets can first form during the very initial stages of the transition in air temperature to below freezing, while the air is still relatively moist. The same tiny pits and new edges on the surface of your outside coil also provide shelter for tiny droplets of water during defrost, making your defrost cycle less effective. And perhaps, although your photo doesn't show it, you've got shrubs or tree limbs nearby, whose growth is reducing or redirecting air flow around the unit unfavorably.

So, in order of cost effectiveness of actions, I'd recommend you fix/clean the gutters around the unit, until you can be sure the ground around the unit is not saturated, and that rainwater is never dripping on it. I'd keep the grass and plants around the unit cut way back to promote sunshine hitting the unit and ground during the day, and getting good air circulation to the unit. And then, you might consider a new outside coil, or better yet (for nearly the same cost), and new, more energy efficient outside unit, or complete system. When I replaced my 20 year old heat pump system last year, my energy use dropped about 20%, and with credits from my utility, the unit will pay for itself in about 3 years, and I cured the winter icing problem I'd been having completely.
posted by paulsc at 6:12 AM on January 26 [2 favorites]

No way that is from overflowing gutters. Unless you have perfectly symmetrical water flowing out the gutters. That unit would have dripping icicles on it if the water came from above the unit.
posted by Gungho at 6:30 AM on January 26

I just recently heard that heat pumps in cold climates should be raised off the ground a bit to allow proper drainage. Otherwise this exact thing can happen. In your picture it looks to my not expert in this field eyes like its own drain water is freezing over itself.

This site confirms and offers some other tips: http://www.hannabery.com/hp101_2_pg2.shtml
posted by mgr at 7:30 AM on January 26

Randomly, my next door neighbor is having problems, too. Different problems, but the guy who came last week is going to be next door. So we might have more info in a while. Thanks to everyone so far!
posted by Stewriffic at 9:31 AM on January 26

gjc writes "It would have to be something like that, because there really isn't enough water vapor in the air when it is so cold out. "

As long as the coil is colder than air temperature you can/will condense moisture out of saturated air. if the coil is below freezing then the moisture will become frost/ice.

Have the coils ever been cleaned? Coils that are dirty will both hold defrost moisture better (in the same way paulsc outlined) and restrict airflow causing lower coil temperatures exasperating the problem.
posted by Mitheral at 11:25 AM on January 26

More info: according to my neighbor, the unit was new in 2007. I have not cleaned it since I moved here in 2009.
posted by Stewriffic at 11:31 AM on January 26

So the HVAC guy didn't come out after all. But he's coming on Monday. My next door neighbor said that his unit and his father's often look like mine does and that it's "normal" as long as there are those spaces for the air to go through.
posted by Stewriffic at 4:17 PM on January 26

For reasons I described above, the advice about cleaning your gutters and managing the moisture accumulation and air flow around your outside unit better is conventional wisdom. But that doesn't invalidate what your neighbor says about partial icing conditions, either.

Some icing was often tolerable in older systems; the coils and fans were simply sized to be effective with some amount (say 20%) of restriction in air flow, and over time, in most climates, the air temp/humidity ratio went either above or below freezing for long enough that a generally fixed defrost cycle of about 4 minutes would eventually melt all the ice, and evaporate the water to the surrounding air. When weather conditions were "just wrong enough, long enough" however (around the triple point of water which is about 32°F and 100% humidity at sea level in atmosphere), many heat pumps would "freeze over" and quit trying to extract energy from outside air altogether (because of the coating of ice on the coil/fins on the outside unit), and "fail over" to a backup setting of "Auxiliary" or "Emergency" heat, even if the "fail over" control mode was nothing more than a printed instruction in the owner's manual to set the switch on the thermostat to "Auxiliary" or "Emergency" when the heat pump couldn't maintain desired inside temperature automatically (usually because it was iced over completely). The "auxiliary" or "emergency" heat setting turns on a separate resistive electrical heat element, which will consume a lot more electrical energy than running heat pump mode will, or maybe it starts an alternative gas or oil heat secondary system, to maintain internal temperature. In some of these systems, the emergency setting will also run the heat pump in defrost mode for a few minutes every hour, to encourage deicing, and may switch back over to heat pump mode automatically, after several hours of running on emergency heat. It really depends on how "smart" the systems controllers and sensors for managing the system are, and what they've been programmed to do when they find that operating the heat pump can't keep the internal temperature of the home at desired temperature.

In a bid for improved energy efficiency, and lower unit costs and size of outside units of heat pump systems, some later units have traded off more sophisticated control systems, which can sense both inside and outside temperatures and even humidity, and use relatively smart controllers and better sensors on the outside and inside coils to detect icing and air flow restriction, and to manage the deicing more efficiently. Hence my previous point about more modern or perhaps more sophisticated systems.

You might find that if you can cut back the area you are heating by closing some room doors and vents, that you might help your heat pump keep a smaller living area warmer, because you're effectively reducing its load, and giving it more time between cycles for defrosting. You might find that lowering your set temperature on your thermostat helps the situation, too. Some heat is better than no heat. But if your heat pump was sized near the minimum for your house size, and is operating in winter conditions where full defrosting is ineffective over time, it is bound to freeze over eventually, unless you either alter the conditions (reduce humidity around the outside unit, lower the duty cycle/load, use emergency heat settings, etc.) or replace the unit with a larger or better managed/controlled unit.
posted by paulsc at 6:40 PM on January 26

One other thing to mention, as a minor point, is that I can't tell from your photo if there may be other sources of moisture, or relatively warm, moist air around your outside unit, like a dryer vent, or maybe just an unintentional leak of warm, moist air from inside your home, through uninsulated space in your walls, like through holes for services, rodent entrances, etc. Sometimes, in a bid to "camouflage" things like dryer vents, bathroom exhaust fans, etc., an installer may put holes for such "behind" or even immediately above the heat pump outside unit. So, you do a few loads of bath towels and winter flannel shirts on a cold day, and you've unintentionally put several pints of water vapor out around the outside unit of your hard working heat pump, further exacerbating its freeze up issues. Or, a raccoon got busy near your foundation last summer, behind your heat pump outside unit, and pulled out some insulation, trying to get into your house, and now you've got a 24/7 leak of moist, warm air, a few feet from where your heat pump is icing over.

It just looks like there is standing water on top of your outside unit, and a lot of ice to the right of it on the ground, as well as heavy ice all over the outside of all the coil surface. In the winter, dry is the friend of a heat pump. In summer, except for promoting corrosion, water mist and humidity generally work in its favor.
posted by paulsc at 7:24 PM on January 26

Thanks paulsc and all.

It is pretty wet around that area, but the ice on top of the unit is gone (there had been freezing rain the night before), and this morning the unit is frozen up still. It's not leaking from above, there are no holes that I can see in the house or foundation, and there is no vegetation within 2 feet of the unit. Not sure how to mitigate the wet ground. The dryer vent is on the opposite side of the house, so I don't think that's an issue.

Yesterday evening I turned it back on, and the heat coming out of the vent measured a nicely warm 95F. This morning it was coming out at 70F. I went outside and there is more ice on the unit's coils, but still some spaces. The coils looked like there was melting water dripping down them. It's 26F outside.

What concerned me was that the unit was making noise as though it were running, and air was coming out of the vents, but the fan was not moving. I turned off the heat again.
posted by Stewriffic at 4:47 AM on January 27