Greetings.  This is my first post/question - many more to follow, I'm sure. 

I live in a rental house that has radiant floor heat and a heat recovery ventilator that pulls warm, moist air out of the baths, kitchen, and laundry room and blows outdoor air warmed through the heat exchanger into the living spaces of the house. 

The  fan started making a high pitched noise this week, so I called and they sent out a service guy (plumber).  The air exchange unit is located in the attic against a south facing wall.  The attic is not heated, but the attic is included within the building envelope (insulation against the roof). 

The plumber, after a bit of effort, pulled out the central core of the exchanger - it looks like a pentagon or hexagon prism with zig-zag filter around the perimeter (not the fan or fan motor).  It was frozen - obviously moisture had gotten in and then the cold make up air froze it.  It's been well under freezing here for several weeks.  The plumber said the ice was restricting the airflow causing the fan motor to work harder.

The plumber changed two small rectangular air filters where the ductwork enters the HRV, put everything back together, turned the fan on, and the sound was gone.  He said the system would be fine. After he left, I checked the air blowing into the house, and it felt like air conditioning - which I really don't need in the dead of winter.

My concern:  When the ice melts, and the water sits within the HRV, will I have a mold problem?

The system looks like it has a light to show that it is frosted up and might auto shut off, but obviously that didn't work. 

Thoughts?  Should I pull the ice core out and let it thaw and dry and reinsert it?  I can live without the air circulation for a couple of days.

Many thanks.

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Is the house tight enough and the occupant load high enough to require ventilation?

Good question. 

It's US government housing on a military base in Germany  - kind of a mix between German and American design (think of committees here combined with language barriers). Most German homes in this area do not have active ventilation systems but do have moisture issues if they aren't opened up frequently.  Locals open their windows for five or ten minutes once or twice a day to reduce humidity and get fresh air in the houses. These US houses were built in the past three years, and I like to think they used some Passivhaus concepts, but they didn't site them for optimal passive design.

House size: 1500 square feet, three occupants (plus cat). Radiant floor heat (central hot water plant). Wall construction is block (concrete) and if it is typical for this area, has foam exterior insulation on the outside covered by a stucco topcoat.  The only wood framing is in the roof, which is tile. 

The west exposure has two large sliding doors 36 sf each and all rooms except the laundry have at least two windows.  The windows seem pretty well insulated but the frames are leaky. Both full baths have operable windows.  The airflow with the HRV doesn't vent humidity well enough and there is no direct exhaust fan from the baths, so we tend to crack the window for a few minutes after each shower.

I really don't think the HRV works as they had hoped/planned.  The vents are 4-inch diameter. Ductwork in the attic is uninsulated metal and all exposed runs in the attic are a good 10 - 20 feet long. The attic is about 20 degrees colder than the rest of the house but much warmer than outside. I think any heat they might have recovered from the kitchen, baths, and laundry dissipates through the ducts in the attic before it gets to the heat exchanger.

We don't own the place or maintain the systems and will be headed back to the states in the summer, but I'd like to be a good steward for the next residents of this house.

As I recall, HRV's have a drain tube to help eliminate this issue & that might have been the main issue / might still be an issue. Personally if you can handle opening the windows for a few minutes each day - I would pull the core & let it all melt / drain away


Very interesting. I would think that you are correct that there might be moisture issues. Paul Raymer whom is on this site would be great person to ask directly. He has probably forgotten more than I know about ventilation.

The system from your description appears to be lacking any defrost mode that would solve this problem. With as harsh as the winter has been in this area this might be an anomaly due to unusual conditions or simply a design flaw

Have you tested your humidity levels?

You might need a dehumidifier in the conditioned space to control the moisture effectively 

very good article about HRV and ERV here

keep us posted on what you find out

You might actually have a tight house there. The typical HRV I see is installed in a large, leaky, custom home with a couple of people living in it, and they're sitting there with their mittens on with the ventilation running continuously. I get funny looks when I suggest they might not need it. 

The installation sounds flawed, and if you're not going to fix it, I would turn it off during conditions cold enough to freeze the core, and crack windows for ventilation.

I am surprised that you are having this issue in Germany. They are well ahead of North America in both efficiency of units, and proper installation. The HRV should have a defrost mode, either with unbalancing motors to provide more exhaust than intake, to warm the core, or a pre-heater. I am also surprised at the installation, which sounds to be poorly done. There are two issues immediately;

1) The system should be checked for balance. By commissioning it, you can make sure that the supply and exhaust flows are balanced. If they are not, the HRV will not perform well, excessive condensation in cold weather can occur, and then frost. 

2) Running ducts in an attic that is up to twenty degrees colder than the house is not good either. Exhaust air is pre-cooled prior to entering the HRV, and supply air is cooled before it gets to the rooms in the house. 

Balancing the system and insulating the ducts in the attic would both be beneficial, and may resolve some of the frost issue, as well as improve performance and result in warmer air coming in to the house. 


This is a combo American-German house.  The US government contracted the design and construction and it ended up being a mix of US vs.German norms/standards -- perhaps a case of too many cooks in the kitchen with an added language barrier and US QA folks overseeing the work who aren't necessarily familiar with German engineering.

That said, the system has a frost warning light (which never lit), so I think it does have a defrost mode which may not be operating properly. Your suggestions make perfect sense too.

Unfortunately, because Uncle Sam owns the house, they maintain it.  If it were my house and system, I would get it commissioned and maybe do something with the ducts. Uncle Sam has a contractor maintaining the houses and we have to rely on what they do.  He changed the filters and it subsequently warmed up outside, so the system seems to be working properly now. I think the unseasonably cold temperatures (-17C) were beyond the design for the system.  Normally the temperature gradient between the attic and house isn't so high - we were in the extreme freeze that hit all of Europe over the past few weeks.

One other thing I noticed yesterday - the dryer vent is located about a foot from the HRV intake vent on the outside wall of the house.  When the dryer is running, the HRV can easily be pull in warm (and moist) air!!  Germans use condensing dryers that don't vent to the outside and wouldn't encounter this issue!

Please let us know if cleaning the filters and the core cured your problem.

There should be a drain for condensation.  The drain tube should have a loop in it with water trapping/preventing air leakage through the drain system.  If yours does not have a drain, since clearly condensation is occurring in your climate, have one installed. 

Some ERV/HRV's have defrost cycles to prevent the core from icing solid like that.  If it ices again the cycle may need to be adjusted.  


When I pull the core, I don't see a drain but there is a tube that runs directly from the base of the unit to the sanitary vent line for the house, so I believe it does drain but don't see a loop. 

I had the flu last week and didn't get up to pull the core until yesterday.  Temperatures were an unseasonably cold -17C here for about two weeks but it warmed up above freezing over the past five days.  When I pulled the core, it was dry, so it must have melted and drained or airflow dried it out. 

The indicator panel on the HRV unit has lights to show how it's operating, and there is a frost light (which wasn't lit when the system was iced up), so I think that may be the failure spot.

To the guys who deal with these units all the time, would an ERV be more of an approiate unit than an HRV?

in general, I would say no based on the climate there, but Martin Holladay did a pretty good write up on this issue here:


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