Ben;

good window, I had them in my house when I was in NW PA it occasionally got to -15 with no condensation.

not sure whats going on with your house, you may need to get a local mechanial engineer involved

A properal designed ventilation system will run nearly full time in the winter and run automatically reset to outdoor temperature. Since sub-zero temperatures are not long lived in the lower 48 there is more to worry about rotting windows than dry floors.

I say it's a simple issue of thermal gradient and dew point. At the low outdoor temps the heat transfer through the glass is so great the interior surface of the glass drops below the dew point of the indoor air. Get a thermometer, Lowes has them, that has a remote contact sensor. Put it on your window and I'll bet you'll easily record glass surface temps below dew point on the coldest days. The only way to bring that temp up is to add more thermal resistance, i.e., insulation, to the heat path through the glass. The best way to do that is storm windows. Think in terms of the cold glass of ice tea in the middle of a hot humid Iowa summer day! Glass temp below dew point, presto, condensation on the glass.

Otherwise the only alternative is to lower the humidity level thus lowering the dew point below the glass surface temp.

dm is correct. However, your problem is short lived-a few days a year- and is most often solved by lowering the relative humidity in the house when the windows would condense. Most of the season, you will have comfortable humidity and no condensation.

Very interesting problem. I have never heard of a high humidity problem indoors in the winter. Around here, northern NJ, it rarely gets down to 0*f, and most houses are not super tight.
From the psychrometric chart, given the indoor air conditions as 68*f, 33%rh, the dew point is 38 deg.
If you were able to lower the rh to 24%, the dew point would drop to 30*f. At 20% the dew point is 26*f

How about running a dehumidifier when it gets really cold ? It would probably be cheaper than installing another HRV.

I don't know if I should drop my rh to much lower than the current 34%.  We've got 2000 sf of hardwood floor that are beginning to show signs of movement.  I don't know if this is relevant, but all the wood floors, baseboard, and window/door trim plus interior doors are reclaimed. 

For now, the temps outside have stayed in the mid teens and up so condensation is minimal.  I have taken the approach of seeing if this phenomenon continues next year, but trying to manage it the best I can now with the HRV and running a dehumidifier occasionally.

Thanks for everyone's help.  Not that I'm not interested in more advice if you've got it though.

Ben

At that high of a humidity rate, it doesn't matter WHAT your heat source is, you'd have condensation on your windows. You have one of two options, and at this point in time, only one is feasible. Option number 1 (in my opinion) would have been to install a heated window. Obviously, that is the one option that is not feasible at this point in time. Alternate number 2 is to change the dew point of the window by reducing the RH within the envelope. Honestly, 33% is high, and if there are any other places within your home that didn't get as well insulate,d it will be a point of condensate generation, possibly allowing mold to also be generated. I maintain a RH of 15% in my home with hardwood floors, furniture etc, and I really don't see any problems with my woods. 33% may be the theoretical "ideal" condition in which to maintain grand pianos, etc, but those are usually kept in an isolated space where the RH can be tightly controlled. Try running the HRV for a longer period, say 40 on, 20 off and see what happens. Try maintaining a RH of around 20% and see if the production of condensation is less If it is, you can tie a simple humidistat to the function of the HRV and maintain a lower RH with the HRV.. In most cases, if you can maintain whatever the background RH was at when the hardwood was installed, then the wood will remain stable. Here in Colorado, background RH is 7%. Information on the heated windows can be seen at rgiglass.com ME

Mark,

This is the first I've heard of heated windows, I'll check them out.

I don't know if it would be possible to lower the humidity to 20%, my dehumidifiers don't register below 35% (though maybe they just keep running with no cut-off below the 35% setting).  I wonder if the drier conditions outside in CO help maintain the lower rh.  According to my weather station outside we're still at 78%.

Ben

Ben
I will add again that this is not a heat system problem. It is a combination of humidity and insulating value of the windows. I would wait until next year until you do any thing major. New houses have a lot of moister in them. If you still have problems. you can either lower the humidity or install storm windows. The problem with casements is that they don't lend themselves to easily install storms. I have lots of wood, and run in the lower 20's without a problem. Yes you will see movement but that is the nature of wood.

Ben, Relative Humidity is the humidity relative to the temperature. If you are at 78% RH outside with a temp of say 32F then that air would be at 21% RH when brought inside and heated to 70F. I'm puzzled why you have to run dehumidifiers to achieve 33%RH. I guess your house is very tight and you must generate a lot of moisture. Your HRV should really handle getting your humidity down. An ERV would be less effective because it would recover some of the humidity so that's not the answer.

So if you run your HRV enough to get the humidity down then you say the floor registers are cold? Others must have this same issue when combining HRVs with radiant heating in cold climates. Probably less of a problem with forced air heat because the HRV probably runs when the forced air heating runs so no cold air.

There must be a standard HVAC solution to this? When the outside temp is down to zero the incoming air from the HRV is probably right around 32F. Maybe adding a hydronic coil to the ductwork to warm the air before discharge? How do you pros handle dumping the cool incoming HRV air into homes heated with radiant???