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ham789 · 01-25-13, 05:06 AM

Been thinking about freeze ups. The previously posted
long concentric tube heat exchanger mentioned pointing
it down to drain out the water.
Problem is that with low outside temps, the output is
gonna freeze up eventually. Then it dams up all the water
and virtually all the moisture gets trapped and frozen.

What if you point it UP and drain the water from the inside?
The water is gonna collect somewhere around
the dewpoint temperature. You run it backwards and drain
it while it's still wet. Yes, the dewpoint varies along the
pipe...there will be continued condensation
at lower temperatures, but it should take much longer
to plug up. And there's no ice dam.

If you know the inside temperature and the lowest
outside temperature, to first order, you can calculate
the position along the pipe that reaches freezing.
Put a V in the pipe at that point and you drain off the
water at the lowest possible temperature that won't freeze
in worst case conditions. Not sure it makes enough difference
to warrant the mechanical complexity.

If you need a defrost cycle, just turn off the incoming air fan.
Or better yet, reverse it.

Another option is to separate the dehumidifier and ventilation.
Use a finned pipe.
Bring outside air thru it and plumb it back into the HRV
fresh air input. Control the air flow to keep the core just
below the dew point you want inside. Gives somewhat
decoupled control of humidity and ventilation. And you
can put the core in a different location to reduce the
humidity gradients.

According to dpcalc.org, a condenser temperature of
around 50F should be plenty low to prevent mildew.
You may not lose too much efficiency by just venting
the 50F air into the space.

Are we having fun yet?

01-25-13, 05:06 AM	#357
ham789 Helper EcoRenovator Join Date: Dec 2011 Location: tigard, oregon Posts: 42 Thanks: 0 Thanked 4 Times in 4 Posts	I think pointing it down is backwards Been thinking about freeze ups. The previously posted long concentric tube heat exchanger mentioned pointing it down to drain out the water. Problem is that with low outside temps, the output is gonna freeze up eventually. Then it dams up all the water and virtually all the moisture gets trapped and frozen. What if you point it UP and drain the water from the inside? The water is gonna collect somewhere around the dewpoint temperature. You run it backwards and drain it while it's still wet. Yes, the dewpoint varies along the pipe...there will be continued condensation at lower temperatures, but it should take much longer to plug up. And there's no ice dam. If you know the inside temperature and the lowest outside temperature, to first order, you can calculate the position along the pipe that reaches freezing. Put a V in the pipe at that point and you drain off the water at the lowest possible temperature that won't freeze in worst case conditions. Not sure it makes enough difference to warrant the mechanical complexity. If you need a defrost cycle, just turn off the incoming air fan. Or better yet, reverse it. Another option is to separate the dehumidifier and ventilation. Use a finned pipe. Bring outside air thru it and plumb it back into the HRV fresh air input. Control the air flow to keep the core just below the dew point you want inside. Gives somewhat decoupled control of humidity and ventilation. And you can put the core in a different location to reduce the humidity gradients. According to dpcalc.org, a condenser temperature of around 50F should be plenty low to prevent mildew. You may not lose too much efficiency by just venting the 50F air into the space. Are we having fun yet?