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Extending/adding cold air return

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  • Jun 16th, 2019 11:45 am
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[OP]
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Jul 7, 2017
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SW corner of the cou…

Extending/adding cold air return

The main bed room in my house does not have a cold air return and suffers from excessive condensation on the windows and outside patio doors during cold nights in winter (pretty much the only BR that is used). Would a cold air return serve to reduce moisture? House is heated by heat pump so air is somewhat humid (~60% which is the normal household humidity in these parts).

The main cold air return duct falls a few feet short of a wall I can use (adjacent/perpendicular to the door). I am thinking I can cut a hole in the covered end of the cold air return box, add a boot and run a duct to underneath the wall. Does one just drill a pilot hole through the sole plate and subfloor (open below) and then cut bigger holes with jig saw? Would a 10" heating duct 90-degree boot do?
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6 replies
Member
Jan 7, 2013
412 posts
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Whitby Ontario
I think 60% humidity is too high for the winter unless you have very good windows.

If the bedroom itself gets to the normal thermostat temperature (ie not a distribution issue) then adding a return will probably not do much to help your issue.

Likely your bedroom door is undercut to provide a (small) return air part.
[OP]
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Jul 7, 2017
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SW corner of the cou…
The high humidity is due to high ambient outside humidity which is usually very close to 100% for much of winter. Temperature differential between inside and outside when condensation is a more significant issue is usually 20C or higher. The maximum temperature difference on the thermostat was 3C but we decreased it to 2C which did not seem to make much of a difference Most windows (not the best) have the same issue (condensation) but to lesser degrees (not inhabited).

The bedroom door does have an undercut and is almost always never closed. The closest return is over 12' from the door.

The door that has the worse condensation is a full-light patio, steel clad on wood core. We have been told the original owner who had the house built usually went with the lowest bid so the doors and windows are probably not the best.
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Jan 25, 2007
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Paris
thriftshopper wrote:
Jun 16th, 2019 1:09 am
The high humidity is due to high ambient outside humidity which is usually very close to 100% for much of winter.
Humidity in Canada outside drops to around 5-10% in the winter. Cold air can’t hold moisture. Where do you live?
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Dec 14, 2011
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I’d probably just add a ceiling fan and see if that helps.
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Nov 18, 2005
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Kingston
thriftshopper wrote:
Jun 15th, 2019 7:38 pm
The main bed room in my house does not have a cold air return and suffers from excessive condensation on the windows and outside patio doors during cold nights in winter (pretty much the only BR that is used). Would a cold air return serve to reduce moisture?
Yes. Without a cold air return in the room your HVAC system will have a hard time pushing heated air into your room as no air is going out of the room. If your door is closed it would be VERY hard for the HVAC system to push heated air into your room. You need air circulation to reduce condensation (although it likely won't eliminate it).

A fan would help with circulation but tends to make the temperature feel colder when it is running which isn't great in the winter so you wouldn't want to run it when you're using the room.
[OP]
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Jul 7, 2017
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SW corner of the cou…
Location is along the coast where it rains a lot during winter. Most of the weather forecast and readings peg the RH on a non-sunny day (rare) at 90-100%. Even today (and it hasn't rained in a week or two) currently at ~ 11-12C RH is nearly 90%.

There is a ceiling fan, and yes, we don't run it because it will make the room feel cool.

There is gap between door and floor of at least 1", but we never usually close it completely (so the cats can go in and out).

The windows and doors are behind heavy curtains at night which probably doesn't help (the other doors and windows which experience less condensation don't have heavy curtains, and are not used/inhabited anyway). My wife found that if she placed the curtains so that there is an air gap between windows/door and curtains, and the heat vents are between, it does reduce condensation.

The steel locking slider for the inactive door is rusty due to condensation and I wonder if the wood inside may be rotting due to damp, and the sheet metal inside rusty (if not coated).

I am thinking the doors and windows aren't sufficiently insulated. Someone posted a chart in this forum of window condensation points at given inside RH and outside temperatures for double and triple pane windows and it's 50 and 58% at 0C.

I do wish there were shutters so provide insulation (the side of the house that gets the most condensation is very exposed).
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