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Reducing house humidity in a mild, humid winter climate

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  • Apr 21st, 2021 11:27 pm
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Reducing house humidity in a mild, humid winter climate

Just wondering what the solution for reducing humidity in a relatively mild winter climate that can be very humid?

I'm in SW coastal BC and none of the houses here have humidifiers (unless some clever HVAC salesperson manages a needless sale) precisely because the climate is so wet (one can assume that RH outside is pretty much saturated for the whole of winter). Home relative humidity was in the mid 60% range but apparently increased to high 60s since some leaky patio doors were replaced. Heating is with a heat pump so the vent air is not dehumidfied.

I was thinking of using a dehumidifier but that can be inefficient. With an air exchanger, what should the RH drop off if the house temperature is 19-20C and outside air temperature is anywhere from 0-7C but pretty much saturated?
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Feb 10, 2006
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Kitchener
thriftshopper wrote: Just wondering what the solution for reducing humidity in a relatively mild winter climate that can be very humid?

I'm in SW coastal BC and none of the houses here have humidifiers (unless some clever HVAC salesperson manages a needless sale) precisely because the climate is so wet (one can assume that RH outside is pretty much saturated for the whole of winter). Home relative humidity was in the mid 60% range but apparently increased to high 60s since some leaky patio doors were replaced. Heating is with a heat pump so the vent air is not dehumidfied.

I was thinking of using a dehumidifier but that can be inefficient. With an air exchanger, what should the RH drop off if the house temperature is 19-20C and outside air temperature is anywhere from 0-7C but pretty much saturated?
If your home has ductwork you might benefit from a whole home dehumidifier. It can sample the air and turn on as required. You might want to check it out if you truly have high humidity.
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Double_J wrote: If your home has ductwork you might benefit from a whole home dehumidifier. It can sample the air and turn on as required. You might want to check it out if you truly have high humidity.
Thanks (and the houses does have ductwork). Didn't know such a thing existed. Aprilaire? Who else makes good ones?
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thriftshopper wrote: Thanks (and the houses does have ductwork). Didn't know such a thing existed. Aprilaire? Who else makes good ones?
It is less common since it has higher up front cost. I personally installed an April aire many years ago because I got a good deal on it. I don't remember off hand what I paid but I think it was a few hundred.

Since I knew of the brand, and got a deal I scooped it up without looking at different companies. Mine is connected to my supply and return. My furnace blower is set to on to help circulate air and clean. I put in a uv light, separate hepa system and carrier air purity (now removed).
Short version is I wanted good IAQ and because I work in the business was able to get the pieces at a reasonable price.

I would do it again even if it would have cost me a few thousand more.
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Feb 20, 2015
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I live in the most humid city in Canada and don't have much of a problem. I run AC in the summer and a Natural Gas Furnace in the winter.

I actually use a humidifier during winter to bring up the humidity to around 40%. With AC on in the summer, humidity is rarely about 50%.

I don't see the issue unless your windows are open and you are letting in moist air year round.

60%? My weather app says 100% right now. How do they survive in Bangkok with 75% humidity and 34C every day?
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Thanks. The website says to contact a pro. Nothing other than portable at the BB stores or my local plumbing supply wholesaler (best they do is HRVs which they have no stock of).

A HVAC website indicates a cost of $2.2k.

https://www.gasexperts.ca/product/april ... -test_tab2

If I get this, I will have to set up the fan to come on so might as well enable low-speed for the Mars Azure motor (need to make a relay).

Fortunately have lots of room in the basement to get to a return duct.
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Gibsons wrote: I live in the most humid city in Canada and don't have much of a problem. I run AC in the summer and a Natural Gas Furnace in the winter.
Maybe summer/all year where you are but the high-to-the-point-of-saturation outside relative humidity where I am is a winter phenomenon. it dries out quite a bit in summer (don't have to run the a/c at all) and outside RH rarely goes above 55% during the day (usually in the 45-50% range).
I actually use a humidifier during winter to bring up the humidity to around 40%. With AC on in the summer, humidity is rarely about 50%.
Yes, as your winter temps are much lower. Average high low where I am in December coldest month of the year) is 7/2C so we don't get the drying effect that a HRV would afford as the outside RH is pretty much at saturation.
I don't see the issue unless your windows are open and you are letting in moist air year round.
Windows (and doors) are usually closed and the building envelope is getting tighter with every window/door replacement. Moisture come from us (2) and 2 cats and house plants. I also should have added we hang dry most laundry items which probably doesn't help. And yes, we have bathroom exhaust fans and use them (I've checked they actually ventilate) as well as 2 kitchen vent fans (which do exhaust outside too).
60%?
That's inside. Outside when it's overcast is usually above 90% (2 hygrometers) and 100% when there's precipitation (more often than not in the depths of winter, as well as a lot of autumn and spring though we have had a dry spring). Called "liquid sunshine" by some and it drives many easterners (anyone from east of the Coast mountain range) away as it's too damp for them..
My weather app says 100% right now. How do they survive in Bangkok with 75% humidity and 34C every day?
Anywhere in the tropics? Stuffed/upholstered furniture can go mouldy. So can paintings made with natural paints. Brass/copper goes green and thus electronics often rot. Other metals oxidize faster. On the converse, furniture made for the tropics will crack when they're moved here, yeah even where I am with high 60% RH inside.

I should add that my house is built on a sloping large piece of lava rock (no radon) and some water does flow through the basement with heavy rain which probably doesn't help. Not damp to the point that corrugated board and other paper products go soft and mouldy though.
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I have to ask. Where in S.Ontario is the most humid city in Canada?
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Beneful1 wrote: I have to ask. Where in S.Ontario is the most humid city in Canada?
Windsor.

Google says it's the most humid in Canada. It's 100% humidity right now.
Last edited by Gibsons on Apr 20th, 2021 10:25 pm, edited 2 times in total.
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As long as the dewpoint outside is much lower than inside, an hrv will dry out the house.

Dewpoint = temperature required for the rh to be 100% at the given moisture level.

To get 40% indoor humidity as 20c, need an indoor dp of 10c , so outdoor dp needs to be around 5c or lower for it to be very effective.

Dehumidifiers don't run very efficiently at the temperatures/humidity levels desired during the heating season.
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Thanks. How efficient (in terms of heat recovery) are HRVs? The house is heated by a HP that can barely maintain temperature (w/o using resistive heating) if the temp drops below 0C so a HRV may result in more electrical energy consumption.

Haven't played with a wet bulb thermometer in close to 40 year and wish there were charts I could look up..

I'd be happy if I can keep the inside RH to around 60%.

Yet another factor, almost all the neighbours (maybe all of them) have and use wood heat which is better for reducing humidity. That could be a reason to avoid a HRV as on some days we go outside and there's a strong smell of wood smoke in the air. Only saving grace is the price of fire wood is so high (I've heard $300-400/cord) that using HPs may be becoming more attractive. The municipality allows 10 trees/acre to be felled either annually or every 12 months w/o a permit.
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This site is great: http://www.sugartech.co.za/psychro/index.php

60% target is way too high for winter, should be 45% max and closer to freezing below 40%.

There are different ways to control hrvs units. You can get a controller that runs it on low speed 1/3rd of the time to supply minimum ventilation required and only bumps up to high speed if the humidity is too high.

You do have to look at the specs for different models and check efficiency. A decent one should recover 70%-80% of the heat from the stale air being exhausted.


One thing to keep in mind is that house building materials absorb a lot of moisture when it's too humid and take a while to dry out.
So if you control the humidity well when it's above freezing from the start of the heating season, you may be able to shut the hrv off entirely during the colder days in your area and avoid running aux heat to compensate for ventilation heat loss. Colder it gets outside, the more the house leaks and the less moisture outdoor air holds.

Yes, most wood stoves/fireplace inserts dry out the house because they use up indoor air for combustion/venting.
The disadvantage is they can over-ventilate; on the other hand, HRVs provide controlled air exchange.
I would take a heatpump powered by hydro-dams with mechanical ventilation over wood heat any day.
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Gibsons wrote: I live in the most humid city in Canada and don't have much of a problem. I run AC in the summer and a Natural Gas Furnace in the winter.
That is an apples and oranges comparison. The physical characteristics of -4 degree air at 70% humidity, common in Windsor winters, are vastly different from the characteristics of 7 degree air at 100% humidity, common in coastal BC winters.

Heat a parcel of -4 degree/70% relative humidity air to 20 degrees and it will have an RH of around 15%. This is why even small amounts of ventilation decreases humidity in cold climates, and why many houses in cold regions need humidifiers to keep indoor humidity levels comfortable.

In contrast, heat a parcel of 7 degree/100% RH air to 20 degrees and it will have an RH of around 40%. That is too humid to keep indoor humidity low enough to be comfortable in the face of indoor moisture production from people, plants, evaporation from sinks etc, cooking, showers, and so forth.
thriftshopper wrote: I was thinking of using a dehumidifier but that can be inefficient. With an air exchanger, what should the RH drop off if the house temperature is 19-20C and outside air temperature is anywhere from 0-7C but pretty much saturated?
A psychrometric chart can tell you this.

Presuming an indoor temperature of 20 degrees, an outdoor temperature of 7, and an outdoor relative humidity of 100%, the physical limit on what is possible with ventilation alone--using any means, including an HRV, a fan, or an open window--is 40% indoor humidity. This presumes no indoor humidity generation, which is not realistic as people and everyday indoor tasks release moisture into the air.

I'm also in coastal BC; in my experience it's possible to keep indoor humidity at around 40%-45% only through the coldest parts of the winter (dec-feb) using ventilation and open windows. Once the temperatures rise into the double digits, however, humidity management is impossible on wet days.

A dehumidifier is definitely the way to go in this climate, especially as it will be able to reduce indoor humidity in conditions too cool to run air conditioning but too humid to be comfortable.
Last edited by middleofnowhere on Apr 21st, 2021 11:57 pm, edited 1 time in total.
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It's better to ventilate when it's effective instead of dehumidify.

HRV is the premium solution.

Maybe best option is to have a basic dehumidifier for the milder, super moist days during the heating season, set it to 45%, use only as needed. If you stick it near a return, should be almost as effective as expensive whole house model.
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Thanks again all.

I will try to convince my wife to try running portable dehumidifiers on the day it's relatively mild but raining like the dickens, or even just drizzling (just as or even more nasty) for next winter. One's a small dehumidfier and one's an A/C unit that can work as a humidifier (and heater). Fortunately e have a couple of mostly-unused shower pans they can drain into.

I think with the wood smoke pollution around here (multiple sources within <200m, more further afield), a HRV isn't a good idea.
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