Conditioning bathroom vent makeup air
What’s the ideal way to do this in low outdoor temperatures with a tight house? The bathroom filling with cold air when you take a shower is sub-optimal, as is the vent fan being underpowered to deal with the static pressure. You could design a whole facet of your HVAC system around extremely peaky loads like this, but that seems expensive.
Here’s an idea I’d like to refine:
1) Use thermostatic shower valves.
2) Run a well-insulated intake duct from an exterior wall (with an insulated damper) to an inline fan on the same circuit as the vent fan.
3) Slot in a heat exchanger at the interior vent.
4) Hook up the heat exchanger to the hot water line before it reaches the shower valve.
If the shower is running, the heat exchanger is hot. If the vent fan is turned on, air is flowing through the heat exchanger and ensuring a fresh supply of warm, dry bathroom air. Any reduction in water temperature is compensated for by the thermostatic valve.
Any downsides I’m missing to this? Any suggestions for improvement? Any good alternatives?
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Interesting idea. A 50 cfm flow with 50F rise in temp only needs 2700 Btu/h, which 2 gpm of hot water can give you with only a 2.7F drop in temp.
But what happens when the water flow stops but the fan keeps running (as it should, to remove residual moisture for 5-15 minutes)? If that incoming air is below freezing, you could freeze up your heat exchanger.
So we use H/ERVs.
Burninate,
Most mechanically vented houses exhaust air from the kitchen and bathrooms, while bringing fresh air into the bedrooms and living areas. The bathroom typically draws replacement conditioned air from the rest of the house through either an undercut door or jumper duct.
Good point there with running fan without water. Also on the scale of the problem... anything below 5000BTU/h would actually be conducive to an electric resistance heater on a normal breaker, which looks like the cost leader right now.
I don't know a lot about how ERVs are scaled into a practical system in a tight house.
How many ERVs and at what cost for what airflow? Would you need one for each bathroom? A high flow ERV is unnecessary unless showering, a steamy shower is likely to test an ERV's exhaust capacity for transferring moisture (and at this point moisture is more of a waste product I suppose), and ERV cores seem very expensive for this limited capability.
I've been spec'ing ERVs (mainly Zehnder, sometimes Renewaire) for whole-house ventilation going on 8 years now, in probably over a hundred jobs. Rarely any issues when they're properly installed and commissioned, and these are doing only 25-30 cfm (in bathrooms) in most cases. If you're trying to vent just one bathroom there are small ones available (Lunos, Panasonic) but looking only at cost-benefit/ROI in that limited scale they may not seem so good, but as a whole-house air quality system I don't think they can be beat.
Your house would have to be very small and well sealed to overpower a bath fan. A 1000sqft 0.6ACH@50PA house would restrict the flow of a 75CFM bath fan down to around 60cfm. You do have to oversize a bit and make sure you don't have any combustion appliances that will have issues with the pressure (or MUA).
The whole house ERV works reasonably well for shower but need a bit more flow to clear a bathroom. If you do go with an ERV, I would aim for being able to do around 50cfm on boost from the bathroom or add in a dedicated bath fan to help.
Since the ERV is also taking stale air from the rest of the house, it is only recycling a small percentage of the moisture from the bathroom it is definitely not a problem in cold climates. I've checked on my ERV, when the shower is running the fresh air supply RH doesn't change (or outside of the sensitivity of my budget RH sensor) in cold weather.
What’s the math on this? Im getting about half the fans CFM, short runs at just below .6 ACH50 but Ive got a lot more square footage and volume than your example.
I was refering to the fan curve of the AE80 from:
http://www.broan.ca/common/productDigitalAssethandler.ashx?id=d98ed4f5-1f6b-4387-8170-27026258c2e5
1000sqft 0.6ach house is 90cfm at 50PA (0.2"). So at 75cfm that around 0.16". Assuming you have 4" exhaust with 20' eq length, so that adds around 0.10" at 75cfm. In case of that fan, it should be right around 60cfm.
Something with a beefier fan like a Panasonic whisper green, would still deliver full rated flow.
Maybe the manufacturers rate their products a bit too optimistic or don't include the restriction from the built in damper and inlet grill.
Maybe, I’ve got a 5’ run of smooth to a broan spring damper off a Panasonic 80 and my flowhood measured 39 cfm.
Doesn't seem right but if my blower door frame is setup I register the fans on my manometer.
The non-ecm blower Panasonic units don't produce much pressure, but even then with a large house, it should not be that restrictive.
Maybe you have a birds nest in your wall cap. I had to clean one out a while ago from mine.
Maybe my flow hood is off, all my fans register less than rated about half. And no birds nests. But they are starving for air.