Simplest way to keep house at slight positive air pressure (or to at least prevent it from going negative)?
I’ve got a leaky house from the 1980s. I live in the Pacific Northwest near the ocean where the climate is fairly moderate. I’d like to keep the house at a positive air pressure (or at least neutral) and to do that with clean filtered air; at least whenever I am running the wood stove or kitchen/bathroom exhaust fans. I’m less concerned about energy efficiency (there are other projects I am taking on that will have a larger impact on energy consumption.) Air flows quite freely throughout the entire house (very open concept and gaps under the few doors.)
The house is sometimes rented to tenants so I need the solution to be something that doesn’t require any action by the occupants. “Open a window” is a common suggestion but tenants won’t reliably do that.
I’m about to undertake a major project to replace an electric air furnace with a heat pump and redo a lot of ducting so this would be a convenient time to do anything that will require new wiring or ducting.
Some ideas:
– I have seen suggestions to add a dehumidifier that takes outside air and use that to pressurize the house. However, I don’t feel like I need a dehumidifier and the one that I looked at needs inlet temperature to remain above 49 f so I’d have to generously mix outside air with air from the house during the winter (which is when I need the positive air pressure the most due to the wood stove.) If I do a 50/50 mix, then I would be down to 35 CFM and also spending a bunch of money each month dehumidifying air. This is the example unit: https://www.santa-fe-products.com/product/santa-fe-ultra70-dehumidifier/
– It sounds like some ERVs can be configured to provide slight positive pressure. That is an interesting idea but it does seem like I would need to move a lot of air in and out of the building just to achieve that slight positive pressure.
– I’m wondering if there is a way to wire up a pressure sensor that would open a damper whenever there is negative pressure. That damper could then open a duct to the outside (and there would be a filter in the duct). That wouldn’t help me achieve positive pressure but would eliminate or reduce the amount of negative pressure when we are running the bathroom fan, kitchen fan, dryer, and wood stove. Is there anything like this “off the shelf” or would I need to cobble it together. Has anyone seen this done in a house?
Any other thoughts or ideas about how to best go about this?
My ideal situation is some kind of a system that is reasonably affordable and will basically guarantee that the house never goes into negative pressure.
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Replies
I'm still waiting for someone to explain to me what's so bad about negative pressure.
But while I wait, the simplest way to achieve this is to have part of the return air for the HVAC come from outside the house. This is not uncommon in commercial HVAC.
Two problems with negative air pressure:
1) It reduces the effectiveness of the exhaust fans. I want to eliminate moisture from my bathroom more quickly after a hot shower (I'm having issues with peeling paint, etc.) One option is to put a more powerful fan in. The other option is to make the fan work less hard by providing make-up air. Same with the wood stove. If there is negative pressure in the house, the stove struggles to burn as effectively and in a worst-case scenario you can even pull air down the chimney and into your house (risk of CO poisoning.) These issues are all much more serious in houses with gas appliances.
2) Air is going to come in either way. It can take two paths: A) It can get in through leaks in my ceiling after filtering through my attic (which is gross, dusty, and was infested with rats at one point) or I could bring it in directly from outside and through a HEPA filter to eliminate allergens.
Re the return air duct option. I am considering that as well. I think the issue is that commercial HVAC is more likely to have long run times. I also think they may put that on a damper (so it would be closed if the system is not running)?
In my scenario, I need fresh air the most when the heat pump is not running (when we are using the stove.) So if the intake was on a damper then we wouldn't get fresh air when we needed it the most. If it wasn't on a damper then it would be like leaving a window open all the time.
Perhaps the best option is a pressure controlled damper?
In addition to Canada Deck's points, I have seen negative pressure suck stormwater in through window gaps, due more to external forces than interior mechanicals, but that experience made me wonder how much other water is getting drawn into assemblies. Negative pressure also encourages radon to enter the home.
Broan makes a pressure sensing damper
I haven't been able to find one yet. There is this one but it's not actually measuring house pressure. You put the pressure sensor in the range hood exhaust and then it triggers the damper when there is positive pressure of 0.05” Ps in the range hood exhaust vent. It also says you can only use it with compatible Broan range hood fans. https://broan-nutone.com/en-ca/accessory/md8tu Have you seen any others?
Sorry it is not broan, https://www.hvacquick.com/products/residential/Makeup-Air/Residential-Makeup-Air-Fans/iO-Controls-MUA-Make-Up-Air-Damper-Kits?gad_source=1&gclid=Cj0KCQjw0MexBhD3ARIsAEI3WHKDCUUjv8WEIPcQ_WICmfw-7ecOUwUagv_hL0PqmVeo9L4Bx5Ds30IaAmREEALw_wcB
Thanks. This is very interesting.