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Power vented crawl

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New 3-story, 2080sf townhouse in Seattle City limits. End unit of a block of 4. Garage is detached.

No exterior crawl vents and at first I thought I had a nice slab. But as soon as everyone arrived and I stepped inside, it was obvious that we had a full crawl. After moving a bunch of 5 gallon paint cans and a large helium cylinder out of a closet I found the hatch. The crawl is for this unit only, standard stem wall between units. Complete, but ordinary moisture barrier and the soil appeared to be bone dry.

At one front corner of the crawl there is a continuously operating fan (see photo). This is venting to the exterior in a niche by the front door. At the rear end of the crawl there is a dryer vent type damper (see photo) connected to a short length of duct that in turn is connected to a forced-air type register mounted vertically in the wall just inside the back door.

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So, besides the annoying noise of the fan clearly audible in the living room, I have a system I've never seen before that seems designed to continuously suck conditioned (heated) air out of the house, which seems a little inefficient. BTW, the home has unitary electric wall heaters only. There is good air flow out of the front vent but after I shut the hatch I checked the wall register with a sheet of paper and, nada, no suction at all. I next get back in the crawl and have the clients close the hatch behind me. The flaps on the intake damper don't move or open at all. So, air is getting sucked into the crawl from elsewhere (or everywhere?).

I have checked the Seattle residential code and, evidently, this set-up is allowed.

Relevant bits only.

SRC R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in sections R408.1 and R408.2 shall not be required where:

• 2. Continuously operated mechanical exhaust ventilation is provided at a rate equal to 1 CFM for each 50 square feet of crawl space floor area. Exhaust ventilation shall terminate to the exterior.

It does not say where the air is supposed to come from!

In conclusion: It seems to me that the system was designed to move air from one end of the house interior through the length of the crawl. What is actually happening is the air is seeping in from elsewhere and I would suspect there are some very dead areas. It also doesn't seem like a great idea to forcibly draw damp NW air into a crawl on a continuous basis. The client is rightfully upset about the constant noise (which appears to be a harmonic rather than the fan itself) but I think the whole set-up is crap, even if it is allowed.


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Hi Richard,

Saw that system about 8 weeks ago at a development of townhomes in Issaquah - D.R. Horton, I think. For some reason, that firm seems to favor these things. Sometimes they work, other times I've found lots of nasty stuff growing. On that last one, the bearings were worn out from running 24/7 and the fan was mounted directly to the floor joists without any kind of attentuation. It was annoying as hell. I recommended they replace the fan and mount it to the foundation wall instead of the floor joists. No damage or funk, just weird.



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Oh...and one final thing. The "filter" for the fan is a piece of mesh taped to the intake duct. It is already being restricted by dust, etc. They did install a light in this area but it is a tough crawl almost the length of the home to get to it.

Aside from the obligatory granite and stainless in the kitchen, this home reeked of minimum standards. For example, the 9 foot long kitchen island (no sink) only had a receptacle at one end. The master bathroom was huge but was mainly empty floor space with only a small fiberglass shower stall stuck like an afterthought into one corner. And, I'm sure, it was cheaper to use a fan rather than install a bunch of perimeter crawl vents.

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I see that setup occasionally. Better if the fan is on a humidistat near the hatch, so it will turn itself off. The last one I saw, the laminate floor was buckling and humping up from high humidity.

We sometimes have an electric baseboard heater installed near the middle of the crawl, no vents, no fans. That seems to work better in our climate. When I see the crawlspace vented to the living space, I suggest people try covering them up and leaving the heat on low in the crawlspace.

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A lot of homes have them in the CS. The majority I've seen are tied to a humidistat or a timer as some codes call for a run time, some call for humidistat control.

If the fan is installed correctly is should have little noise - the last one I installed we just hung from the joists and isolated it the best we could - it has little noise above the floor.

It all depends where you are and seems to work fine. I don't how one could blame a fan for buckled floors though......

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