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Old 10-23-2010, 03:09 PM   #1
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Running HRV F/A supply to crawl space?


30 year old 2000 sq ft. rancher in PNW :

Looking to close off our crawl space vents to outside and make the area more conditioned. The 3' CS is all concrete, including floors (vapour barrier under). Insulation in subfloor joists.

I will need some sort of air circulation to replace the open vents and was thinking of tapping into the HRV supply trunk and running a small vent to the CS, and also putting in a small exhaust fan at one of the existing and opposing CS vents.

The HRV unit is situated in the garage and the insulated mechanical ducts are run throughout the long attic. All living space 4" vents (fresh air & exhaust) are in the ceiling.

What is your opinion on how I can best do this without screwing up the HRV application, and perhaps potentially contaminating the living space above?
Also, what minimum size of duct would be needed to move some of that supply air into the CS? The HRV is currently balanced.

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Old 10-23-2010, 03:57 PM   #2
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Running HRV F/A supply to crawl space?


You don't want to intake fresh air or exhaust any air from the crawlspace to the outside.

A small amount of air to and from the HRV is all that is needed.

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Old 10-23-2010, 04:45 PM   #3
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Running HRV F/A supply to crawl space?


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Originally Posted by beenthere View Post
You don't want to intake fresh air or exhaust any air from the crawlspace to the outside.

A small amount of air to and from the HRV is all that is needed.


So are you suggesting I should run both ducts, HRV fresh supply and HRV stale air exhaust ..perhaps into opposite ends of the CS space ?

If there is stale exhaust air returning to the HRV from the CS won't there be a risk of contamination of main living air quality during the exchange, as well as temperature inefficiencies in the main living area itself?

How would I maintain overall HRV balance when this one complete ventilation process is covering such a large CS area (2000 sq. ft.) ?

Last edited by Daler; 10-23-2010 at 04:54 PM.
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Old 10-23-2010, 05:08 PM   #4
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Running HRV F/A supply to crawl space?


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Originally Posted by Daler View Post
So are you suggesting I should run both ducts, HRV fresh supply and HRV stale air exhaust ..perhaps into opposite ends of the CS space ?

Yep, opposite ends.

If there is stale exhaust air returning to the HRV from the CS won't there be a risk of contamination of main living air quality during the exchange, as well as temperature inefficiencies in the main living area itself?

The crawlspace will become part of the conditioned space. So it won't be able to contaminate the homes air.

How would I maintain overall HRV balance when this one complete ventilation process is covering such a large CS area (2000 sq. ft.) ?
Your HRV will need to run a little longer. You need to add a damper to the return and supply vent in the crawlspace. It will need about the same CFM per volume as the rest of the house.
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Old 10-23-2010, 05:38 PM   #5
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Running HRV F/A supply to crawl space?


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Your HRV will need to run a little longer. You need to add a damper to the return and supply vent in the crawlspace. It will need about the same CFM per volume as the rest of the house.
Thanks -- Does that mean it will be a huge volume -- about 1/2 that of the upstairs (same sq footage but 1/2 height)?
How do we determine the proper CFM (without getting professionals involved with all their gear)?


"The crawlspace will become part of the conditioned space. So it won't be able to contaminate the homes air."

..does that mean it also have to become the same temperature as the main living area too?

As I don't trust any crawl space especially ours (potential mouldspores, etc) how can I then protect it via any HRV application.

Essentially what I want is some sort of air exchange in the CS without opening it to the outdoors again. I figured our HRV could supplement that.
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Old 10-23-2010, 06:35 PM   #6
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Running HRV F/A supply to crawl space?


In the winter.With the HRV running. The crawlspace won't be damp. And mold won't be a problem.

It does not have to be the same temp. It will warm up, because you don't have that fresh cold air coming in any more.

The crawlspace will probably only need 50 CFM max.

Without gear, hard to balance, or adjust. other then trail and error.
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Old 10-23-2010, 06:48 PM   #7
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Running HRV F/A supply to crawl space?


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Originally Posted by beenthere View Post
Your HRV will need to run a little longer. You need to add a damper to the return and supply vent in the crawlspace. It will need about the same CFM per volume as the rest of the house.
Quote:
Originally Posted by beenthere View Post
In the winter.With the HRV running. The crawlspace won't be damp. And mold won't be a problem.

Well, we do get our rainy seasons

It does not have to be the same temp. It will warm up, because you don't have that fresh cold air coming in any more.

The crawlspace will probably only need 50 CFM max.

Without gear, hard to balance, or adjust. other then trail and error.
I hear ya. I've been using a large garden leaf bag and stopwatch guage a fill on the outside exhaust, then placing it over the outside intake and timing it to deflate. I adjust a damper inside. Without the proper meter, that seems to approximate it.


However, I'm still concerned about the affect only 2 vents will have on the HRV performance/balancing when venting such a large room volume as the CS. In the rest of the house there is either type of vent located in every room !
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Old 10-23-2010, 06:55 PM   #8
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Running HRV F/A supply to crawl space?


Crawlspace doesn't need a lot of air. Your not down there breathing out CO2.

If you want, you can install a small portable dehumidifier down there, and set it to run if the humidity gets to 60%.

Do you run your HRV continuous, or is it in a timer.
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Old 10-23-2010, 07:07 PM   #9
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Running HRV F/A supply to crawl space?


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Originally Posted by beenthere View Post
Crawlspace doesn't need a lot of air. Your not down there breathing out CO2.

If you want, you can install a small portable dehumidifier down there, and set it to run if the humidity gets to 60%.

Do you run your HRV continuous, or is it in a timer.

We usually run it continuously during the fall and winter, on "low". Call-for-air (high speed) is done manually through some nifty digital on-demand controlled powered vents that are at strategic locations (bathrooms). No humidistats were installed at the time of install.

hmm.. so what then to do about summer, when the vents are closed and the HRV isn't running?

As for a dehumidifier ..did that over last week to establish a baseline as there was a lot of pooled hi-humid air trapped in there -- it seems to have stabilized from 85% down to 65% now. Otherwise, using a dehumidifier on a regular basis will be expensive. So, if I can make use of the mechanical equipment we already have that will be a plus..
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Old 10-23-2010, 07:12 PM   #10
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Running HRV F/A supply to crawl space?


When the HRV isn't running in the summer. The air from the upper levels will still travel through the duct to the crawlspace. natural convection if it isn't connected tot eh central system.

Only use the humidifier to keep the humidity under 60%.
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Old 10-23-2010, 07:31 PM   #11
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Running HRV F/A supply to crawl space?


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When the HRV isn't running in the summer. The air from the upper levels will still travel through the duct to the crawlspace. natural convection if it isn't connected tot eh central system.

hmm.. won't that process then contaminate the living area upstairs? -- maybe I should just leave the HRV running even during the summer..

Only use the humidifier to keep the humidity under 60%.
Ok. I should probably finish my project of reinsulation of the joists before I settle on the RH figure. Currently, the floor is wide open (no insulation under it -- replacing with higher R rating). So the convected heat transfer from the upstairs is now probably affecting the RH in the CS, huh? -- CS is warmer and RH is dropping a tad.

Hopefully when the insulation is done, the vents remain closed, and a couple HRV vents are routed to the CS I will get a better idea of the true RH down there..
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Old 10-24-2010, 03:23 AM   #12
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Running HRV F/A supply to crawl space?


As the crawlspace warms up, its RH will drop. After you insulate the joist, it will probably go up. Until the HRV vents are installed, and it runs a couple times.

The HRV should be ran by a timer in the summer. And only allowed top run for short periods of time. Other wise. If it runs too long, it will bring in too much moisture into the house and crawlspace.

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