Hi again, peeps.

So I know that the accepted way to change air is to take the a stale air from the wet rooms and push the fresh air to the living and bedrooms by having two vents, one for incoming fresh air and one for extraction. Fresh in, stale out.

Ok, in my extended bungalow I have 3 rooms from the original cottage that have timber suspended floors. I've ripped up the suspended floors, and replaced joists etc, and insulated everything from below. All these room have wall vents to allow crossflow and I've added holes to allow more air to flow through ducts up in to the loft. I've not fitted the ducting yet though.

Here's my question:

How bad of an idea is it to use the air from the underfloor area for the fresh air supply?

Or, alternatively, is it a bad idea to instead use the area to extract the stale air in to?

(I'm only thinking about doing this because I'm looking for ways to not have drill through 3 foot thick stone walls, or 1 foot thick concrete walls. Any other ideas on that would also be appreciated)

Cheers.

You have posted in MVHR but assume you are not talking MVHR.

To it from under floor are sounds like a poor idea. Why through the roof?

What ventilation system are you discussing, dMEV, MEV, intermittent fans?

7 hours ago, JohnMo said:

You have posted in MVHR

Hi

Yep I'm talking about MVHR.

The idea behind using the under floor (or roof) is so I don't have to core drill through 3 feet of stone wall for the incoming vent, and 1 foot of concrete wall for the extraction vent.

Cheers.

8 hours ago, Dazz71 said:

How bad of an idea is it to use the air from the underfloor area for the fresh air supply?

Very bad if the air contains Radon. Otherwise just really bad - air may well be humid with the potential for other contamination depending on location - road fumes, for example.

8 hours ago, Dazz71 said:

Or, alternatively, is it a bad idea to instead use the area to extract the stale air in to?

Really because it will raise humidity levels below the floor in general and, in winter, increase condensation in the void.

While the floor may have ventilators, the air will often be pretty static, so if you're extracting and exhausing into the same void, that's going to lead to a build-up of moisture and cross-contamination of the supply with the exhaust air.

The whole idea is no doubt against Builing Regs too.

8 hours ago, Dazz71 said:

I'm only thinking about doing this because I'm looking for ways to not have drill through 3 foot thick stone walls, or 1 foot thick concrete walls. Any other ideas on that would also be appreciated

Through the roof, if you have one.

If not, the only sensible option is to drill the stone or concrete. Mine were core-drilled through 450mm of stone & brick within around 25mm of the targeted break-through points, with the crew suspended on abseiling ropes.

Edited 2 hours ago2 hr by Mike

9 minutes ago, Mike said:

Radon.

Hi, and thanks for the reply.

There is no radon. Or at least that's what the survey and radon map showed.

There's also no road fumes etc due to us living on farm land.

And I was only thinking of using the void for either fresh air or exhaust. Not both.

However, with all that said, by the sounds of it then it's not a good idea either way even if everything is perfect under the floor. I'll just use the duct holes I left in the floor to vent up in to the loft and out from there.

Looks like I'm going to have to do some drilling.

Roof:

Would it be better to draw fresh air through a roof vent then extract through the wall?

Or draw fresh from the wall and vent out the roof?

Cheers again.

Do both through the roof, do both on the same roof plain with suitable seperation. No need to go through wall.

+1 to both through the roof - that's clearly going to be the easiest. They should both go through the same surface so that both intake and exhaust are subject to roughly the same wind pressures, which then keeps the airflow through the machine more-or-less in balance at all times.

Intake & extract are generally recommended at least 1.5m apart from each other, but most important is that the intake must be well away from chimneys and vent pipes.

Thanks both of you for the info and advice.