Hi Chris- Can't argue with you there, the 240v one does seem surprisingly higher m^3/hr in free air given the similar power rating. As to which is best with around 50Pa pressure drop across it, I couldn't say. If you do get the RS one, be prepared to end up taking the finger guard off to fit a cowling closer to the fan blades.

I made a 400mm diameter 400mm length, tube out of stiff material to help make the airflow uniform before measuring airspeed. This was placed inside the house around the fan (which was blowing to outside, depressurising the house), then I found airflow was easy to measure, and it was easy to calculate the flowrate.

Tim - Mrs RobL shuddered when I showed her three fans in your door - it brought back memories she'd rather forget!

I think I was just searching for EBM Pabst fans (maybe EC) - I was looking for one for work (a computer room free-air cooling project). I did get very lucky with these (job lot of 4, for £20 - I was sole bidder), as RS has a similar model for > £300 each. Have only seen one similar come up since (and that in the US). They are W3G400-CA22-71:

http://www.ebmpapst.co.uk/assets/PDF/Catalogue%20Data/epM_Axial_S_EC084.pdf

At full power, they do 5500 m^3 per hour in free air, 350 Pa at zero-flow. They have their own internal speed controller, and a single 0-10k ohm pot controls all three at the same time.

Once the bay window roof is blocked up on Monday, I'll probably only need one of them.

It's probably best to look for a fan which has a high static pressure, rather than high m^3 per hour (unless you suspect you have hundreds of tiny leaks I suppose), as when you block holes up, the pressure (at full power) will rise, allowing you to find smaller ones more easily.

BTW, have ordered one of the Wizard Stick (as sold by EBS etc. for the best part of a ton) hand-held smoke machines on eBay from the US. Will give a review when I get it:

http://www.ebay.co.uk/itm/350700114724

http://www.ebay.co.uk/itm/350700359312

circa £25 inc shipping for the toy-shaped version.

The other end of the tubing was attached to a Magnehelic gauge which had a range of 0-60pa (bought from ebay for 25 quid).

The MVHR was then turned on. As the supply duct isn't connected it sucks air from the house and expels it out of the exhaust duct causing the property to depressurise. I then adjusted the fan speed of the MVHR to get a 50 pascal pressure drop.

Have only had it up and running for a few hours today but have already found and fixed a few major air leaks. I currently have the MVHR fan at 50% which is approximately 150m3/hour air flow according to the manual. I think the voulume of my property is about 375m3 so I might have an air tightness of about 0.4ACH.

Plan to use this to find and fix any major leaks before getting an official air test of the property.

Posted By: tonyI am afraid that the ability of your fans to shift air greatly diminishes as the pressure difference increases.

Isn't that good news? If the fan's shifting less air than Adi thinks then his airtightness is better than he thinks. Or are you suggesting that the quoted flow rate for the fan is at a higher pressure difference?

The neat thing about using the MVHR is it's using an orifice which would otherwise be sealed up during testing. A normal blower-door test won't test for leakage on the door it's replacing.

Consider a limiting case then

Thought experiment: put the MHRV in a polythene house with only one outlet, the MHRV outlet, run the fan, it will find an equilibrium position where it is shifting no air due to the pressure difference that it has achieved.

Posted By: tonyThought experiment: put the MHRV in a polythene house with only one outlet, the MHRV outlet, run the fan, it will find an equilibrium position where it is shifting no air due to the pressure difference that it has achieved.

Indeed, but looking at his curve 50 Pa is nothing like the pressure difference you'd get in that case. At 100% it seems to be capable of holding very nearly 150 Pa without reduction of airflow. I.e., below 150 Pa it's something other than the pressure difference which limits the airflow, just the geometry of the blades and the volume they “bite” in each rotation I imagine - in effect, the flow rate at which the angle of attack of the blades approaches zero, I think.

Anyway, the point that any back-pressure will tend to reduce the airflow which will indicate better airtightness still stands.

Posted By: adiIf I was to take a pressure reading at 25%, 50% and 75% fan setting I should get a pressure reading of 25Pa, 50pa and 75pa if the relationship is linear.

We've discussed this before (GaryB or djh can probably say something authoritative) but my recollection is that the flow rate vs pressure is not linear. Be interesting if you could do some measurements. Any way you can measure the actual fan speed (rpm)?

From Wikipedia! Building leakage is described by a power law equation of flow through an orifice. The orifice flow equation is typically expressed as

Q = C * ∆P^n

Q =Airflow (m3/sec)
C = Air Leakage Coefficient
∆P = Pressure Differential (Pa)
n = Pressure Exponent

The parameter reflects the size of the orifice, the ∆P is the pressure differential across the orifice, and the n parameter represents the characteristic shape of the orifice, with values ranging from 0.5 to 1, representing a perfect orifice and a very long, thin crack, respectively.

Fan setting m3/hr ∆P
20% 60m3/hr 19
30% 90m3/hr 30
40% 120m3/hr 40
50% 150m3/hr 50
60% 180m3/hr 59

solving the equation looks like n=1 (makes sense as I would expect leaks to be via very long thin cracks) and C=3 in my building.

So the only unknown is if a fan setting of 50% is actually 50% flow e.g. 150m3/hr? The MVHR is a German engineered top of the range unit so it probably does. I know the MVHR control board does monitor fan RPM when controlling the motor. So knowing what German engineers are like they have probably engineered the unit so 50% setting is 50% maximum flow rate so long as you don't have an excessive pressure restriction.

But 50 pa isn't that much of a restriction for most fans! 50pa = 0.007psi = 0.5cm water.

Good spot Ed, Typo and me doing the maths using the %flow instead of m3/hr value. Have edited post.

I plan to get a proper blower door test done next year once the property renovation is complete. It will be interesting for me to see how accurate my estimate is and I will try to remember to update the post with the results.

I've bought one of these fans:
http://www.ebay.co.uk/itm/Stoney-Racing-16-120W-Electric-Radiator-Fan-Universal-12v-Slimline-/181428101106

Where do I get a 12V 10A power supply, preferably variable?

How long does it typically take to build up enough pressure and do you just walk around the house blowing smoke into vunerable areas?

I can feel where leaks are with my hands and hear it whistling if I crack the window but the smoke machine seems to be waste of time. I can't see it moving. Do I need to full up house and look for it outside?

Thanks Tony. Ended up filling the entire house with the fog machine and had people outside spotting. Spotted one leak through a vent tile; vent hadn't been sealed properly. And a leak through our sliding folding door flush threshold, which we were expecting to some extent; it's inherently vunerable and needs a new seal. Other than that, no other leaks. Smoke came rushing out anywhere I took off the temporary seal.

Might try another test one in next few days once I have some tube for a manometer.

This thread is really interesting, I like the idea of people being able to rig up their own kit for these tests.

Car mechanics use an additive and a smoke machine with a UV lamp that can detects very small air leaks.

I bought a UV lamp recently very cheaply which works great.

Could something be added into the smoke machine liquid could make it glow under UV, something like washing detergent maybe?

Posted By: tonyWe depressurise then look inside for air coming in!

Genius.