Green Building Forum - MVHR and air tightness

MVHR and air tightness

tony — Thu, 01 Sep 2016 09:46:22 +0100

There is a popular notion that MVHR is not worthwhile in leaky houses.

1) we should not have leaky homes.
2) we should not throw heat away through fans or through draughts.
3) all new homes should no be built air sealed and with MVHR

On a calm day there is little difference between an air tight home and a leaky one

The benefits of MVHR are more than just the economic ones, air quality improves, heating bills will be lower in all cases, any condensation problems go away.

It has been claimed that MVHR in a leaky home can push humidity into the fabric, this is no more the case that the amounts the home can do for itself without MVHR.

MVHR and air tightness

gravelld — Fri, 02 Sep 2016 11:49:57 +0100

You're forgetting the stack effect though, which causes heat loss on even a still day.

MVHR and air tightness

ringi — Fri, 02 Sep 2016 12:18:51 +0100

We have a leaky large 1930 bungalow, our MVHR transformed the air quality and removed all of the mold we were getting due to condensation.

MVHR and air tightness

tony — Fri, 02 Sep 2016 13:50:11 +0100

Happy to hear that positive story, thanks.

MVHR and air tightness

Sigaldry — Fri, 02 Sep 2016 16:04:40 +0100

I heartily endorse all three points Tony.

The issue with MVHR and 'leaky' houses tends to come from the fan power required to run the system compared to the savings that come about from heat recovery, which are reduced when much of the heat is going out through uncontrolled ventilation.

From various SAP analysis I've done in the past, the savings are largely lost from a typical MVHR when the air tightness gets much poorer than around 6 to 7 mÂ³/mÂ²/hr @ 50 Pa. That's not to say however that's always the case and as you say, there's a lot more to it than cost savings for winter heating.

Best of all is when you have the combination of well insulated, attention to detail at junctions, low air tightness, with an appropriately designed and installed MVHR unit having a low fan power and good level of heat recovery.

There are definite benefits to having adequate ventilation in any building - most older ones (and some newer ones) don't. In fact I'd go as far as to say that a good level of ventilation (extract and input) is essential for any building to perform well.

MVHR and air tightness

ringi — Fri, 02 Sep 2016 16:28:00 +0100

In our case, the MVHR preheating the incoming air (â€œfor freeâ€) means we have less cold drafts than we would have had with another system, hence may allow us to run the heating less.

However PIV is also a good option, and if it was hard to install ducts I would be very happy to use a PIV system.

MVHR and air tightness

fostertom — Sat, 03 Sep 2016 18:07:01 +0100

Posted By: tonyOn a calm day there is little difference between an air tight home and a leaky one

Not so sure - according to Fraunhofer Inst, the main driver of air change is stack effect, not wind. So if the interior's warmer than outside, the warm air rises and squirts out of any gaps near top, draws in cold thro gaps near bottom. A 2-storey house makes a taller stack than a bungalow, hence higher exchange rate.

MVHR and air tightness

tony — Sat, 03 Sep 2016 20:04:21 +0100

Yes but that effect is nothing like the horrid things that wind can do in terms of stealing away heat.

MVHR and air tightness

fostertom — Sun, 04 Sep 2016 22:48:21 +0100

So we'd think, but their research apparently justifies treating wind-caused loss as insignificant. Maybe because strong wind is relatively rare, while stack-effect keeps going 24/7. Anyway, best, when explaining leakage loss, to avoid mentioning wind, stick to stack effect (greater in 2 storey than bungalow - that makes an impression!)

MVHR and air tightness

gravelld — Sun, 04 Sep 2016 22:55:31 +0100

If you want to make an impression, ask your client to hold their hand over a down lighter in a cold loft.

MVHR and air tightness

PeterStarck — Mon, 05 Sep 2016 08:19:04 +0100

Posted By: gravelldIf you want to make an impression, ask your client to hold their hand over a down lighter in a cold loft.

+1

I installed some LED pivoting downlighters in my existing bungalow as an experiment for future LED lighting in my self build. The draught through them is surprisingly strong even when it's not that windy!

MVHR and air tightness

Gotanewlife — Mon, 05 Sep 2016 09:49:56 +0100

Yes all very good but, with respect, irrelevant to the conversation. MVHR provides ADDITIONAL ventilation when (ie always) and where it is needed (in a leaky house). Careful placement can provide airflow across problem areas like windows and warm those top outside corners that are prone to mould. It removes humidity from those small volume UK bedrooms with 2 people sleeping in them far more effectively than air leakage (be that wind driven or stack effect) and PIV cannot do this.......which is why there is now MIV! That's before we start on kitchens and bathrooms where non-MVHR ventilation strategies require additional room specific ventilation fans with run on or ideally humidity control.

Any mention of saving energy/saving money is also irrelevant (or so far down the list as to be unimportant) to the conversation. Any ventilation is better than none (in a leaky mould prone house) so it should be about comparing the available solutions and in my view PIV is always second best to MVHR but when ducting is impossible (too ugly or too expensive) then PIV, with additional bathroom and kitchen fans, is next on the list.

MVHR and air tightness

djh — Mon, 05 Sep 2016 11:32:31 +0100

What is MIV? http://acronyms.thefreedictionary.com/MIV

Or did you mean MEV?

MVHR and air tightness

gravelld — Mon, 05 Sep 2016 15:19:26 +0100

PIV. Positive Input Ventilation. Often from the loft although not always.

MVHR and air tightness

djh — Mon, 05 Sep 2016 15:42:51 +0100

Posted By: gravelldPIV. Positive Input Ventilation. Often from the loft although not always.

I know what PIV is and I expect gotanewlife does too. The question is what he meant by MIV.

MVHR and air tightness

barney — Mon, 05 Sep 2016 16:39:12 +0100

Multiple Input Ventilation I suspect

Barney

MVHR and air tightness

Gotanewlife — Mon, 05 Sep 2016 19:18:18 +0100

Posted By: barneyMultiple Input Ventilation I suspect

Correct, ducted PIV

From PIV to MIVÂ® - Positive Input Ventilation has Evolved:

http://www.envirovent.com/blog/from-piv-to-miv-positive-input-ventilation-has-evolved/

So to me the only advantage PIV has over MVHR is that it doesn't need ducting, so I really don't see MIV catching on.....

I like this bit though "multi zone destratification where the warm air at ceiling level is redistributed evenly around the property", sounds to me like a vent in the ceiling. Sarcasm aside it is worth remembering that whilst dead cheap generic vents are widely available, vents specifically designed for the extract and for the input ducts are also avaialble.

MVHR and air tightness

gravelld — Mon, 05 Sep 2016 20:54:05 +0100

Posted By: djh
I know what PIV is and I expect gotanewlife does too. The question is what he meant by MIV.

Sorry, I missed the 'M'.

MVHR and air tightness

Gotanewlife — Tue, 06 Sep 2016 01:17:47 +0100

Actually my fault for not spelling it out, MIV is not a normally used abbrv, has never been mentioned on here before and, indeed, appears to be trademarked! Some people guessed though!