Green Building Forum - Does PIV work?

Does PIV work?

delprado — Mon, 06 Mar 2017 17:00:37 +0000

This, presumably reliable, report suggests it does not. The idea of solar gain in a roof space in winter is clearly wrong - when properly insulated at ceiling level the roof space should be colder than ever. Leaking moisture into loft, which is the reports mention, is also clearly avoidable.

http://projects.bre.co.uk/positivevent/

Does PIV work?

tony — Mon, 06 Mar 2017 18:12:22 +0000

Yes

BUT think about it, blowing cold air into a home results in warm air being pushed out,

The occupant pays the bills for the fans to run and for the additional heat losses too.

Does PIV work?

ringi — Mon, 06 Mar 2017 20:20:04 +0000

But the occupant does not have to leave windows open for hours, and a damp free house needs less heating. So yes PIV does work and is cheap to install.

PIV works like magic if you have a tenant with lots of condensation due to their life style, these tenant are mostly not willing to pay Â£50 pcm more rent to get MVHR.

Does PIV work?

djh — Mon, 06 Mar 2017 22:18:47 +0000

"The test house was very airtight, with an air leakage rate of 4.5 ach @ 50 Pa having been measured by a fan pressurisation test (Reference 1)."

So they think that ten times as leaky as my house is "very airtight". Doesn't say much for their standards or expectations really.

"In both the test and occupied houses, the roof space was consistently more humid than outside (excess vapour pressure of about 0.1 kPa), implying that moisture was being transmitted to the roof space from the rooms below."

Seems to imply that the test house did not have an effective VCL.

To be fair, this is a rather old study, from 2000, but this report

http://www.building.co.uk/positive-input-ventilation/4598.article

exposes a bit of the BRE bias - it says "relatively airtight", which is a more reasonable assessment sadly.

Does PIV work?

ringi — Mon, 06 Mar 2017 22:36:16 +0000

One of the problem is that PIV systems reduce real life condensation a lot more then any logic says they should given how little they reduce measured average RH.......

No one with half a brain would consider putting a PIV system in a "very airtight" home, they tend to be fitted into homes that are well below park L standards. The PIV makers do say you should seal all down-lighters and any holes in the ceiling so that steam from the bathroom does not go into the loft - but I don't know how often this is done. I was told to just turn the lights on in all the rooms, and check at night if I could see any light coming into the loft!

Does PIV work?

delprado — Fri, 31 Mar 2017 13:40:19 +0100

Can anyone comment on how they work in the summer? We have a slate roof and so the loft gets ludicrously hot. I can't have that heat being pumped into the home...

Does PIV work?

tony — Fri, 31 Mar 2017 13:54:59 +0100

Not a good ides, better to reverse the fan

Does PIV work?

delprado — Fri, 31 Mar 2017 14:05:24 +0100

What do I do Tony? I am at a loss as to what to do in my retrofit project. I can't have ducting everywhere.

Does PIV work?

gravelld — Fri, 31 Mar 2017 15:10:23 +0100

It's one of the big conundrums of retrofit.

If you want heat exchanging you have to either duct and box in, or use a ductless system, as discussed in other threads.

Also:

- PVHR
- PIV
- MEV (with optional exhaust air heat pump)

Does PIV work?

gyrogear — Fri, 31 Mar 2017 15:20:13 +0100

Posted By: delpradoCan anyone comment on how they work in the summer? We have a slate roof and so the loft gets ludicrously hot. I can't have that heat being pumped into the home...

Surely, in summer the house would be open anyhow (doors and/or windows), so would you be needing the input from a PIV in any case ?

(I throw water onto the deck and use evaporative cooling to get a cooler airflow up my stairs from the basement etc.Â°).

And at night, you ought to be drawing cool air into the bottom of the building anyhow, and letting it out at the top ?

I'd respectfully suggest that a period of observation is required (say one year, to see how the house works...) before diving in to a particular technical route...

gg

Does PIV work?

ringi — Fri, 31 Mar 2017 21:02:01 +0100

Posted By: delpradoCan anyone comment on how they work in the summer? We have a slate roof and so the loft gets ludicrously hot. I can't have that heat being pumped into the home...

Most of the units have a thermostat that just turns them off if the loft is too hot. People do after all open windows in the summer.....

Remember that a PIV unit is a cheap way to stop mold without having to open windows in winter (that no tenants do in real life), they don't claim to give you the same air quality or energy saving that a correct design and installed MVHR system does.

Does PIV work?

tony — Sat, 01 Apr 2017 07:58:04 +0100

Not cheap for the tenants as they have to pay to both run the fans and for the consequential deliberate heat losses involved.

Does PIV work?

ringi — Sat, 01 Apr 2017 12:17:18 +0100

Posted By: tonyNot cheap for the tenants as they have to pay to both run the fans and for the consequential deliberate heat losses involved.

Cheaper for them then leaving the windows open all winter or losing their deposit due to mold. When tenants start telling me they will pay more rent to get MVHR systems (or even loft insulation), I will consider if the rate of return make it worth while providing MVHR systems.

Does PIV work?

ferdinand2000 — Sat, 01 Apr 2017 15:50:23 +0100

The fans are around Â£5.00 per year to run in electricity - so truly negligible.

I haven't got any numbers on extra heat loss, but I expect it is -ve compared to the extra cost of heating up humidity in the air or heat lost through open windows and trickle vents.

Ferdinand

Does PIV work?

tony — Sat, 01 Apr 2017 16:50:12 +0100

I can't see that, the cost must be quantifiable in terms of blowing cold air into a home, an equivalent volume of hot air is being expelled.

Does PIV work?

CX23882 — Sat, 01 Apr 2017 17:40:58 +0100

Yes, MVHR is more efficient than PIV, but I recall discussions on here where the opinion was that MVHR doesn't move sufficient air to distribute heat around a property. So if that's the case, how can PIV, which moves even less air, cause such a big heat loss? My cigarette packet calculations suggested that the difference in capital cost is much more than the cost of the proportion of gas usage that can be attributed to ventilation losses, but maybe I missed something out of the admittedly rudimentary calcs.

My property is very small. I wonder if things work out more in favour of MVHR as property size increases?

My gas bill has gone down since fitting a PIV unit. Maybe it would have been even lower with MVHR, but the payback period is ridiculous. The upfront cost is significantly higher. The electric running cost is higher. The ongoing maintenance costs are higher. I'm not against moving up to MVHR in the future, but at the moment, it doesn't seem to be worth it for me.

Does PIV work?

djh — Sat, 01 Apr 2017 18:17:02 +0100

Posted By: ferdinand2000I haven't got any numbers on extra heat loss, but I expect it is -ve compared to the extra cost of heating up humidity in the air or heat lost through open windows and trickle vents.

I agree with this assessment. The point not explicitly stated but implied is that you DO have to open the windows and lose the heat if you want to avoid mould, or else run the whole house hotter so condensation doesn't form.

Posted By: tonyI can't see that, the cost must be quantifiable in terms of blowing cold air into a home, an equivalent volume of hot air is being expelled.

Agreed, so why don't you calculate those costs? Ferdinand didn't say they were impossible to know, simply that he hadn't done the sums.

Posted By: CX23882My gas bill has gone down since fitting a PIV unit.

And there's some actual evidence of what Ferdinand said.

Does PIV work?

djh — Sat, 01 Apr 2017 18:28:35 +0100

For a comparison with MVHR, here's my back of an envelope. I typically run the unit at 125 mÂ³/hr for 8 hours overnight during winter to allow the post heater to work and at 50 mÂ³/hr for the other 16 hours. (The volume of the house is 440 mÂ³ for PHPP purposes.) At the moment I'm running it at 125 mÂ³/hr for 8 hours whilst its daylight (so it's powered by the PV) and 50 mÂ³/hr for the other 16 hours. I'm not entirely sure what I'll do this summer so lets assume the same.

At 50 mÂ³/hr it consumes 9 W. At 125 mÂ³/hr it consumes 26 W.

So 8 hours at 26 W and 16 hours at 8 W. I make that (9*16+26*8)*365/1000 = 128.5 kWh/year. At an arbitrary 15p per unit that's Â£19.20 per year. In practice most of my 26 W periods are cheaper - E7 night rate in winter and free PV otherwise.

Does PIV work?

DandJ — Tue, 04 Apr 2017 20:44:25 +0100

Posted By: delpradoCan anyone comment on how they work in the summer? We have a slate roof and so the loft gets ludicrously hot. I can't have that heat being pumped into the home...

I've been looking at fitting a PIV system recently and the envirovent unit has an outside feed which will switch to external air source when the loft gets too hot. Its also available with two outputs and a heater for when the loft is too cold (as many of them do).

Still looking at which one to fit myself, it was originally going to be MVHR but that is too much work and expense for me.