Green Building Forum - MVHR

MVHR

XT600 — Thu, 11 Jan 2018 21:14:35 +0000

I've yet to decide how to ventilate my house (1760 mid terrace) but will probably not go with MVHR due to there being two chimneys with unsealed woodburners downstairs. I guess a PIV system may work better. However,I'm currently fitting some new stud walls upstairs and considering putting in a 50mm pipe (which will fit down through the studwork) from the loft space to the downstairs room. Would this pipe be any use with an MVHR system should I change future plans or is it too small? All the systems I've seen use much larger diameter ducts.

MVHR

snyggapa — Thu, 11 Jan 2018 22:04:25 +0000

smallest I have seen is semi flexible 75mm outside diameter, about 60mm internal. sometimes you use two 75mm pipes to keep the noise down - I would guess that 50mm would be very noisy (higher velocity air movement)

MVHR

SteamyTea — Thu, 11 Jan 2018 22:04:43 +0000

Seems a bit small to be, it only has 0.002m^2 area.
A litre is 0.001m^3
So to shift 1 litre per second you need a air flow speed of 5 m/s, which is 11 MPH.
It may whistle a bit.

MVHR

XT600 — Thu, 11 Jan 2018 22:12:12 +0000

I could probably stretch to 75mm, will take out most of the studwork but do-able. I've had several lengths of 50mm wastpipe I've been trying to find a home for ages :-)

MVHR

DarylP — Fri, 12 Jan 2018 06:14:04 +0000

use the semi-rigid oval? ....designed for this sort of job........ :-)

MVHR

SteamyTea — Fri, 12 Jan 2018 08:34:19 +0000

Posted By: XT600I could probably stretch to 75mm

That will give you a windspeed of 5 MPH to move 1 litre a second.

How much air are you looking to shift?

MVHR

tony — Fri, 12 Jan 2018 17:24:35 +0000

Why not try rectangular plastic duct say 100x50 ?

MVHR

XT600 — Fri, 12 Jan 2018 21:07:21 +0000

Posted By: SteamyTea
Posted By: XT600I could probably stretch to 75mm
That will give you a windspeed of 5 MPH to move 1 litre a second.

How much air are you looking to shift?

I've no idea. When the wind blows through my open keyhole I can feel the draught from some distance, so a 75mm hole seems massive in that respect! :-)

MVHR

XT600 — Fri, 12 Jan 2018 21:12:47 +0000

Posted By: tonyWhy not try rectangular plastic duct say 100x50 ?

Good idea, worth considering thanks...

MVHR

goodevans — Fri, 12 Jan 2018 23:19:27 +0000

For MVHR in a kitchen would require 10 l/s - a 50mm pipe requires a flow speed of 5m/s for that capacity (Steamy is a factor of 10 out in his calcs I think). The flow speed needs to be down to less than 2m/s ideally.

MVHR

SteamyTea — Sat, 13 Jan 2018 10:39:05 +0000

Posted By: goodevans(Steamy is a factor of 10 out in his calcs I think)

Could well be

Pi x 0.025^2 = 0.001963m^2

Litre = 0.001m^3

Flow rate = V / Pipe Area

0.001 / 0.001963 = 0.509

So yes, I must have got an extra zero in somewhere (or left one out I think)

So a flow rate of 10 l/s will be 5.1 m/s which is 11 MPH, which is still fast.

MVHR

djh — Sat, 13 Jan 2018 11:45:29 +0000

For laminar flow - which you need for quiet operation - the flow velocity profile is parabolic not uniform. So you need to double the peak speed. So a 50 mm pipe is way too small for 10 l/s.

See e.g. https://www.pipeflowcalculations.com/pipe-valve-fitting-flow/flow-in-pipes.php

MVHR

XT600 — Sat, 13 Jan 2018 20:12:59 +0000

Am I barking up the wrong tree even considering MVHR with the existence of two woodburners? (one of these is inset into the original surround, so I don't see myself ever changing them for direct air supply versions)

I've been looking at a simple piv system but not sure I like the idea of very cold air being pushed into the house, or the idea of a 400w heater heating this air ...

MVHR

tony — Sun, 14 Jan 2018 08:52:11 +0000

PIV should be banned, like you say it pushes cold air into the house,

It does work in terms of no mould or condensation, probably would make wood burner glow red.

The bad thing about it is that it continuously pushed warm air out through all gaps and cracks!

MVHR

XT600 — Sun, 14 Jan 2018 22:09:54 +0000

Posted By: tonyPIV should be banned, like you say it pushes cold air into the house,

It does work in terms of no mould or condensation, probably would make wood burner glow red.

The bad thing about it is that it continuously pushed warm air out through all gaps and cracks!

The other thing about PIV is they normally get put in the landing ceiling, so how does that help circulate air around the bedrooms?
I'm wondering whether I could modify a PIV output by splitting it and ducting the air into each bedroom and also one downstairs. That way the air will have a small chance to warm up a little as it gets pushed around the duct. It will also get to where its needed rather than one icy blast from the landing!

MVHR

tony — Mon, 15 Jan 2018 16:12:08 +0000

And the bedrooms would become very cold and more noisy

MVHR

gravelld — Mon, 15 Jan 2018 19:26:38 +0000

Posted By: XT600The other thing about PIV is they normally get put in the landing ceiling, so how does that help circulate air around the bedrooms?

The bedrooms have trickle vents which is where the positive pressure is pushed out.

MVHR

XT600 — Fri, 19 Jan 2018 20:28:06 +0000

Posted By: gravelld
Posted By: XT600The other thing about PIV is they normally get put in the landing ceiling, so how does that help circulate air around the bedrooms?
The bedrooms have trickle vents which is where the positive pressure is pushed out.

My bedrooms are now highly insulated and airtight apart from gaps left under the doors. The windows are triple glazed without air vents. This is to reduce traffic noise from the road, and to avoid pollution from the road side. My plan is to push fresh air into the rooms from the loftspace, which will circulate through the room, under the doors, and leave through the 'open' chimneys downstairs. The issue now is to decide how to control the amount of air pushed into the bedrooms, without introducing extra noise :-)

MVHR

XT600 — Fri, 19 Jan 2018 20:31:05 +0000

Does anyone know if I can get a 100mm filter box with some washable filters to try out?

MVHR

djh — Fri, 19 Jan 2018 22:17:52 +0000

Make a box out of cardboard or hardboard etc and tape it in place? Filter material from http://www.fairair.eu/en/

MVHR

DarylP — Sat, 20 Jan 2018 09:25:19 +0000

http://www.justfans.co.uk/duct-mounted-panel-filter-495-495-22mm-p-2146.html ?

MVHR

XT600 — Sat, 20 Jan 2018 12:51:21 +0000

Thanks for the links guys, will take a look at them.
Do you think that by using in-line filters in each room duct this will help avoid noise transference between rooms?

MVHR

djh — Sat, 20 Jan 2018 13:01:50 +0000

Posted By: XT600Do you think that by using in-line filters in each room duct this will help avoid noise transference between rooms?

No, not really. You need attenuators for that purpose.