MVHR is Largely Bogus

[Jeremy Harris]

10 minutes ago, DavidHughes said:

Hi Jeremy,

 

I'm looking at your graphs now and can't quite make my calculations match yours though I am in the same ball park.

• Please can you confirm your mean annual continuous flow rate, mean annual outside temperature and mean house temperature.
• It looks like your MVHR is delivering about 90% efficiency, can you confirm?
• You might add the density and SHC of the air which you are assuming and your cost/kW.hr for re-heating air lost with no MVHR. This will help me align my calculations.

David

 

 

 

All the data is in the spreadsheet linked to earlier in this thread.  IIRC I used an MVHR efficiency of 85%, which is pretty typical for most units.  The ventilation rate is as given, 0.43 ACH.  The temperature differential is shown on the plots, and the spreadsheet includes monthly data for our location (from the nearest Met Office weather station, although it's in error slightly for our sheltered location).  SHC can be considered pretty constant, as it makes very little difference when comparing heat recovery ventilation versus non-heat recovery ventilation.  Again the figure used is in the spreadsheet, 1210 J.m³.K.  You can easily change this if you want to see the impact - the cells aren't locked.

 

I didn't bother with cost, as that is very highly dependent on the heating fuel used, the efficiency of any heating system, plus things like variable tariff rates (almost all our heating is from E7, via an ASHP, for example, so the cost is ~ 2.5p/kWh, so cheaper than pretty much any other fuel I suspect).

[DavidHughes]

>Again the figure used is in the spreadsheet, 1210 J.m³.K

Finally found it amongst the posts. Thanks. That, the 0.43 ACH and  and the 85% efficiency figure has gobbled up most of the discrepancy.

 

[Oz07]

I think you'd be looking at 250 or less for the test 

[SteamyTea]

41 minutes ago, DavidHughes said:

Have an air tightness test done

How much does this cost for a 100 m^2 bungalow

Probably around £250. I think that is what the guy that did @joe90 said he charged.

Or make your own test rig. 

[Ed Davies]

29 minutes ago, SteamyTea said:

Or make your own test rig. 

 

Making a test rig to help find leaks is pretty easy - quite a few people have done it including a few on this forum. Car fan, bit of MDF and some sticky back plastic. Some pressure measurements to compare before and after leakage is a bit harder but quite doable. Getting actual flow rate measurements to a sufficient level of calibration to be confidently comparable between houses or against standards (building regs or Passivhaus) is a bit trickier I think.

[SteamyTea]

22 minutes ago, Ed Davies said:

Getting actual flow rate measurements to a sufficient level of calibration to be confidently comparable between houses or against standards (building regs or Passivhaus) is a bit trickier I think

Isn't that when the MVHR, air flow meter comes in handy.

On the rig @joe90's the guy blocked off holes, of known area, to settle the fan speed. Measuring the airflow through the holes would give the volume.

A few hours testing and for any given fan speed and pressure difference, a chart could be made up.

Edited December 22, 2019 by SteamyTea

[joe90]

Yes my chap charged £250. 

[Jeremy Harris]

51 minutes ago, Ed Davies said:

 

Making a test rig to help find leaks is pretty easy - quite a few people have done it including a few on this forum. Car fan, bit of MDF and some sticky back plastic. Some pressure measurements to compare before and after leakage is a bit harder but quite doable. Getting actual flow rate measurements to a sufficient level of calibration to be confidently comparable between houses or against standards (building regs or Passivhaus) is a bit trickier I think.

 

 

Spot on, this is what I made up to find air leaks in our old house:

 

 

It's made up from an old car radiator fan, fitted to a bit of MDF that was cut to be a tight fit into an open window.  I had to replace the motor, as the original was rusted up, so fitted a model aircraft brushless DC motor, with speed controller.  This gave pretty fine control of fan speed, which turned out to be quite useful.

 

I did try to make a differential pressure measurement unit, together with a pitot-type flow sensor, with the intention of being able to display and log leakage, but the leakage from the old house was so great, even after replacing all the doors and windows, and sealing up around all the leakage areas I could find, that I felt it just wasn't worth the effort.

 

The unit was very useful for finding all the many leaks, though.  Some made their presence felt by the noise they made, others had to be pinned down using smoke (I used some joss sticks for this).  After weeks of work I never did manage to make our old early 1980's built, block and brick, bungalow very airtight, as there were just too many big, inaccessible leaks.

[ProDave]

And this was my effort, an old 12" desk fan, some cardboard and a roll of duct tape,

 

It was stuck in the door between the house and the garage which is the only one that was not a proper sealed outside door.

 

It could build up a reasonable pressure, enough to go looking for leaks (not many found) and get an idea of performance.  The telling thing was if you opened a door or window with the blower running you could feel the blast of air escape and the pressure change.

 

[Onoff]

Finally, a use for this! ?

 

Edited December 22, 2019 by Onoff

[Ed Davies]

@ProDave, what are the pipes taped bottom left of your cardboard? Field-expedient manometer?

[ProDave]

39 minutes ago, Ed Davies said:

@ProDave, what are the pipes taped bottom left of your cardboard? Field-expedient manometer?

Yes that's the manometer from my drain test kit. One hose inside the house, the other through the cardboard to outside to try and measure the pressure differential achieved.

[Ed Davies]

Did it work? 50 Pa is only a head of just under 5 mm (unless I've got the arithmetic wrong).

[MikeGrahamT21]

On 22/12/2019 at 15:44, DundeeDancer said:

To clarify my understanding of when it is not worth installing an MVHR system is when there are to many gaps in the fabric of the building that allows a certain percentage of cold air to be sucked into the building through the fabric and  not via the MVHR inlet manifold.

 

So this external cold air is just seeping into the building without going past the MVHR heat exchanger.

 

In this scenario a few undesirable processes are occurring:-

1. Cold air is seeping into the building without being warmed up by the MVHR system so cold drafts occur in the dwelling.

 

2. The stale air does not move so readily between the rooms to then be used by the MVHR system to heat up the fresh air from the MVHR inlet.

 

These points mean the efficiency of the system is significantly reduced and with the volume of air that goes through MVHR system it may well be possible that the MVHR system then starts to significantly chill the dwelling.

 

The only way to stop this drop in efficiency of the system is to ensure there is a good airtight barrier in the dwelling apart from the MVHR systems inlets and outlets. 

 

The only way to get to the point of sufficient airtightness is to install an airtight barrier behind plasterboard and ceilings etc.  So this takes a deep refurbishment project to install correctly or to be installed on initial build.

 

In summary I believe installing an MVHR system will always help with air-freshness and de-humidifying a dwelling but if the air-tightness of the dwelling is not up to standard then in effect the system will be bringing to much cold air into the property via the gaps in fabric of the building and chilling the building.

 

This is my laymen’s understanding of why installing an MVHR system may be folly. I do wonder if I am missing any other points?

 

Many thanks, DD.

I personally don’t see why it’s not achievable, a wet plaster finish is no different to an airtight membrane as long as it’s free of cracks, dot and dab would be the thing causing the problems. I may well find out through my retrofit that my assumption is wrong, but my instinct is that once there is a fan strapped to the door I’ll be able to resolve any minor leaks relatively easily.

 

i also read a research paper done by Leeds met (IIRC) and they fitted mvhr to houses which had gaps filled and plywood over floorboards, final airtightness test came in around 12ACH and all of the houses had a reduction in energy usage, so I still think there’s worth even with poor airtightness test results.

[SteamyTea]

1 minute ago, MikeGrahamT21 said:

a wet plaster finish is no different to an airtight membrane as long as it’s free of cracks

Isn't the problem all the parts that are not plastered, like between the floors, 2 inches above a floor, around windows and doors, ceiling corners, all of the loft, under stairs, behind kitchen cupboards...

And you really want the airtight layer on the outside or you have just made a tent.

 

[joe90]

59 minutes ago, SteamyTea said:

Isn't the problem all the parts that are not plastered, like between the floors, 2 inches above a floor, around windows and doors, ceiling corners, all of the loft, under stairs, behind kitchen cupboards...

And you really want the airtight layer on the outside or you have just made a tent.

 

 

In my case....all parts plastered.....between floors was parge coated before floor/ceiling installed. Plaster down to floors. Window and door reveals plastered. Upstairs ceilings plastered to walls (but warm roof anyway). Nothing was left without plaster apart from the warm loft which had OSB and an airtight layer foamed to plastered walls . 

 

My airtight layer is on the inside so the wall can breathe to the outside.

[SteamyTea]

45 minutes ago, joe90 said:

In my case

Yes, but you knew what you wanted to achieve.

45 minutes ago, joe90 said:

My airtight layer is on the inside so the wall can breathe to the outside.

If there is mineral wool insulation, air can easily travel though it.  Not so bad with sheets of properly fitted and sealed PU.

[Hastings]

On 19/12/2019 at 12:16, vivienz said:

I've had the UFH on for a while now as we are in an exposed and very windy spot. The MVHR helps but to maintain a steady temperature somewhere between 21 and 22 definitely needs some heat input in a twice daily basis. The common feature that I'm aware of with the houses of @jsharris and@mvincentd is that they both receive a lot of shelter from being in the pocket of a hill side. Ours is the opposite and behaves very differently as a result.

 

We still have no functioning UFH upstairs but it currently holds steady at about 19C. A little on the cool side of things for me if I'm working but an oil filled radiator sorts that out until the UFH gets connected up in January.

Where did you locate the two vents on the house?

Asking because I am about to install MVHR in a house also very windy and exposed. 50-60mph days are common and we are miles from nearest tree or hill.

Because house has 600-700mm stone rubble walls (cladding a 150mm inner airtight timber frame) fitting ducts in walls is far more difficult than going through roof but I worry that the roof location will be severely affected by strong wind.

[joe90]

I am also in a windy location and mounted the vents in the north wall under the eaves, some trees to the North and winds from that direction rare. I am yet to balance the system, we have no heating upstairs apart from towel rads in the bathrooms and not over warm (19’) but comfortable fir sleeping. I prefer to use an electric blanket and decent tog duvet in the cold weather.

[mvincentd]

For clarity, while @vivienz is correct re our houses being sheltered, in my case my vents come out of the flat roof which is totally exposed to the prevailing wind.

 

Another observation re benefit of mvhr; nye party of 33 people dancing eating and drinking for 7 hours, detritus left overnight.....following morning wife comments on freshness of atoms’ in house.

 

[vivienz]

My vents are on the north facing part of our roof. The prevailing wind is from the south west but it blows in from the north with reasonable frequency. The position of our vents was largely determined by the location of the unit itself and not wanting the vents through walls.

[MikeGrahamT21]

On 23/12/2019 at 18:41, SteamyTea said:

Isn't the problem all the parts that are not plastered, like between the floors, 2 inches above a floor, around windows and doors, ceiling corners, all of the loft, under stairs, behind kitchen cupboards...

And you really want the airtight layer on the outside or you have just made a tent.

 

 

My wet plaster is straight up on the blockwork, so there are no gaps for the tent effect. Ceilings have coving all round which is free of cracks, though there is always the concern that there could be a route through to the wall/ceiling joint, will have to see what shows up on an air test for this. Ive paid a lot of attention to round the windows and doors, and as I live in a bungalow, there is nothing to cover in between floors, and the loft isn't of concern as its a cold loft, which is currently ventilated, i've made sure any gaps between the living space and the loft are sealed, such as spot lights and wire entry points. Hopefully it works as planned anyway ? time will tell, but I won't be installing MVHR until there is a need for it, ascertained by an air test.

[joe90]

38 minutes ago, MikeGrahamT21 said:

won't be installing MVHR until there is a need for it, ascertained by an air test.

surely if you don’t have MVHR you will need trickle vents in your windows?

[MikeGrahamT21]

5 hours ago, joe90 said:

surely if you don’t have MVHR you will need trickle vents in your windows?

 

Yeah absolutely, what I meant is I won't go out and blow £2000 before I know I definitely need it. Soon as the airtightness works are done, I'll leave a couple of windows on vent, book an air test, and if it shows its low enough, then proceed straight away.

 

System design and spec has already been done from a few different suppliers, so I know what I'm buying, just a case of adding to the basket and hitting order.

Edited January 3, 2020 by MikeGrahamT21

[jack]

On 19/12/2019 at 11:33, Jeremy Harris said:

May be we should start a thread to try and stem the use of MVHR, and get it changed to MHRV...

 

I will undertake a rare abuse of my mod status and delete any such thread on sight.

 

There is literally (literally!) nothing wrong with 'MVHR' as a term. It's an abbreviation of a sequence of words that form a perfectly adequate and accurate description of the equipment's function.