DIY retrofit MVHR design for beginners

[Ommm]

I've been challenged to sort out some air quality issues in our 1960s 120m2 chalet bungalow:

 

• No moisture extraction in the bathroom beyond a window; there was previously a fan which just blew straight into the loft, with no ducting anywhere
• No extraction in the kitchen: the cooker hood just recirculates via a filter
• No effective ventilation upstairs (loft conversion) beyond useless trickle vents, hence window condensation (made worse by good thermal blinds so temps behind the blinds drop overnight)
• Occupant who suffers from noise and air quality sensitivity - we run room HEPA filters a lot
• ASHP installed with the possibility of fitting a ducted water fan coil for cooling
• Tight loft spaces (crawl only, not conditioned)
• House is quite leaky, especially into the loft (holes for pipes, electrical fittings, access hatches), and some old vent holes (todo to block these).  It's a windy area so much more leaky on windy days and less on still ones.
• Chimney with wood stove (the vents are largely left wide open for optimal burn)

 

I had originally considered going for dMVHR to address the first with a Vent Axia Tempra in the bathroom, but having listed the others wondered about ducted MVHR.  I realise we get some amount of fresh air from the leaks (although I hope to plug those) but hope to improve moisture control and air quality.

 

Does central MVHR make sense for this?  I suppose I can improve the moisture situation with maybe 3x dMVHR units (bathroom, kitchen, upstairs), then the other rooms have their own leaky ventilation.  That doesn't get filtering or cooling, but maybe a central air filter/fan coil doesn't make sense in a leaky house?  Maybe the physical limits (not much flexibility in duct positioning in each room) make it ineffective?

 

Cost is a big factor so looking at ebay parts and DIY install (also the physical design constraints are hard to convey), but not really sure how to start with system design.  Searching around brings up lots of puff pieces for design services, but not so much about design principles.  I did my own ASHP design and there were decent resources for that. Any pointers on how to begin? 

[JohnMo]

People think that MVHR is the best and only solution, but you need something fit for purpose above all.

 

You already have trickle vents, but they are rubbish on their own, they rely on a fan somewhere else. To give room/house cross ventilation.

 

dMVHR units are only any good in a single room, unless to get the forward reverse ones in pairs. The prices start to stack up really quickly.

 

My view is this

Replace trickle vents on all windows with humidity sensing units, these are closed or open in response to humidity being sensed in the room. Then either dMEV in the wet rooms or MEV. MEV again you get or specify you want humidity sensing extract terminals.

 

The above will give you ventilation only when and where needed.

 

Look in the ventilation section on here, loads of stuff there on design.

[Ommm]

There are only trickle vents on two of the upstairs windows, the rest don't have them.  For the 'humidity sensing units' I assume you mean fans, ie cut a hole in the wall?  I already have one ceiling and one wall hole in the bathroom (both plugged) and could make a ceiling hole + duct + soffit vent in the kitchen (there's a plugged wall hole but at the opposite end to the cooker).  But that wouldn't address moisture/CO2 elsewhere, eg bedrooms?  Without a heat exchanger, won't that make any room with a fan cold?  (yes I know there are backdraft flaps on cowlings, but they didn't work well in my last property).

 

(basically, moving from a 'window open all the time with cold draughts' model to a 'warm fresh air all the time' model is the motivation to look at MVHR)

[gregcope]

I DIY fitted a MVHR.   It works really, really well, however do not under estimate the work in installing.   We went from a house that had mould in the bathrooms (no extractor fans) to much better air quality.

Running the ducts in the loft was the easiest, for a given value of easy in the loft.   Running ducts down to the ground floor took a while as hiding ducting in cupboards and also ensuring the loft end worked was more effort.  Fitting the intake/extract vents in the roof was straightforward for a roofer!  It took five weekends; One for the intake/extract/condensate drain/unit setup.  One for the upstairs rooms (two bathrooms, three bedrooms).  Another for an extension bedroom (should have hired a core drill for going through an outside wall that is now internal).  Another for downstairs ducts to kitchen and lounge.  Another for a downstairs loo that involved dropping part of the ceiling to fit the ceiling duct as I could not access above it.   I think I could do it in half the time or less as now more confident.

 

All my parts came from ebay, isells, BPC, Screwfix.  I did the design myself with a spreadsheet of room volumes and flow rates.   I went slightly below the suggested flow rates for "Normal" rates and over for Boost.  I overspeced the ducting to reduce noise (ie twin ducts to all rooms as running these is not much work relatively).  I fitted noise silencers.   I would go higher for boost, to clear damp air quickly, however above 90% flow rate it is noticeable noisy.

 

We have virtually zero condensation.  The odd, tiny bit behind some plantation shutters on some cold nights.   Some condensation on a door lock that is a cold bridge in a yet to be vented room (its the hardest to get to) and sometimes on the letterbox flap (needs more insulation!).

If you have reasonable loft access and only need to do runs in the loft this will help reduce install effort allot.   I assume you could have the intake/exhaust through a gable end which should be quite easy (given value of easy).   Think of the condensate drain and where that goes (I suggest a soil vent spigot to keep it warmish).  

It is great, but allot of work.   I would have a strong think of doing it again, but on balance would do it again.

I did consider four individual room MVHR as these would be quite allot of work to install (core drilling, running electrical feeds) and these are expensive units.   I think this would be cost neutral compared to my MVHR.   Maybe slightly less work, but not much.   Running costs would be more due to more fans.   It would not have improved the air in non-vented rooms compared to MVHR.

Your air leaks might be an issue although depending on units you can set them to run intake fans faster than extract to positively pressure the house which will help with internal air quality.  This would be slightly less efficient as you are losing air that is not going via the heat exchanger.

On either method consider the filters the units have.   Mine goes upto M5 (A "Pollen filter" ie 50% of Pm10).   Newer models have HEPA level filters.   You could DIY a HEPA filter on the intake from BPC/Isells bits.

Sorry that turned into an essay...

[patp]

Following.

 

Our build is finished the hot bedroom syndrome is causing us huge problems. Trickle vents are useless and we live in a bungalow so opening windows would probably negate our home insurance.

[Simon R]

32 minutes ago, patp said:

Our build is finished the hot bedroom syndrome is causing us huge problems. Trickle vents are useless and we live in a bungalow so opening windows would probably negate our home insurance.

Have you considered a small air to air AC unit? there are lots of options in the 2.5kw power range are very efficient, cheap to install and run.

[JohnMo]

1 hour ago, patp said:

opening windows would probably negate our home insurance

Probably - not

[Mike]

There's also the option of adding a ceiling fan.

[patp]

Thanks for all the most helpful solutions above. I am going to investigate the ones I have not tried already. These are shutting the doors and closing the blinds during the day and opening them at night. This does not work when it is 26C outside. Once the room gets to 20C or more, which it did most of last summer, there is not way it is going to cool down unless we use. air conditioning. Building Control do need to take this in to consideration in sunny climes like East Anglia. We have three large South facing windows and five, large, West facing windows. 

 

MVHR is a possibility as husband (a time served heating engineer) says he could fit it easily. I cannot see how moving hot air around the house is going to help us?

 

I like the idea of window shades. More expense - sigh. Ceiling fan is also a possibility.

 

Does anyone have any experience with tinted windows? We have lovely views over countryside so do not want to spoil those.

[MikeGrahamT21]

39 minutes ago, patp said:

Thanks for all the most helpful solutions above. I am going to investigate the ones I have not tried already. These are shutting the doors and closing the blinds during the day and opening them at night. This does not work when it is 26C outside. Once the room gets to 20C or more, which it did most of last summer, there is not way it is going to cool down unless we use. air conditioning. Building Control do need to take this in to consideration in sunny climes like East Anglia. We have three large South facing windows and five, large, West facing windows. 

 

MVHR is a possibility as husband (a time served heating engineer) says he could fit it easily. I cannot see how moving hot air around the house is going to help us?

 

I like the idea of window shades. More expense - sigh. Ceiling fan is also a possibility.

 

Does anyone have any experience with tinted windows? We have lovely views over countryside so do not want to spoil those.

 

MVHR can help, but it will only be slight, you can get units which fully bypass the heat exchanger to bring in the cooler air overnight, but as the air flow speeds are so low this is a minimal effect. I too have a bungalow and find it best if i can drag myself out of bed early enough, to open a bi folding door which is at the back of the property, and some velux windows which are at the front, even just a tiniest breeze and the house becomes like a wind tunnel, funnelling all of the heat back outside, then as soon as the temperature charts on my sensors level out and i've lost all the heat i can, everything gets shut again which keep things manageable. The problem is when we get consistently hot days and warm nights, thats when AC would come in very handy.

 

Window shading is a very good option, all of my velux windows have awnings on them to limit the amount of heat coming in, and the window film i have had a sample of before, but make sure you get external window film (its rather expensive!) to stop the heat entering the pane altogether, and there is solar control glass too which is another option, neither of which should change your view too much

[Mike]

2 hours ago, patp said:

Does anyone have any experience with tinted windows? We have lovely views over countryside so do not want to spoil those.

Only from an office. It's a bit like wearing sun glasses - everything looks normal through the glass despite the tinting, until you see 'reality' through a non-tinted / open window. The biggest downside is that they cut the light transmission into the room significantly, which could make the room gloomy in winter. I wouldn't do it.

[MJNewton]

On 12/12/2023 at 07:46, patp said:

Our build is finished the hot bedroom syndrome is causing us huge problems. Trickle vents are useless and we live in a bungalow so opening windows would probably negate our home insurance.

 

Whilst MVHR might help with heat-related 'stuffiness', it won't do much for the heat itself. The combination of the low heat capacity of air and the relatively low flow rates means it is not a good solution.

[MikeGrahamT21]

3 hours ago, Mike said:

Only from an office. It's a bit like wearing sun glasses - everything looks normal through the glass despite the tinting, until you see 'reality' through a non-tinted / open window. The biggest downside is that they cut the light transmission into the room significantly, which could make the room gloomy in winter. I wouldn't do it.

 

I was thinking more along the lines of this stuff: https://www.windowfilmsuk.com/product/sl50-osw-window-film/

[IGP]

I’ve put window film on it in one of rooms, it helps but isn’t a wonder product.
 

I’ve just DIY’d an MVHR install in my 30’s semi. It’s made a huge difference in air quality. 
 

It’s no longer stuffy, there’s no condensation on any windows as we used to have humidity 65-80% regularly. 

I feel I sleep better with the fresher air in the house which is something I didn’t expect. Also my partner hasn’t used her inhaler since we fired it up, but that be coincidence. 
 

All in all, I’m super happy with it but it was loads of work! I’ve now fully insulated the unit (photo taken before) and has helped a lot with warmer supply air. 

 

I can recommend MVHR. 

[IGP]

Also in terms of the design, I used BPC ventilation and they gave a rough design for me to follow with approx placement locations, numbers of pipes that connect to each plenum etc. 

[Roger440]

2 hours ago, IGP said:

I’ve put window film on it in one of rooms, it helps but isn’t a wonder product.
 

I’ve just DIY’d an MVHR install in my 30’s semi. It’s made a huge difference in air quality. 
 

It’s no longer stuffy, there’s no condensation on any windows as we used to have humidity 65-80% regularly. 

I feel I sleep better with the fresher air in the house which is something I didn’t expect. Also my partner hasn’t used her inhaler since we fired it up, but that be coincidence. 
 

All in all, I’m super happy with it but it was loads of work! I’ve now fully insulated the unit (photo taken before) and has helped a lot with warmer supply air. 

 

I can recommend MVHR. 

 

Interesting stuff.

 

I will in the next couple of years doing a mini refurb on the house and i keep pondering this. Im not a fan of the complexity it brings, but obviously there are plenty of benefits.

[Ommm]

Thanks all for the interesting comments.  Since I posted the OP I'm gradually developing a strategy:

 

For the bathroom (downstairs), the problem is mostly shower moisture but also we leave the window open for fresh air.  Fit a non-counterflow single room MVHR, the current frontrunner is the Vent Axia Lo-Carbon Tempra.  This location is very noise sensitive, and so I like that somebody has identified that it seems the fan is just a 24V motor which takes a 0 to 5V speed control input and so it's potentially possible to modify it to fit linear speed control with a slow ramp rather than the fixed speeds (also, I don't like the counterflow ones because changing direction may cause noise from cycling).  I have a blocked up 100mm wall opening so it should drop straight in.

 

For the kitchen, there's currently a cooker hood with a recirculating filter.  This is useless because we never fry things so it's only pushing damp air straight back into the room.  The thinking for this is just a regular extractor (possibly a ducted fan in the loft) for the 10 minutes a day something is bubbling on the hob.

 

There are two bedrooms upstairs and three rooms downstairs, that would be good to have both fresh air for air quality and also cooling.  I haven't really come up with a strategy for this; I have two small lofts front and back, and each loft can only reach 3 rooms; I might be able to 100mm duct under the upper floor front to back but that would be  complicated. I'm also not super keen on cutting giant holes in ceilings or (stud) walls, especially for noise carrying reasons.  Maybe there is some way to combine MVHR (low flow rate) and a ducted fan coil from the ASHP for cooling purposes (recirculated air), or do they need separate ducted systems because the air volumes are so different?

[Russdl]

On 12/12/2023 at 07:46, patp said:

Our build is finished the hot bedroom syndrome is causing us huge problems.

 

Micro Louvre’s are a solution we contemplated for one of our troublesome overheating rooms. We got a sample from them, it wouldn’t be an invisible fix but probably would be a goodish fix. In the end we didn’t go for it as we managed to control the problem by closing the curtains and isolating the room whilst the sun was beating in the east facing window. 

 

https://www.smartlouvre.com/