DIY install of an MVHR on a 1930s Semi-detached House

Mechanical Ventilation and Heat Recovery (MVHR) is technology that’s typically only installed on highly air-tight and insulated new build Passivhaus standard houses. Even bulk house builders will revert to trickle air vents for ventilation rather than install this technology. So is it possible to perform a relatively inexpensive DIY installation on an old leaky house and how well does it work?

If you’ve stumbled across this post you might want to read my earlier post on my experiment with a Single Room Mechanical Ventilation and Heat Recovery Unit.

Our house is a typical 1930s semi-detached house in Langley Road, Chippenham. We’ve worked over a number of years to insulate the house and keep out the drafts with the ultimate goal of removing gas and installing a heat pump. I’d been very much inspired by Passivhaus design where houses are extremely well insulated with little thermal bridging, and extremely air tight. The air tightness is taken to the point where joints are taped and every detail sealed such that the overall air leakiness of a house is less than the size of a saucer, or thereabouts. The end result is a home with extremely low energy loss, such that very little heat energy is needed to warm the home. But with such little natural air flow a Mechanical Ventilation and Heat Recovery (MVHR) system is integral to the design ensuring air quality is high and condensation removed.

The diagram above from the Passivhaus Institut illustrates the typical ventilation and heat recovery strategy in a Passivhaus. Warm air is extracted continuously from a room such as the bathroom and kitchen. This warm air is passed through a heat exchanger warming the exchanger before being expelled outside of the house.

At the same time cool fresh air is drawn in and filtered before passing through the heat exchanger, warming the fresh filtered air. This is passed into living spaces and bedrooms. There are intentional gaps under internal doors and this creates a circulatory flow of air through the house. The vents and routing are designed such that a balance of air flow is achieved and typically over 90% of the heat is recovered.

In some Passivhaus designs the MVHR is combined with a small heat pump to provide gentle warming or cooling of the air. There is often no need for a radiator system and underfloor heating can be used instead.

So why would you try this in a leaky 1930s house?

A very good question. Despite our best efforts our 1930s house is not even mouse proof let alone air tight!

My quest to make our own house more draft proof meant that at the most obvious we needed to keep our windows closed to avoid the heat simply going out of the window.

Similarly in my son’s box room bedroom his cast iron air vent next to his bed led to cold North air blowing in throughout the winter, so he’d close it preventing ventilation. This led to mould growing on the ceiling where cold spots were on the north side of the house. You can read how I greatly improved this with single room mechanical ventilation.

Our bathroom had the same issue as the hot water from the shower would condensate on the ceiling. Though I had a timed overrun extractor fan it had several practical issues. Firstly we needed to set the fan to run for around 20 minutes to remove the steam by switching on the light. But at night if the light was turned on the fan was noisy and would annoy us, so we’d turn off the isolation switch outside. Then we’d forget to turn it back on in the morning. So every 18 months or so the ceiling would bubble, go mouldy, and peel, no matter what anti-mould paint I used. Very frustrating!

So i changed several things.

Firstly I had already insulated as best I could above the bathroom. Whilst this helped a bit it didn’t fix the mould or paint peeling issue.

Secondly I changed the fan from being timer trigger to condensation triggered. I had considered changing the fan to a single room MVHR unit but the location of the fan stopped me. It was above the shower and therefore needed to be safe extra low voltage supplied via a transformer. All of the single room MVHRs available were mains 230VAC and therefore not suitable to be mounted within touch location of the bath. So I installed a condensation triggered low voltage fan with the control unit mounted in the corner of our small bathroom the correct distance from the bath. This worked extremely well and removed the bulk of the steam from the shower. It also meant we could simply leave it on as it didn’t trigger if the toilet was required in the middle of the night.

Thirdly I painted the bathroom ceiling with the Dryzone anti condensation paint used in my son’s bedroom. It’s very thick and goes on like tooth paste. But it is really good and so far I haven’t had to repaint it in three years – unheard of!

Finally, to actually dry the bathroom of the remaining humidity I decided to install a relatively low cost Blauberg Komfort Ultra D-105A MVHR unit in the loft.

I chose this unit on the basis that it was relatively cheap (just under £400) and was sized to supply a small apartment of two rooms. I decided to stretch it to three rooms for our remaining two bedrooms and the bathroom. This would allow us to keep upstairs windows closed in the winter and not feel too stuffy maintaining air quality.

To achieve this I also bought insulated pipe lengths and joints as well as a ceiling vent kit and a ventilation roof tile kit. All in about £550 for the installation.

The installation

If you’re a professional MVHR installer please stop reading at this point!

So of course I should have perfectly straight pipe runs with minimal air flow interference. My vents should be placed for perfect air flow and the system design balanced.

But I didn’t do that, I did what I could achieve in a crowded loft with eaves that cut off the available space.

So the first step is to put two vent tiles replacing the outside roof tiles. I achieved this by putting a step ladder on a flat roof. I wouldn’t have done this if I hadn’t had easy access to this part of the roof. On the inside of the roof I ran two insulated flexible pipes to the MVHR unit, one inlet pipe, and one exhaust pipe. The other side of the unit needed to connect to an extraction vent and outlet vent in each room. I put both vents in each room as I didn’t consider that I could really create a balanced air flow through the upstairs given a mix of carpets and less than airtight house.

Though the unit was designed to cope with two rooms, I decided to extend the pipes to three rooms. However the third room with the longest pipe run did have much reduced air flow and in reality probably needed a more powerful unit with a much better design of pipe runs.

The unit comes with a prewired speed control with settings of 1,2, and 3.

My wife described my brown trunking for the three speed controller and ceiling vents as ‘like living in a bad 1950s hotel room’, but I think she’s forgiven me!

The loft unit is powered from a 13A socket and comes complete with a mains plug. It’s a very low power unit as they are designed to continuously run on a trickle ventilation. The unit needs a drain connection as the condensation extracted from the bathroom air runs into a drip tray. I connected a flexible pipe and utilised an old pipe left over from a loft tank which runs through the eaves outside.

How did it work?

Though professional installers might be crying at this point, it has worked very well. The extraction and circulation in the bathroom removes the excess moisture that the condensation fan doesn’t quite remove, and so far the bathroom has stayed mostly mould clear for three years with only the occasional anti mould wipe down required. The ceiling hasn’t flaked at all. All this with the bathroom window closed throughout the winter months.

Our bedroom windows have also remained closed in the winter. Where we have shutters we do still get condensation forming behind them which we have to wipe down every few days in the winter, but the air quality is good.

How much heat does it recover?

So this is a fairly simple unscientific test…

At 9-30am in the morning it’s 4 deg C outside. The Bathroom ambient air temperature is 16 deg C. Whilst that seems cold we have a low temperature electric tile heating.

The inlet air temperature to the bathroom is warmed from 4 deg C to 12 deg C, so an 8 deg C rise.

What mistakes did I make?

Well, the loft unit has to be perfectly level. This is in the instructions but I ignored it, and as soon as the very cold weather hit condensation from the bathroom dripped through our bedroom vent onto my wife’s head whilst she was asleep!

So I levelled the unit and positioned the water drain pipe correctly!

The longer pipe runs to my younger son’s room mean that his air flow is weaker than the other two rooms.

I should perhaps have tried to place the extraction pipe in the bedrooms farther away from the inlet, but I was constrained by the eaves shape.

Does it save money?

That’s hard to say. The Blauberg Komfort unit isn’t the most energy efficient MVHR available. It’s a low cost unit and the reality is it’s a tin box with some insulation and a plastic heat exchanger. But it does the trick. I’d estimate it perhaps costs me £40 a year to run the unit. Whether it recovers that in heat is hard to say, but having clean filtered air, hardly any black mould, and a dry bathroom is well worth it I think.

It does need the filters cleaning every 3 months which requires climbing into the loft, so that’s definitely a consideration.

Does it Save Carbon?

That’s even harder to say. It obviously recovers what would be lost heat at around 85% efficiency. But it also runs a low power electric fan continuously. It also allows us to keep our windows closed throughout the winter thereby retaining heat.

So I don’t have any specific numbers, but i’d say as an overall strategy for saving energy and maintaining a good habitable environment I’m satisfied. I suspect it probably evens out carbons saved by heat recovery with carbon to run. We have solar and battery, so I could argue that our energy source is very green and therefore we recover more than we use running the unit.

I’ve also saved the extra carbon i was using by continually repainting the bathroom ceiling. But there is embodied carbon in the unit manufacture.

What are the alternatives?

Some people say Positive Input Ventilation (PIVs) work well for them. This is an easier installation than an MVHR as it is simply a fan taking filtered loft air and feeding it through your ceiling via vents. The outlet is through your leaky house. This would be a solution for ventilation for mould or damp and doesn’t do any heat recovery.

The Single Room ventilation unit I put in could also be paired wirelessly with a wall unit in another room synchronising input and extraction. This is a more expensive route but doesn’t require any loft or roof work.

A friend of mine had an assessment carried out on his 1930s house by our friends at the Centre for Sustainable Energy including a house Air tightness test. They advised that a continuous running trickle ventilation fan would be more suitable than MVHR for his house.

Building Regulations

Ventilation is covered under building regulations, so that may be worth researching further. I didn’t worry as I considered I was improving ventilation in a house that wasn’t working well on that front, but a professional installation would cover building regulation requirements.

It should be noted special requirements apply if your room has a combustion vessel – so if you have a gas boiler, wood burner, or chimney in your room you may need to investigate further before undertaking a diy installation.

Noise is also a consideration. I found the noise of the Bluberg Komfort to be akin to the background noise of the engine in Startrek the Next Generation. Not a very scientific assessment, but I found it to be low noise and quite soothing, with no sleep disturbance.

Conclusions

I hope this short case study on my DIY install is useful. Overall i’m pleased with the results which have run over two winters now, allowed me to keep upstairs windows closed, and significantly reduced condensation and mould issues. The air quality feels better and removes that stuffy feeling when windows are closed at night in a bedroom.