Hello lovely people!

We bought our house (84m2, three bedroom, mid-terrace, 1950s build in Norwich) about two years ago, but have been experiencing damp problems ever since on cold days, with windows dripping with condensation in the mornings and mold starting up in cold corners of the walls and around windows.

The only ventilation seems to be one air brick each in the kitchen, the toilet and 2/3 bedrooms, but not in the bathroom. We have one dehumidifier unit, but it struggles to keep up and is too noisy to run at night.

To sort out these damp problems, improve our air quality and improve the energy efficiency of the house, I would really like to put in some kind of heat recovery ventilation.

Other options (extractor fans, PIV or PIV with heating) all seem like they run against every advice for energy efficiency, by swapping warm air from inside against cold air from outside (or even heating air with a resistance heater). Energy-wise none of those seem like a huge improvement compared to shock ventilating by opening all the windows briefly, which is what we currently do.

When I look for heat recovery ventilation online, I find lots of information on ducted, centralised MVHR and some on single-room/paired/decentralised MVHR, but weirdly, none of the companies I can find around Norwich do decentralised systems. I don't really understand this, as putting in and wiring a few of those seems so much easier and less costly than ripping through lots of walls and ceilings to fit all the ducts for a centralised system, all of which then need additional work to cover them up. I can find plenty of websites selling all sorts of single-room MVHR units, but no companies that seem to specialise in putting them in!? I don't have the skills or knowledge of building codes to design and install a proper ventilation system, so I would really like to find a company that can do it.

Hence my questions are:

- am I missing something about single-room/decentralised MVHR that makes it not the obviously better option for a retrofit compared to centralised MVHR?
- are there any better options we should consider?
- can anyone recommend companies that could design and install a suitable system?
- Given the size of the house, can we get away with just putting fairly powerful single-room MVHRs into the kitchen and bathroom (or, say, a single-room one in the kitchen and paired units in the bathroom and toilet, which are on opposite sides of the top floor) or do we need one in each room - I have seen differing advice on that?

For background information: We are generally happy with the fabric of the building. It has had cavity wall insulation and double-glazing put in at some point, but the seller could not tell us when (he was handling the sale for his mother who was moving to a care home, and she could not remember). Anyway, the house seems pretty tight - none of the windows or doors have drafts, but I don't have an air permeability measurement. There was a gas fireplace which we removed and blocked up, following the gas engineer's recommendation. We are keeping the heating very low to save money and GHGs, so room temperatures fluctuate between 15 and 17.5C in winter. I would imagine the previous owners used to have the heating on much higher, so probably had less damp because of that.

Thanks for all your help!

Hello and welcome to the forum.

I have a centralised MVHR system, put in when we built the house, so I'm probably biased It works well. I think if you want to investigate decentralised systems then you probably want to contact some of the manufacturers and ask them about installation as well as their particular products and the system design that they all seem to offer.

Lunos has a good reputation, I believe. There's a list of PH-certified devices at https://database.passivehouse.com/en/components/list/ventilation_decentralised_single to give an idea of the best/most expensive stuff. There are various other brands - other people here may have more experience with them.

You will probably not be able to eliminate condensation etc as long as you keep the temperature so low. More insulation would help to raise the temperature.

Posted By: LittleRadRidingHoodHello lovely people!
...wiring a few of those seems so much easier and less costly than ripping through lots of walls and ceilings to fit all the ducts for a centralised system, all of which then need additional work to cover them up.

I have (DIY) retrofitted centralized MVHR using rigid ducting so like djh I am probably biased as the system works as intended and I am very happy with it.
Fitting the inlet/extract spigots in ceilings is an easy job when using a dust catcher around a drill mounted circular saw: practically dust free and no remedial work required. I needed to fit 2 risers at the first floor, and core out 2 large holes through internal brick walls. Inlet and exhaust holes you need for all types of systems, more in number for a distributed system. Any dry liner worth his/her salt can box in and cover up riser ducts in no time.

I considered distributed MVHR. An overlooked advantage of a central system is the whole-house air flow plan that comes with it. This is tricky to achieve and balance using distributed units. When I totted up all the cost & work for 2 distributed units inc chasing in the required cables etc and the larger numbers of holes through external walls it was hardly cheaper or simpler than a centralised system, and certainly less efficient and harder to calibrate.

Retrofitting MVHR is tricky, but that goes for both centralised and distributed systems alike. Looking back it was well worth it. Amongst other things it eliminated the condensation problems on/around our N facing DG windows.

Do you have spot temperatures round the window reveals below 12C?

It should be very easy to fit a small MVHR in your loft to upstairs, there may be places that you could run ducts to downstairs behind doors, in corners of rooms,through the airing cupboard.

Central system will solve problems , but will increase your energy use ( it can be very efficient but will use energy)

Whispered more info

A decentralised MHRV uses about 3x less energy to overcome duct friction than a centralised MHRV does, so saves energy, *only if* both options have similar heat exchanger % efficiency and similar motor efficiency.

But more importantly IME, a decentralised MHRV allows you to boost the ventilation in just the room that needs it - eg boost the bathroom ventilation while showering, or the kitchen while cooking, without having to unnecessarily boost the guest bedroom ventilation at the same time. Also you don't have to keep all the rooms at the same temperature at the same time, eg the upstairs can be a bit cooler than the downstairs during the day and vv in the evening, if you want to save heating. Depends a lot on the individual house, and how the occupants like to live in it.

LRRHood, we have similar project to yours and went with kind-of-decentralised MHRV for same reasons - difficulty running ducts and no space for big manifolds. We are living in the house and slowly renovating one room at a time, which doesn't fit well with installing ducts everywhere all at once.

Eg we have vent axia Tempra dMHRV in utility room to dry clothes (very effective), by opening connecting doors it also ventilates the kitchen and/or the downstairs shower room and has fixed the condensation in those. Bit cheap and noisy for bedrooms though.

Also look at "two room" dMHRV units (eg HR100, though again pretty basic).

The push-pull units seem to be catching on in Germany.

Edit to add : dMHRV units are a lot cheaper to retrofit than ducted systems, so I haven't seen any specialist companies doing that, their overheads would kill the project cost. Tends just to be job for general builders or electrician to fit the units, they won't have particular knowledge to size the system. Possibly the online ventilation super store type retailers could do that, but their mindset is Building Regs flow rates for new houses which are overkill most of the time. I'd start with units in kitchen and bathroom and see if that fixes the problem.

Further edit: if you are saving heating by having the house cold, beware you need a lot more ventilation, because the cold air carries a lot less humidity before it condenses.

Thank you all for the helpful advice!

<blockquote><cite>Posted By: Mike1</cite>If you choose decentralised units, you want them on the same wall of the house. If they're on opposite walls you've created a wind-tunnel.</blockquote>

Very good point, I was thinking that's exactly what you'd want, but it's not if you are ventilating actively, as the fans will either be working against the wind pressure (meaning less throughput) or have the air rush through faster than it should (meaning less heat recovery).

<blockquote><cite>Posted By: WillInAberdeen</cite>Edit to add : dMHRV units are a lot cheaper to retrofit than ducted systems, so I haven't seen any specialist companies doing that, their overheads would kill the project cost. Tends just to be job for general builders or electrician to fit the units, they won't have particular knowledge to size the system. Possibly the online ventilation super store type retailers could do that, but their mindset is Building Regs flow rates for new houses which are overkill most of the time. I'd start with units in kitchen and bathroom and see if that fixes the problem.
</blockquote>

Yes, that sounds very plausible! I think I will just write to a couple of manufacturers and see what they say. I suppose another benefit of the single-room systems is that I can just put them into the kitchen and bathroom and see if that's enough.

I am less concerned about the efficiency differences between centralised and decentralised MVHR systems, as both have got to be way better than all the energy lost when ventilating without HR.

And maybe I'll have to just heat more... That will be easier once I can get an ASHP installed, but that seems to be bizarrely difficult with our very ordinary house.

half a kWh per day, or perhaps about the same as lighting? There's definitely a danger of worrying too much about the consumption of an MVHR system.

So about 1kWh/m²a, and duct friction is actually helpful towards the 15kWh/m²a budget, for part of the year. Agree that the actual air heat losses from unnecessary ventilation are more interesting than the electricity, if the SFP is good.

wind tunnel

I've not seen any issues with wind tunnel effects. The theory sounds plausible*, but in practice the air speed squirting out of the vent terminals is fast enough to overcome the wind pressure that we get in housing areas, and the vent terminals are angled down to deflect the wind. Our vents are on one side of our house and opening a window on the other side doesn't upset them.

(*the fluid dynamics of the wind over houses are interestingly counterintuitive, but very geeky! The wind sucks roofs off houses, rather than blowing them off, and sucks air out of vents and chimneys.)

Our sr-mhrvs have much smaller terminal outlets than the equivalent 'duct' diameter, so the air comes out of them much faster than 2 m/s. Because there's no appreciable length of duct (perhaps 500mm) they run happily at much greater air speed than would be acceptable in a lengthy duct system. The airflow in the heat exchanger core is even faster, I suspect this is the main pressure gradient that resists 'blow back' through the system.

The stagnation pressure rise is a misconception (overestimate) about wind and houses, because the wind doesn't stagnate (stop) - it blows over and around. It can't stop, because more wind is always arriving. (Similar to the Betz limit with wind turbines). In fact the opposite happens - the house deflects the wind upward, squeezing the stream lines together and so accelerating the wind, which reduces the pressure over the house in compliance with Bernoulli's energy balance, so the roof is sucked upwards, like a wing. Hopefully it only flies in extreme winds. I did mention geekiness!

I've done a little more reading around this issue. It appears that there might be a problem that specifically affects single-room ventilators rather then centralised MVHR systems, and especially SRMVHR systems that use axial fans rather than centrigual fans.

https://www.airtechsolutions.co.uk/blowing-in-the-wind says "Ventilation systems specialist Airtech commissioned independent tests from BeataFlow Limited, a leading British ventilation/airflow R&D company, to examine a typical single room HRV unit of the type known to have been approved by EST. Test results revealed that – even on boost – with an external wind speed of 5 m/sec no air whatsoever was extracted by the HRV unit. In boost mode with an external wind speed of 7 m/sec – a wind state that occurs frequently in the UK – the air flow through the unit was actually reversed and would be blown back into the room unless it were (unrealistically) hermetically sealed. In trickle mode with an external wind velocity of 1 m/sec, i.e. barely a breath of wind, almost no air was extracted: at wind speeds of no more than 3 m/sec the air flow was again reversed and blown backwards." Oops, although I've no idea how accurate it is.

And then https://www.sciencedirect.com/science/article/abs/pii/S0378778820334757 which I believe is a refereed journal says "Room ventilation units (RVUs) with heat recovery represent an alternative ventilation solution for renovated residential buildings, allowing simple installation through the façade. However, wind pressure on the façade can have a substantial impact on fan performance and thus affect the airflows through the RVU. Airflow imbalances between supply and exhaust negatively affect total heat recovery and supply air temperatures." I've only read the abstract since that's easily available.

There's also https://www.mdpi.com/1996-1073/12/13/2633 (from 2019, not 1996!) which says similar things and which points out, as the previous one also mentioned, that stack effect is also relevant as well as wind effects. "The laboratory measurement results confirmed, that the axial fan used in the ventilation unit was not capable to work in typical pressure conditions occurring in multi-story building in cold periods, in order to achieve sufficient air change rate, heat recovery and supply air temperature, with noise levels under acceptable limits."

My personal conclusion is that I'd avoid axial fan units at all costs, and I'd strongly prefer centralised systems over multiple single room systems even if they use centrifugal fans. And personally I'd trust things that are PH-certified a lot more than national tests.