MVHR - advice

[TDK-C60]

I need to put an extractor fan into a bathroom which doesn't have any proper extraction and hence suffers from bad moisture build up and the only current solution is to leave a window ajar.

I was looking at fitting just a normal extractor of some sort (into the loft, venting externally) but I'm also considering whether some sort of MVHR system - to generally improve house ventilation as I upgrade loft, cavity wall and windows would be a good option, not least because it can better manage heat recovery.

Any recommendations? Prices seem to vary a lot. The ones that sense humidity automatically look useful, with boost settings tripping in when demand to remove moisture maybe high - and running on a very low setting other times (seem to have fridge like wattages - offset by the heat loss benefits perhaps). I also see that efficiency varies - some claim only 75% or so, others 95%+.

As this would be a retrofit I'm thinking just a couple of vents in the upstairs - the bathroom and (open) landing maybe - would this help circ in the whole house a good degree or do you really need to go to the full faff of piping to each room? It feels like even the former would make a big difference and hence be worthwhile as I can't see me doing every room easily.

Any advice welcome.

[OutInTheShed]

MVHR needs quite a powerful fan to push the stale air through the heat exchanger.

So the energy input to the fans in significant.
In the (South) UK, where out door temps are not that low most of the year, the numbers didn't look overwhelmingly favourable last time I looked at specific units. Bearing mind electricity is about 3x the price of gas, it seemed cheaper to just run a simple extractor fan and put a bit more heat into the rads.

In Scandinavia when it's -10 outside, it's a different playing field of course!

When we're cooking on the hob, that seems like a lot of heat going out of the extractor hood, but of course that's about half an hour a day.

But since I looked into this, energy has got expensive and the kit has got more common, so have the economics changed?

[Minsky]

I am waiting for RobMcDonald of this parish to give me a quote.

I've had a couple of other quotes and was surprised at the low cost compared to what some were talking about at the home building show. Will install it myself - firstly we have a bungalow which makes life easier and secondly I have installed HVAC before.

The cost (and compromise) involved in putting in trickle vents in the (triple glazed) windows we're about to order and also the cost of kitchen extraction made me go for a MVHR. It also helps with your EPC rating.

[tux850]

OutInTheShed said:

So the energy input to the fans in significant.

Be careful with statements like that as it could give the wrong impression.

Our MVHR consumed 125kWh over the last 12 months so that's £40 at today's rates which is nothing given the benefits it gives us. Of course, there are capital and installation costs to factor in - which are still worth it in my view - but in terms of running costs it's not something to be overly concerned about.

[springfan62]

OutInTheShed said:

MVHR needs quite a powerful fan to push the stale air through the heat exchanger.

So the energy input to the fans in significant.
In the (South) UK, where out door temps are not that low most of the year, the numbers didn't look overwhelmingly favourable last time I looked at specific units. Bearing mind electricity is about 3x the price of gas, it seemed cheaper to just run a simple extractor fan and put a bit more heat into the rads.

In Scandinavia when it's -10 outside, it's a different playing field of course!

When we're cooking on the hob, that seems like a lot of heat going out of the extractor hood, but of course that's about half an hour a day.

But since I looked into this, energy has got expensive and the kit has got more common, so have the economics changed?

MVHR isn't just about heat its about air quality. A well configured system in a suitable property will reduce condensation and manage humidity levels as well as providing fresh air, it also remove the need for dreaded trickle vents. But to be effective properties need to be airtight.

[tux850]

springfan62 said:

MVHR isn't just about heat its about air quality. A well configured system in a suitable property will reduce condensation and manage humidity levels as well as providing fresh air, it also remove the need for dreaded trickle vents.

Absolutely. The cost benefit of heat recovery seems to overshadow all the other benefits for some reason in common conversation, perhaps because it's the easiest to quantify. It is only part of the story, as you say.

springfan62 said:

But to be effective properties need to be airtight.

That seems to be a popular misconception.

Here is some Passivhaus research that makes the case for MVHR in nearly all property types.

[OutInTheShed]

springfan62 said:

MVHR isn't just about heat its about air quality. A well configured system in a suitable property will reduce condensation and manage humidity levels as well as providing fresh air, it also remove the need for dreaded trickle vents. But to be effective properties need to be airtight.

This is the classic.

The point of MVHR over plain old MV is to recover the heat.

When you work out that it's not economic to recover the heat, move the goal posts.....

[Minsky]

OutInTheShed said:

This is the classic.

The point of MVHR over plain old MV is to recover the heat.

When you work out that it's not economic to recover the heat, move the goal posts.....

What’s the ROI on your car?

Or when does your kitchen pay back?

Just curious how some seem so opposed to something based on only financial parameters.

[OutInTheShed]

tux850 said:

That seems to be a popular misconception.

Here is some Passivhaus research that makes the case for MVHR in nearly all property types.

Thanks for the link.

I don't disagree it's a good idea for most new builds.
My measure is unfortunately more about cost than CO2, so the crossover point where heat recovery becomes economic is a little different.

Houses vary a lot, but unless you're doing a full renovation, it's going to be hard to retrofit properly.

I do have a potential opportunity for a small system, extracting from a shower room which we use daily and the kitchen, they happen to be accessible to loft space. I'm willing to spend a modest amount of cash, one goal is to have a reasonably effective cooker hood that's very much quieter than the existing, at least on the low and medium settings.

One big issue is that in my location, the outdoors is often on the humid side.

I'm working on the airtightness, I don't know what figure I can hope for though.

[OutInTheShed]

Minsky said:

What’s the ROI on your car?

Or when does your kitchen pay back?

Just curious how some seem so opposed to something based on only financial parameters.

It's not something I'm going to specially enjoy owning.
It does a job. A job which can be done in other ways, like trickle vents, dehumidifiers , the existing extractor fans etc.
It adds complication to the house, anther thing to go wrong.

The units I've seen are a lot of money for a simple heat exchanger and a fan.

Unless it's going to do something for me and not be expensive in the long run, I see no point.


But my question was, have the economics changed in the last few years?

It 's not just the economics though, these things seem to take at least twice the electricity to change the air in e.g. my shower room, compared to a simple ducted fan? So it's not necessarily very green either.

[tux850]

OutInTheShed said:

The units I've seen are a lot of money for a simple heat exchanger and a fan.

Yes, they do seem to command quite a high price point, but I've always assumed that a competitive marketplace (afterall there are many vendors now offering products) would keep excessive profits somewhat constrained and so perhaps some of the component parts are actually inherently expensive. All that said, I've never bought my units (well, the two that I've owned!) at retail price - they've both been 'as new' purchases off eBay at around a third of the off-the-shelf price and I'd recommend that route to others too. Would I still be content to pay full price? Yes, but I'm always one for saving money if I can and MVHR units are commonly bought this way by those that have fitted them.

OutInTheShed said:

Unless it's going to do something for me and not be expensive in the long run, I see no point.

I think you are underestimating the difference between an extractor fan and MVHR system. I don't want to comes across as an evangelist, and nor am I trying to convince you to go down the MVHR path, but I think it is worth pointing out some aspects that might qualify for 'do something for me' - but of course it depends what you actually value and want. Aspects that I value are:

- Whole house ventilation
- Heat recovery from air that would otherwise be expelled from the property
- Reduction of relative humidity
- Quick drying of indoor-hung washing
- Fixed and predictable performance regardless of outside weather conditions (e.g. no difference in effectiveness between windy and still days)
- Near-silent operation (avoidance of intermittent extractor fan noise and noise through trickle vents)
- No need for a cooker hood (some do have one for carbon-filtered recirculated air but we've opted without)
- Reduced pollen and dust
- Slight cooling on still summer nights
- Constant feeling of air 'freshness' (whatever that means - but it is so noticeable it deserves mentioning)

As for 'expensive', then yes I've spent nearly £1500 each time (not factoring in DIY labour time) and then £40/yr running costs as mentioned so I agree it's not cheap but I consider it worth every penny. I've spent similar on a good mattress before, but not for any financial benefit but for the comfort it brings.

Edited by tux850 on Monday 19th December 14:52

[TDK-C60]

thanks all,

i’ve seen some “thru wall” 100mm single room “decentralised” bathroom jobs at £200-300+ but these seem to have the heat exchanger in the short pipe in the wall and rely on the flow alternating. Says 70% efficient and only uses 2W of elec but they appear a bit unconvincing?

in loft whole house units seem to be £900 up - with ducting cost on top but that doesn’t seem a massive cost driver.

i could do three vents easily enough - main bathroom,
landing and perhaps one down to the kitchen via the hot water tank cupboard (which is above kitchen hence easy to duct 100mm/125mm pipe from loft).

It is a bit of an outlay but suspect it will remove all moisture probs. Plus as above putting trickle vents in triple glazing seems a shame if it can be avoided There is also some condensation in our loft I need to sort and assume it will help with this too.

Might be another subject but how do you make a 1980s house more airtight? just caulk up the drafts is an easy answer but how do you really find them? My IR camera only really finds largish cold sink areas.

Any recommended brands or types by someone who has “done the research” welcome. Vent axia etc.

Edited by TDK-C60 on Monday 19th December 15:13

[OutInTheShed]

tux850 said:

I think you are underestimating the difference between an extractor fan and MVHR system. I don't want to comes across as an evangelist, and nor am I trying to convince you to go down the MVHR path, but I think it is worth pointing out some aspects that might qualify for 'do something for me' - but of course it depends what you actually value and want. Aspects that I value are:

- Whole house ventilation
- Heat recovery from air that would otherwise be expelled from the property
- Reduction of relative humidity
- Quick drying of indoor-hung washing
- Fixed and predictable performance regardless of outside weather conditions (e.g. no difference in effectiveness between windy and still days)
- Near-silent operation (avoidance of intermittent extractor fan noise and noise through trickle vents)
- No need for a cooker hood (some do have one for carbon-filtered recirculated air but we've opted without)
- Reduced pollen and dust
- Slight cooling on still summer nights
- Constant feeling of air 'freshness' (whatever that means - but it is so noticeable it deserves mentioning)

As for 'expensive', then yes I've spent nearly £1500 each time (not factoring in DIY labour time) and then £40/yr running costs as mentioned so I agree it's not cheap but I consider it worth every penny. I've spent similar on a good mattress before, but not for any financial benefit but for the comfort it brings.

Edited by tux850 on Monday 19th December 14:52

That's interesting.
Where in the UK are you?
How do you get to £40 a year? is that running 24/365?

Looking at the passivhaus document, I'd need ~100W of fan power to get the 0.5 air changes an hour.
I think that's £280/yr at 32p/kWh.

[tux850]

OutInTheShed said:

That's interesting.
Where in the UK are you?

Wiltshire (South West).

OutInTheShed said:

How do you get to £40 a year? is that running 24/365?

Yeah, it runs 24/7. Realtime steady state consumption is around 12W and periodic boost 40W. The annual summary figures are from a smart relay that monitors power consumption, and is fairly consistent year-on-year.

OutInTheShed said:

Looking at the passivhaus document, I'd need ~100W of fan power to get the 0.5 air changes an hour.

You wouldn't need 0.5 ACH in practice - most find it perfectly fine to operate lower than this (for steady state, not boost) - and if your house isn't to Passivhaus spec level of airtightness then there'll be background leakage too.

Whilst I balanced my systems on first install - partly to get building control signoff (which uses some rather odd figures; they could really do with an update) - I subsequently tweaked (lowered) the overall rates to more what feels 'right' rather than aiming for a notional model-derived target.

Edited by tux850 on Monday 19th December 17:09

[TDK-C60]

tux850 said:

OutInTheShed said:

That's interesting.
Where in the UK are you?

Wiltshire (South West).

OutInTheShed said:

How do you get to £40 a year? is that running 24/365?

Yeah, it runs 24/7. Realtime steady state consumption is around 12W and periodic boost 40W. The annual summary figures are from a smart relay that monitors power consumption, and is fairly consistent year-on-year.

OutInTheShed said:

Looking at the passivhaus document, I'd need ~100W of fan power to get the 0.5 air changes an hour.

You wouldn't need 0.5 ACH in practice - most find it perfectly fine to operate lower than this (for steady state, not boost) - and if your house isn't to Passivhaus spec level of airtightness then there'll be background leakage too.

Whilst I balanced my systems on first install - partly to get building control signoff (which uses some rather odd figures; they could really do with an update) - I subsequently tweaked (lowered) the overall rates to more what feels 'right' rather than aiming for a notional model-derived target.

Edited by tux850 on Monday 19th December 17:09

Very interesting

Which unit do you go for any way? Does the system go into boost automatically? eg detects humidity going up as shower on?

What unit is that power for - m3/h? Whats the rough duty cycle between 12/40? 50/50? or more occasional boost?

With a home battery using EV elec that consumption / cost will be pretty small.

[OutInTheShed]

If you're only playing at it, with a much lower flow rate, then you won't be recovering much heat either.

If I was looking to keep the electricity spend on MVHR down to £40 a year, then maybe I'd just work on the cooker hood and shower vent, where the unwanted air is hotter.

A lot of the time, the temperature difference between indoors and out is really quite small. Right here, right now, in December, it's just 6 degrees.

[TDK-C60]

OutInTheShed said:

If you're only playing at it, with a much lower flow rate, then you won't be recovering much heat either.

If I was looking to keep the electricity spend on MVHR down to £40 a year, then maybe I'd just work on the cooker hood and shower vent, where the unwanted air is hotter.

A lot of the time, the temperature difference between indoors and out is really quite small. Right here, right now, in December, it's just 6 degrees.

Surely the point is you recover 70-90% of the heat that otherwise would go straight outside via an extractor fan, vent block, general leakiness, open window, or trickle vent? That is just throwing kWh away.

Thinking about flow a bit more I'll need to consider how if I extract in one place it needs to be put back somewhere to create a whole "house flow". It looks like that is often ground level - or at least removed from the extract point if in same ceiling. Hmm. Bit harder than I hoped for, for a retrofit. Any clever ideas welcome!

[tecplumbing]

check out vortice, they have a good range of extraction and heat recovery, there technical will advise you. retro fitting heat recovery is a huge job.

[tux850]

TDK-C60 said:

Very interesting

Which unit do you go for any way?

I've used Titon 1.25 Q Eco Plus units both times and have been really pleased with them. Bottom of the range controls-wise but I've rolled my own Pi-based controller (first time as I was looking to save money, second time as I realised it gave me complete flexibility as to how it operates).

TDK-C60 said:

Does the system go into boost automatically? eg detects humidity going up as shower on?

Yeah. Mine has a built-in humidity sensor but it's a fixed threshold and so prone to incorrect triggering with seasonal variation so I've disabled that and use my controller to boost if there's a 5% within 5 minutes. This was intended as a 'strarter for ten' threshold to get things going but I've not witness a single false positive or negative over the years. It also detects pans on the hob too. You can manually boost too, and I've put momentary trigger switches in the kitchen and outside each bathroom. Can also do it via a phone app, just because.

TDK-C60 said:

What unit is that power for - m3/h?

Max output is around 200m3/h but steady state (background/idle call it what you will) we run it at around 25% of that. Boost is around 60-70%. If you ran it at 100% you'd hear it so the unit does need to be sized appropriately.

TDK-C60 said:

Whats the rough duty cycle between 12/40? 50/50? or more occasional boost?

Pretty much 12W all of the time, just with the occasional 30 minutes of showering/cooking-induced boosts:



During the summer we might boost for several hours in the late evening / overnight and activate the bypass to cease heat recovery and instead just get the cooler outside air moving through the house. Don't get me wrong: it's not air conditioning, but it does seem to help (I've sometimes wondered if there might actually be a psychological component to it).

TDK-C60 said:

With a home battery using EV elec that consumption / cost will be pretty small.

The running costs isn't something we think much about, not least given the benefits.

Edited by tux850 on Monday 19th December 21:05

[OutInTheShed]

TDK-C60 said:

Surely the point is you recover 70-90% of the heat that otherwise would go straight outside via an extractor fan, vent block, general leakiness, open window, or trickle vent? That is just throwing kWh away.

Thinking about flow a bit more I'll need to consider how if I extract in one place it needs to be put back somewhere to create a whole "house flow". It looks like that is often ground level - or at least removed from the extract point if in same ceiling. Hmm. Bit harder than I hoped for, for a retrofit. Any clever ideas welcome!

That is indeed the idea, but if you want to recover that heat, you need to be MVHR for the vast majority of your ventilation.
If you're only using £40 a year to power your MHVR and therefore you're either living in a fetid atmosphere or most of your ventilation is by other means.

I think you need several degrees difference between indoors and out to get any efficiency from it.
So it looks like a winner about 3 months of the year?

In my house, I have a couple of rooms which are less frequently used and could benefit from a bit more airflow.
I'm trying to improve insulation and draught proofing now.

Retrofitting is a bugger,
Getting airlow from upstairs ceiling to ground floor level isn't trivial, there's a boxed-in soil pipe where the box might be used, I'm idly wondering about the void behind the dry-lining plasterboard?