I too was about the by the Kair K-HRV150 but was concerned about the fact that we would have to mount it on a wall that receives the prevailing wind- as we are relatively exposed and in Cumbria, that can be alot! I asked Kair about it and the guy said this:
‘I would look at the Kair Smart Fan, which is a continuous running fan that runs on a low trickle speed and will boost when humidity is high. There is also a setting on it for constant flow, which will adjust the fan speed to keep the trickle speed at its rate if there is any resistance. As it is continuous running, it is unlikely that you will get wind traveling back up the duct run.’
When I went back to him and said I was really interested in the heat recovery element, he backtracked and said the Kair K-HRV150 was my best option but couldnt comment on how it would fare in high winds- now I’m not sure what to do- and wouldnt the HRV150 be on a constant trickle run as well anyway?
(Don’t have the budget for MHRV).
Ventilation units come in 2 varieties with regard to airflow and constant ventilation, constant flow and constant pressure.
The average run of the mill fan is constant pressure and if the back flow pressure is greater than the flow pressure the air will move in the wrong direction (but slowed by the amount the fan is pushing back).
Constant flow systems increase fan speed to counter the effects of counter pressure, within the limits of their capabilities of course.
In a simple extractor with a round or quadrangle duct you can get anti backdraught flaps to block reverse flow. I doubt that similar exists for the arrangements in a single room single duct MVHR. A more robust two duct system could have a pair of such flaps.
If the system specification doesn’t specify that the system is constant pressure you will need to go back to the supplier for clarification before deciding whether the system is suitable for you.
@pottyone72 how have you got on with your Kair K-HRV150? We finally managed to get an electrician round to look at quoting on fitting one and he basically thought I was mad for not putting in a normal extract fan…losing confidence in my choice now…he will put it in but has pointed out we’ll have a larger than normal hole if it doesn’t work out, and doesn’t see the point.
@Phoebe_Spence @Alan_Mould how noisy are they and what if its humid outside, do you just turn down the humidistat? Do you run it all summer (we would usually just have all the windows open then as relatively secure and quiet here)? thanks
Hello Lisa, i would just say background fan hum, not noisy. There is an option to turn off during the night, when dark.
I just set the humidistat to 65% so it only comes on when necessary. So you could open windows as you wish, it doesn’t turn on unless humid, so you don’t need to be adjusting settings.
I would definitely say heat recovery is worth it. I didn’t notice that the hole in the wall was that large.
But my experience is that the fitter hadn’t read the instructions properly so had to get them back as the side inlets were blocked and the external vent/outer cowl wasn’t correctly fitted. Electricians need to become more familiar with developments. Not easy to find builders/electricians who are interested …
You’d be welcome to visit to see one in action - I live in Stockport
Ignore the electrician unless he happens to be a qualified retrofit coordinator. My plumber hates pipe insulation but I only employ him for his plumbing skills.
If the windows are open in summer you can safely turn the unit off.
My plumber hates pipe insulation too!! 
Hi Lisa
We’ve not had it fitted yet. Only because we’re staging the work in the bathroom and can’t put the fan in until our bath is removed.
Currently we are using our Ebac dehumidifier in the bathroom to dry clothes - once the ventilation unit is in, we won’t need to use the portable dehumidifier in the bathroom .
Hi Lisa,
I agree entirely with Phoebe here. The fan on “standard” mode makes little noise - it is less than the hum you get when a fridge is on for example. We have 2 including one in our ensuite and we always sleep with the door to the bedroom open. The noise never disturbs our sleep and the fan provides some air flow. The fan only goes into boost (which is about the same noise as a normal extractor fan) when the humistat says it needs to - ie when you have had a shower or put up clothes to dry or whatever. As soon as it is below whatever level you set it at it turns back down to “standard” mode.
Sorry but I don’t understand your comment about it being humid outside - the mode changes depending on the humidity in the room, it is nothing to do with the humidity outside.
Like Tim, I would not worry about what the electrician said. Our constant and unaltering experience of a year long retrofit is all contractors want to do what they are used to doing and hate doing anything else, no matter how sensible it is. So plumbers hate pipe insulation; electricians hate any devices they are not used to wiring, builders hate putting in more insulation that they are used to putting it, and hate any taping you want; builders won’t stop dropping mortar in the cavities, etc etc etc. We found it is a constant battle against the contractors you employ. Of course employing specialists in retrofit should get round these problems, but certainly when we need it (6 years ago) they were as rare as hen’s teeth.
All I can say is that the Kair fans we have are excellent and one of the best things we did. Like Phoebe you are welcome to come and see ours if you want, we live in Fallowfield.
And still are. That’s why I bought into People Powered Retrofit.
@Tim_Gilbert @pottyone72 @Phoebe_Spence @Alan_Mould thanks all for your replies, I was just having a wobble!! Thanks for the offers to look as well, as we are in Cumbria we’ll not take you up on it, but you’ve given me the confidence to go for it!
I have wobbles about my choices all the time too 
This group and the people in it provide me with the facts, the science and peace of mind to have more confidence in my choices.
Hi, we have lived with our humidity-controlled heat recovery fans for 4+ years now and think they are great. Once they have dealt with excess activity generated humidity and move into trickle mode they are switched off. We do not run them 24/ 7 as they are not needed and during the warmer months, we just leave window open except at night time when insects are attracted to lights.
Interesting. I appreciate trickle mode for pre-warmed fresh air during the winter.
Agree windows can be opened in the warmer weather.
Good to have all these options and see which works for you.
Hi Lloyd Is your system whole house or are you using single room vents with heat recovery?
Hi, we have three single room vents with heat recovery- bathroom and kitchen.
Thought it better to jump on this thread rather than start a new one. We’re looking for a single room heat recovery extractor for the utility room as there is a lot of heat generated by the tumble dryer that I don’t want to waste when it is in use. Our building inspector is telling us that the rate of extraction must be 30 litres per second and I’m struggling to find a HRU that will extract at more than 15 litres per second. Does anybody know, do they exist?
Welcome to the forum @the_rubik. I think 30l/s is going to be quite noisy. Do you know why such a high figure was specified?
Apart from fitting two 15l/s units at opposite ends of the room I don’t know how you could achieve that level of extraction.
So long as you are ok with the humidity going above recommended levels from time to time and then rapidly returning to normal I should think a high performance 15l/s system would be fine. You would also have to keep the room warm enough to avoid getting near the dew point, but the tumble drier probably does that for you.
(November 2022 price)
Hello @the_rubik, looking at the numbers I wonder if there is mix up with units? 10 litres/second is roughly equivalent to 30 meter cubed/hour. D
Thanks for the reply Tim. So approved doc F of the regs states that the minimum intermittent rate (mechanical extraction) for a utility room is 30 litres per second. The continuous rate is 8 litres per second. My building control officer has quoted the first rate but I’m wondering if he’s got that wrong as I understand that heat returning units work on a continuous basis rather than the type of extractor fan that is connected to the light switch.
Yes possibly. That seems more realistic. The figure he’s quoted me is actually the litres per second figure (30 lp/s), I used a conversion tool (on a website) to do the arithmetic.