Part Thirty Two – More On MVHR And Nasty Smells

This thirty second entry was published originally by JSHarris on the 21st July 2014 and received 2,047 views on the closed forum


Just a quick update, as I’ve not had too much time to work on the house over the past week or so, due to having had my car written off (and then having to spend hours on the phone sorting out all the paperwork etc).
One small problem we had, that I only found after I’d commissioned the MVHR system and done all the flow testing, was that there was no built in way to automatically switch the MVHR to boost mode when the bathrooms or kitchen are in use. The boost can be manually turned up, either by hitting the “party button” on the Optima controller, or by manually selecting a different fan speed, but I thought it’d be nice to have this done automatically. I looked at the Genvex wiring diagram, and it seemed there was an option for fitting a hygrostat, essentially a sensor that closes a pair of contacts when the humidity increases in the extract duct (as it does when using a shower, running a bath or cooking). A quick few phone calls revealed that these things were a silly price, and that even a really dumb, basically fixed point mechanical switch type unit was around £80. I really wanted something better than this, so was going to design and build one myself, when I came across a Polish company that sells some very neat little control and sensor units, including exactly what I needed to automatically control the Genvex MVHR.

The Genvex isn’t at all unique in using a boost input switch, virtually all MVHR units have such provision built in. In almost all cases all that is needed is a way of closing a pair of no-volt (i.e. not mains or voltage fed) contacts to activate boost. Some fit a manual boost switch near to the bathroom or kitchen and just manually turn the MVHR up as required, but as I’d already finished the second fix wiring I didn’t much fancy doing this. The logical thing to do (as I see it) is to sense the humidity in the extract duct close to the MVHR and if it exceeds a pre-set level switch the thing to boost. The MVHR is always “sucking” from bathrooms and kitchens, etc, normally at the trickle rate, so it will suck in damp air if the humidity in those rooms increases.

Sensing in the duct near the MVHR makes things nice and simple, as just fitting a sensor inside the duct and wiring it to a humidity controller allows this to close a pair of contacts at a user-defined level and boost the MVHR to increase the extract rate and get rid of steam etc quickly (and recover all the heat from it in winter).

My solution was to buy this unit from Poland:…latory/dhc-100/

Yes, I know that site is only in Polish, but Polish isn’t that hard a language if you have a few Polish friends around to make sure you absolutely do not accidentally use a Russian pronunciation of some common words shared between both languages………….. Anyway, to make life easier they sell these units on eBay and they come with an English manual, all for around £43 including postage from Poland and the remote sensor. I took a risk and bought one, and fitted it to a surface mount double gang box, screwed up under the duct silencer on the intake side. I opted for the 24V version, as they had one in stock and the Genvex has a 24V supply available for ancillaries (at up to 500 mA). This hygrostat has isolated contacts, so I could have bought the 240V version (same price) and wired it to the mains had I wished. Here’s some photos:
This shows the installation, with the controller mounted under the duct silencer and the sensor cable (the thin grey cable) leading up to the extract manifold. I fixed the sensor module just inside the main extract duct to the MVHR, using a double sided foam sticky pad.

This shows the controller, which is what gives this unit such flexibility. Not only does it display the current relative humidity in the extract duct all the time, but you can very easily programme when it switches the built in isolated relay (to operate the MVHR boost), how much hysteresis you want (the difference between the turn on point and the turn of f point, in 1% RH increments) and whether you want an over-run timer, to keep the MVHR on boost for a time (from 0 to 99 minutes) after the humidity has dropped below the trigger level.

I have to say this is a fantastic solution, as it is easy to see what humidity you need to set to trigger the thing to boost from the display (just nip in and watch it increase when the shower is running, and select the level you want). Being able to set a delay time is also useful, although you can use the hysteresis setting to do much the same (i.e. set it for 5% and the relay will remain on until the MVHR has dried the air in the duct down to 5% below the trigger point, by running at a boosted fan speed). Best of all it was cheap (compared to the offerings from the MVHR people) and very simple and easy to fit. It should work with pretty much any MVHR in the same way.

Some may remember that I was planning an experiment with removing odours from source (toilet pans) using the MVHR extract. I’ve now fitted this system, but cannot yet say whether or not it works! What I did was run a length of 40mm waste pipe along the eaves stud wall in the service space, and connect it up to the extract manifold (it’s connected to the rear of the manifold box in the photo above). This 40mm pipe connects to the plastic built-in cisterns in the bathrooms, via a home made adapter (I just opened up the alternative water inlet hole to accept a 40mm pipe, cut the end off a glued fitting and made a special elbow that could be glued to the cistern). Here’s a photo that should be self-explanatory (the extract is the bigger elbow on the left):

The way this works is that, with the lid on, the air space at the top of the cistern is always at a low pressure, because of the slight suction from the 40mm pipe. The overflow pipe from a built in cistern like this (the vertical pipe just to the left of centre) is connected directly to the flush channel that runs around the upper inside edge of the loo. This means that the MVHR is gently extracting air directly from the loo bowl all the time, so any odours in there will be extracted directly, hopefully before they’ve even had a chance to rise above the rim. The idea is that this will remove odours at source, and not rely on the main room extract to do it once they’ve escaped into the room.

Whether it works remains to be seen, but if it does I shall be well-pleased, as it was a pretty simple thing to devise and install.

With luck I should get a new car at the end of the week and my build schedule can return to normal. It’s been very frustrating having to spend so much time sorting out insurance claims, finding a new car etc, when all I really want to do is get on and finish the house!

Calvinmiddle 21 Jul 2014 08:18 PM:

 You could do a smoke test with the toilet pan and see how much I the smoke gets pulled into the vent.

 A bit of burning sage to “cleanse” the house maybe?


 jsharris 21 Jul 2014 08:31 PM:

 Good idea!

 I still have some joss sticks (given to me as a present from my hippy wild life artist sister, http://cornwallartis…nnabel-harris-0) that would test the system pretty well. Not sure what flavour they are, but I’m sure they’d do the job at seeing if this system works or not.

 There are commercial units that work is a very similar way (like the Odour Buster: but add a fair bit of cost, whereas adding a bit of pipe to an existing extract system is relatively cheap (and in my case), very easy to do.



22 Jul 2014 07:27 AM

Have seen the Odour buster before and they actually have a smoke test on their website so can’t take the credit for the idea. But really interested in If this works as if it does I will likely do something similar.

 May have asked this before but can’t remember your answer, did you include this extract in your balancing of the MHRV system, or add it later and let the house run at a slight negative pressure?



22 Jul 2014 07:48 AM

I didn’t include it in the original commissioning, and I’m going to have to re-balance the system. I’m also going to add two more fresh air feeds, one to each bedroom, to counter the tendency for warm air to rise. I can (very luckily!) do this as the service space and the services eaves void allow me to run extra ducting in. The main issue is that the East window in the spare bedroom allows a lot more solar gain than I’d expected. I’m a bit surprised by just how much sun that side of the house gets. Luckily it will be very easy to add another high level fresh air feed (around 2.5m up on a wall just above and adjacent to the East facing window) and I then plan on throttling down the down stairs fresh air feeds to the two studies and the one in the hall above the kitchen door, as at the moment the ground floor is around 2 or 3 deg cooler than the first floor. I’m hoping that putting more fresh air in upstairs, at as high a level as I can, will reduce this temperature differential.

 Having said that, the bedrooms have only been up to about 23 deg C in this warm spell, whilst down stairs rarely gets over 20 deg C. The killer is if I have to open the bedroom window (which I have to to poke the aerial for my temporary mobile phone repeater out of), as then warm air from outside comes in. Yesterday I had the MVHR off and the bedroom and bathroom (where I was working) was up around 26 deg, too warm to be comfortable and the sort of room temperatures we’ve been living with in our current house for the past week or two.


 DeeJunFan 22 Jul 2014 08:16 AM :

 There is also the Pan Fan

 Seems to be a bit cheaper. They are around £50 on ebay i think.


 sarahsouthwest 22 Jul 2014 12:51 PM:

 Really interesting, thanks for posting.


 joiner 22 Jul 2014 02:11 PM:

 I’m thinking of running a book on how quickly it takes someone to stumble on this blog, copy the hygro-sensor idea exactly and sell the resulting article on ebay for a nice little profit.


 jsharris 22 Jul 2014 04:25 PM:

 I don’t think it’ll take long!

 I hunted high and low for hygrostats that would do this job. There are a few about, but the cheapest I found was around £80, had a fixed switching point and was really intended for switching a bathroom fan on and off. The price of the ones made for duct sensing are crazy, £200 plus. This one was a tad under £44 delivered, took maybe half an hour to wire up and get working, and seems to have a great deal more functionality than even the really expensive ones that a specifically marketed for MVHR use.

 If MVHR installers read this and supply and fit the same units as part of a package, then as long as they don’t rip off their customers by charging a silly price I think it’d be a good thing. In the meantime, anyone who wants programmable automatic MVHR boost control can just copy what I’ve done. If they can’t work out how to wire it up they can ask me and I’ll help them for free.

 We need to take all the mystique out of this stuff, and make more people aware of how it works and what it should cost. That way, even if people don’t want to DIY a solution they will at least have an idea of what something should cost, and how it might work, so will be in a better position when it comes to choosing a supplier/installer.


 Calvinmiddle 23 Jul 2014 09:01 AM :

 I agree with you Jeremy wholeheartedly – I’m still trying to understand how several different companies quoted me £1,500 for installing my MVHR system. THis price didn’t change even when I specc’d the semi rigid ducting that is promoted to cut the installation time by up to 70% (from https://www.airflow….Airflex-Ducting)

 I know you have said that doing a two storey house took you a couple of days and a lot of that time was going up and down between the floor, so can’t see how £1,500 can be justified for a single level dwelling.

 It’s as if there is a “eco” cartel and the prices are just a way to gouge people and make more profit.

 I think this is made worse by building control who just want certificates to show that a “competent” person has done an install. I thought that the job of building control was to check everything, but if a competent person had done it then they didn’t need to check it. But surely that doesn’t mean that they can insist you have a competent person to do everything (by BCO mentioned something in passing about the UFH pipes) as they can check things themselves.


 Calvinmiddle 23 Jul 2014 09:17 AM :

 As something separate to my gripe above.

 I will likely be using the Brookvent Aircycle 1.2 unit that has a integral humidistat which eliminates the need for either a separate boost switch or an in-duct sensor.

 I contacted the manufacturer direct and they will only sell you to a supply and install package so they “can maintain quality of installation” and that install they priced at £1,344.14, also they would only use rigid ducting whereas I want to use the semi-rigid options.

 However I found the unit I want on ebay which works out at £50 less than the manufacturer was looking to charge after the VAT is taken off.

 So looks like I’ll be doing the install myself and then hopefully the BCO accepts that I am competent when it comes to the commissioning.


 jsharris 23 Jul 2014 09:28 PM:

 I suppose the price hikes in the eco/green/low energy stuff is all down to the market exploiting consumer ignorance, which is nothing new, snake oil salesmen have been around for centuries.

 The installation prices being quoted for things like MVHR and ASHPs are just daft, given the work involved. Most ASHP installs are less work than fitting a boiler, yet all I’ve seen seem to cost around double this. There’s no reason for it, other than greed and the desire for some in this market to rip off consumers.

 In many respects, government grants aimed at tackling energy efficiency have contributed to the problem, by distorting the market. As I found recently when looking to buy a car that attracts a £5k government grant, this is still rife. The grant just goes as additional profit to the supplier and the car remains at the original sale price without the grant.


 alanc 24 Jul 2014 12:22 PM:

 Jeremy, Four Candles Alan here. Just a quick note to say what an interesting and lucid read this is. Nice to see that you have effectively finished your project.



 jsharris 25 Jul 2014 07:09 AM :

 Hi Alan, thanks for the kind words. Not sure about “effectively finished” though; building a house is a bit like building a boat. With a boat you get the hull finished and are tempted to think “nearly done now”, only to find that 90% of the work still remains to be done. It’s the same with a house.

 It’s slow going fitting out the kitchen, utility room, WC, bathrooms etc. For example it’s taking me on average at least two or three weeks to do each bathroom, when a proper tradesman would probably have them done in a few days.

 Today is going to be a day spent clearing out the sand in the borehole with a big airlifter, so fingers crossed that by the end of the day we’ll have resolved our ongoing sand in the water problem.


 vk2003 20 Feb 2015 05:25 PM:

 Jeremy, having lived with your MVHR ‘odourbuster’ for ages months now, how do you rate it’s efficacy?


 TheMitchells 27 Feb 2015 02:13 PM:


Can I ask why you did not go for the FreshR ventilation system which is advertised on the Viking website? We will certainly be looking at Viking for our build (following your blog recommendation) but am querying the MVHR system they use. The other one we have been recommended was a Paul system as used by Touchwood.

I’ll have a look at the forum on MVHR to see if an answer is there but there’s a lot to read!

with thanks,



 jsharris 27 Feb 2015 06:16 PM:

 Sorry for being late replying to the last two comments, for some reason I missed the email notifications.

 The house isn’t yet occupied, I’m still working on it, so although I know that the “odour buster” direct toilet bowl extraction works (in that I can measure airflow going up the flush holes around the rim) I don’t know if it works in practice.

 With regard to the Fresh R system, then I think the idea is fantastic, and having seen the units installed I think they are a really good idea.

 There is a very, very major fly in the ointment though, they don’t have any form of certification and cannot comply with UK building regs even if they did, for the following reasons. UK whole house ventilation system regs require that minimum extract rates be achievable from specific rooms, like kitchens, WCs, utility rooms and bathrooms. The FreshR system relies on diffusion to ventilate the house and so cannot comply with the regs. In order to comply conventional extract fans would need to be fitted to bathrooms, kitchens, WCs and utility rooms, which would massively reduce any potential heat recovery (most of the heat recovery comes from recovering waste heat from warm, moist, air, as it’s the moisture that has most of the heat that can be recovered).

 Paul units are pretty good, on a par with the Genvex we opted for, and available with active exhaust air heat pump recovery, I believe, and maybe even with the reversible heating/cooling system like we have in the Genvex.

 I’m still in vehement disagreement with the Touchwood approach to putting the VCL on the outside, and have modelled a few odd corners of the house/outside air temperature/humidity envelope where interstitial condensation could form. Although I think they are a good company, the scientist in me tells me to never, ever build a vapour permeable wall like this. The VCL HAS to be on the inside, as that way way no matter what the conditions long term interstitial condensation cannot occur.


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