MVHR unit poorly installed options?

[tomds]

Hi Everyone,

 

We moved in to our house in 2011. The property is a 3 bed new build with a Vent Axia sentinel kinetic V unit installed in the loft. Our house has always been extremely hot in the summer, but I always put it down to good insulation. I appreciate these units are not 'aircon' but it always feels very stuffy with no airflow. Early last month I was talking to a neighbour about how hot the houses are, and he brought up the MVHR unit. He mentioned that his had been getting poor airflow and he had someone out to look at it. He said this improved things with the heat, so I decided to follow his course of action and had a company come out and 'service' it. 

 

The results from the service were pretty shocking. The guy measured out house and said that our unit should be performing at an input and output of 35 litres of air per second. From his device he stated it was performing at 12... Even on the boost function he could only get it up to 17, which is still over 50% less than what it should be. The guy said that it had been installed poorly, with flexi ducting being used rather than rigid ducting. 4 inch flexi ducting had been fitted to the larger ducting to supply the individual rooms. He said that ultimately it would need a refit if I wanted it to improve at a cost of up to a few thousand pounds..

 

I immediately contacted NHBC to see if the unit was covered under the warranty, but they advised it isn't. I have also contacted the builders (Hillreed/now Persimmon) to see if they can help at all. 

 

My question is that based on the above, and if I don't get anywhere with Hillreed, how hard would it be to do myself? From looking at it, I think I could quite easily replace the ducting in the loft and also ducting the supplies the first floor rooms. Would this make a significant difference? I'm guessing ground floor rooms would involve taking up floor boards on the first floor? I can add some pics to show you how it looks at the moment if needs be.

 

Thanks in advance,

 

Tom

Edited July 10, 2017 by tomds

[Nickfromwales]

Hi tom and welcome. 

A lot of guys on here will be along shortly, but please hold off doing anything until you had some replies. 

The flexi ducting is extremely popular here and by no means a bad / incorrect product. . 100mm is also quite big so no problems there TBH. A camera survey of all the duct runs would be where I'd first spend money to ensure no kinks or flat spots. 

The company you've had come out and condemn it sound like they know very little about this system TBH. 

Hang around and we'll steer you in the right direction. ?

[jamiehamy]

Couple of things to check - did you keep the old filters or see them? First thing I would expect on an inspection that resulted in such a drastic recommendation was to see the 3 year old filters... 

 

Secondly,  did they do any form of inspection of the ducts? It's perfectly possible,  although unlikely,  they are blocked partially. 

 

Thirdly,  what checks did they do on the unit itself? Could have a worn motor (again unlikely). 

 

The piece around flexible ducting is bollocking frankly - used all over the place. 

 

Is the company that did the inspection the same one proposing to do the works? 

 

Until they provide more evidence of the problem (taking their airflow readings as read) I wouldn't be dreaming of the planned course of action. 

 

I would add that a perfectly functioning system cannot stop a house feeling warm - it can reduce with summer bypass mode but all depends on outside air temperature. 

[Jeremy Harris]

Welcome.

 

I'm afraid I'm not the slightest bit surprised to find a new house with problems, I get the very strong feeling that house build quality is getting poorer, especially with some of the big name builders.

 

There are several issues that could be contributing to your problem, and it may well not be the MVHR that's the major issue, either, or at least not directly.  In general, good insulation keep heat out as well as it keeps heat in, so it's not automatically a given that a well insulated house will get hot.  Additionally, mass house builders don't build well-insulated houses anyway, so you may well find that the insulation level is such that it just barely meets the building regs requirement (and my experience is that often it may not even meet that standard).

 

Flexible ducting means many things.  Semi-rigid ducting is fine, and is not "flexible ducting" in the general sense of the term.  The really poor flexible ducting is the stuff that has an alumised (shiny) or possibly flexible PVC, skin with a wire spring to try and maintain it's shape.  Long lengths of this are bad news, as it does impede air flow a lot when compared with other types of ducting.  Big builders love it, as it's cheap and quick to install.  The only place it should ever be used (and then only as a last resort) is as a short length to act as a vibration and noise isolator at the duct junctions to the MVHR unit.  Even then I'd not use it, as there is an alternative, in the form of the concertina aluminium ducting that isolates noise and vibration just as well but has much lower flow resistance.  A bit more information on what you have would help.

 

It's also important that the MVHR is sized correctly for the size of your house.  It's quite possible that the builder has installed a system that's either too small, or right at the lower limit of what might be acceptable.  If you can give the total internal floor area (all floors) of your house, plus the model of MVHR, then it's easy to do a quick check.

 

When were the MVHR filters last changed/cleaned?  Generally these need to be cleaned or replaced at around 6 month intervals, as they clog up with dust which then reduces the air flow rate.

 

MVHR won't have much impact on house temperature in summer, and if the MVHR unit and ducting is located somewhere hot, like the loft, and not well insulated, then it may contribute to daytime heating a little bit, rather than decrease it, but again it won't be a massive effect, as MVHR just can't move large amounts of heat, even when all the air flow rates are set properly.

 

Your overheating problem may well come from solar gain through the windows, and, perhaps, solar gain through the structure of the house, especially if the insulation level either isn't that great, or if the insulation and structure of the house combined has a low decrement delay.  The latter gets a bit complex, but if your house has somethign like a brick or block outer skin then the decrement delay should be reasonably long, and the problem is more likely to be elsewhere. 

 

Reducing solar gain can be achieved in several ways, from adding external screening in some form (shades, slats, a brise soliel, solar blinds etc) to fitting external reflective window film (we have this, it's very effective, but not cheap).

 

 

[Stones]

@tomds Can you confirm what you mean by flexi? Many on here have used a semi rigid flexible duct which is really easy to install. At 100mm I assume you have a flexible aluminium foil type duct (insulated?). Can you confirm how the main ducts are run from the MVHR unit? Do you have distribution boxes with individual duct runs to each room, or a main trunk duct with branches off to each room?

 

Builders often use 100mm for single extract fans duct runs. The Vent Axia has 150mm spigots so I would expect 150mm for either the main duct run or to the distribution box.

 

What state were the filters in?

Edited July 10, 2017 by Stones
Cross posted

[ragg987]

Flexi ducting is not always bad. The type that has concertina effect on the inside (like you used to get with tumble dryers) is very bad for airflow, though the type we used is lined internally so it has a (relatively) smooth bore and low losses.

 

Have you checked the inlet and outlet for blockages (leaves etc), and also have you tried to replace the air filters? Also might be worth checking the heat exchanger for blockages (with care it is delicate).

 

One other pointer, does your system need rebalancing? This is done by adjusting the room valves to achieve a balanced in- and out-flow, and also adjust for the fact that long duct runs lose more pressure than short ones. Requires an air-flow calculation and meter to do properly.

[Jeremy Harris]

5 minutes ago, ragg987 said:

Flexi ducting is not always bad. The type that has concertina effect on the inside (like you used to get with tumble dryers) is very bad for airflow, though the type we used is lined internally so it has a (relatively) smooth bore and low losses.

 

 

 

I agree, but the stuff I described, that uses a steel spring to try and keep it open, is bloody awful, and that is often used by mass builders because it's cheap and quick to install.  The floppy stuff with the steel spring inside also collects duct and dirt, because the higher surface friction causes the air flow to slow right down and allow dirt to drop out of the air flow, I think.

 

The semi-rigid ducting used for radial flow systems is fine, as it has a smooth liner, and the test data shows that it has a flow resistance that is near-identical to rigid ducting, with the exception of slight losses at bends (but then the losses on rigid bends are higher anyway). 

 

The other form of flexible ducting, the stiff aluminium stuff that has very shallow concertina grooves, has a far lower flow resistance to the floppy stuff with the spring steel spiral support.  It's not as easy to use, but seems fine for the short isolating lengths at the MVHR unit itself.

 

 

[tomds]

14 minutes ago, ragg987 said:

Flexi ducting is not always bad. The type that has concertina effect on the inside (like you used to get with tumble dryers) is very bad for airflow, though the type we used is lined internally so it has a (relatively) smooth bore and low losses.

 

Have you checked the inlet and outlet for blockages (leaves etc), and also have you tried to replace the air filters? Also might be worth checking the heat exchanger for blockages (with care it is delicate).

 

One other pointer, does your system need rebalancing? This is done by adjusting the room valves to achieve a balanced in- and out-flow, and also adjust for the fact that long duct runs lose more pressure than short ones. Requires an air-flow calculation and meter to do properly.

Thanks for the tips, the ducting used has the concertina effect on the inside. The guy rebalanced the air-flow but it made little difference. I clean the filters every 6 months, but I haven't checked the inlet and outlet for blockages though. 

[Triassic]

Could we see a photo of the ducting?

Edited July 10, 2017 by Triassic

[tomds]

25 minutes ago, Triassic said:

Could we see a photo of the ducting?

 

[jack]

I haven't time to post on the calamity that is the ducting you've shown, but a base rate of 35L/second seems high to me. We have ~290m² 4 bed house, and from memory our unboosted flow rate is something like 20 L/second.  We regularly run it at less than that with no ill effects.

 

 

[Declan52]

That ain't a pretty setup. As your unit is installed in a cold roof all them ducts should have insulation over them at the very least. The inlet and and outlet are fine without it but any duct serving a room needs to be covered or you will get condensation in it.

Can you show a pic of the distribution box where all the ducts join up???

The actual unit would need wrapping in insulation as well.

What is the Y connector doing in that pic??

[Nickfromwales]

They look like they're all 100mm nominal vs 150mm with insulation to me. Strange that the inlet and outlet aren't 150/200mm eg upsized. 

The junctions should have been insulated and it looks like the branches tee off from the first runs, not good. Probably either not balanced or will be difficult to do so whilst keeping the noise levels down. 

 

[PeterW]

Hmmm ... I don't even think that is installed to the MIs to be honest ..!! If not you could tell NHBC to sort as it is a defect that should have been corrected. 

 

Lots to sort there - but not difficult with a bit of effort if you are handy. 

[ProDave]

With that set up, I would expect the extract runs from bathrooms to be full of water from condensation by now.

[Jeremy Harris]

That is indeed as expected, totally crap ducting - the cheapest, nastiest possible way of doing the job.  That stuff is technically "insulated", but frankly the insulation is a bit of a joke, and the airflow resistance is high.

 

However, I very much doubt if the MVHR has any bearing at all on the overheating problem.  An MVHR system simply doesn't shift anywhere near enough heat to effect over heating, plus if the air outside is warmer than inside any ventilation, be it MVHR or just opening windows, will make the house hotter.

 

The key to reducing internal temperature is to shade windows on the outside, keep windows and doors closed during the hottest part of the day (and turn off the MVHR) and then open windows and turn on the MVHR at night, when the outside temperature is cooler (the "night purge" system that's very common in Mediterranean countries). 

 

You could also have a look to see if the insulation level is both reasonably good and free from voids.  Mass housebuilders are generally poor at fitting insulation properly, and good insulation will help to keep the house cool, by reducing the amount of heat that gets in from outside.

 

 

Edited July 10, 2017 by JSHarris

[richi]

33 minutes ago, JSHarris said:

The key to reducing internal temperature is to shade windows on the outside.

Agreed. We got two of these for our new south-west-facing Veluxen:

 roofwindows.co.uk/blinds/by-type/solstro-sunscreen-awning-blinds-for-velux-roof-windows

So far, they seem to work well.

Edited July 10, 2017 by richi
fmt

[tomds]

Thanks for the replies, I don't believe there is a distribution box, everything is run off the ducting coming from the unit. The Y connector is feeds off the first runs going to different rooms. It looks horrendous in comparison to other setups i've seen on here. If I get no where with the builders I think I may have a go at a refit myself...

[Nickfromwales]

Take photo and video evidence plus the testimony of the company that wants to rip it all out and start again and tell the house builders you'll go to the courts for compensation, and explain that you feel that you should discuss this with all of the neighbours to make them aware that they may also have incorrectly / poorly fitted systems. 

Explain that you'll only use a 3rd party contractor of your choice to carry out reparations and see with what / how they respond . 

Dont threaten them, just a polite letter of how you intend to proceed. If a camera survey shows moisture and mould growth in the bathroom / kitchen ducts then it's also a health hazard so maybe get the video survey done first for peace of mind ( and ammunition ? ). 

 

[Jeremy Harris]

It looks as if the system could be properly re-installed easily enough, using rigid ducting, and with much better duct insulation, plus insulation around the MVHR unit.  Ideally, the MVHR and as much of the ducting as possible should be inside the heated envelope of the house, but failing that it needs to be very well insulated.

 

As mentioned earlier, I very much doubt that doing anything to the MVHR will change the house overheating problem, as MVHR just doesn't shift enough heat around to make a significant difference.  The overheating is almost certainly being caused by solar gain, most probably through the windows.  Modern glazing units work as partial one-way radiated heat transfer paths; they have an internal long wavelength IR reflective coating that reflects heat back into the house, but which lets heat in from the sun.

Edited July 11, 2017 by JSHarris