MVHR Duct Design

[Triassic]

I've just had a couple of quotes back for a MVHR system for the new house.

 

Looking at the quotes and prices online I'm finding them all to rather inflated, for example, the MVHR unit in one case is £1,100 more than I can buy it online. In the second quote, the ducting is the inflated element, by about £830. In both cases, they want around £300 up front to design the ducting layout.

 

So the question is, how difficult is it to design the ducting layout and do a DIY install?

[Jeremy Harris]

I didn't find it hard at all to design the layout, the only slightly difficult bit was pulling the ducting through the webs of the Posi-Joists on my own; it's a job that would have been a lot easier with two people, just because the stuff tends to snag on the edges of the metal webs.

 

The basic design rules are to locate extract ducts above sources of water vapour, and make the path length from the fresh air terminal in any room to the point where air will be extracted (typically under a door) as long as possible.  I aimed to try and get the fresh air supply terminals diagonally opposite the door where air would get extracted from, within the bounds of practicality.

[willbish]

I will PM you the guy I am using.

 

He is £500 upfront for the design which seems a lot. Especially as Ive already designed all the duct runs, valve locations, unit location etc.

However the total quote for all the equipment and design is cheaper than Ive been able to source elsewhere.

CVC were the closest on price for the identical products. (Incidentally very helpful advice there) but couldn't beat the price.

 

Mine will be a DIY install with support if necessary

 

[Triassic]

@JSHarris   I recall that there are some rules on extraction rates, where can I find these? Are there any other technical details I need to address?

[Jeremy Harris]

The rates are in Part F, the section dealing with continuous mechanical ventilation.  There's worked examples in there to help in Appendix C.

[Gone West]

2 hours ago, Triassic said:

So the question is, how difficult is it to design the ducting layout and do a DIY install?

Can't be that difficult as I did all mine . I've got layout drawings you could use as a basis for yours if you want.

 

I found the building regs. extraction rates too high and after the house was signed off I reduced them to PH levels. Again I've got my tables of PH ventilation rates if they would be of any use to you.

[Jeremy Harris]

3 minutes ago, PeterStarck said:

I found the building regs. extraction rates too high and after the house was signed off I reduced them to PH levels. Again I've got my tables of PH ventilation rates if they would be of any use to you.

 

 

Me too, I wound our ventilation rates down a fair bit after I'd had the test report accepted by building control.  I'm not sure where the building regs figures came from, but they seem to demand much higher continuous rates for a house with continuous mechanical ventilation than would be the case for a house fitted with just trickle vents and extractors. 

[ProDave]

If using the radial duct system, the "design" is really easy.  You just have to find a route to get ducts from all outlets and inlets back to plenum boxes then a large pipe from there to the mvhr unit.

 

I found that easiest to work out on the hoof with a complete bare shell.  then it was barely a days work to pull all the ducts through.

 

I have worked on a building where they guys were trying to follow a detailed drawing that showed every single joint and bend overlaid onto the timber frame drawings, and in many places it was blindingly obvious there was an easier and simpler route to run the ducting.

[dpmiller]

The guys at BPC did a design for me for a few quid against a future order ( gave the BCO a nice official drawing with flow numbers on...) and they got it pretty much spot-on. I'm doubling-up on the studs in two small wall areas to give the ducts ( and rising wiring and plumbing) plenty of space and moving one pair of ducts to negate boxing in  but their 3d drawing is pretty close and darned helpful...

[Alexphd1]

I have tried speaking to a few companies for design and supply of a system minus the mvhr unit. As soon as I mention I will not be buying a mvhr unit  I never hear from them again, I would have been happy to pay a small premium for their service and I told them this, their loss! It going to be the basic flow calculations I may struggle in so I may be asking some simple questions shortly! 

[Triassic]

Just a thought, from what I’ve read the flexible ducting comes in two sizes 75mm and 125mm. I assume as I’ve got a large footprint house 310m2 I’ll need to use the larger stuff?

[willbish]

Room size is the factor not total area. If you've got big rooms you may need to double the 75mm, 2 ducts to a single valve adapter. If you've got really big rooms then the larger duct may be necessary.

[willbish]

As the 75mm duct is so much easier to work with, if you can't reach the required volume of air with 2x75mm instead of increasing the duct size it might be easier just to add another terminal.

[Jeremy Harris]

If using 75mm semi-rigid ducting, then, as a reasonable rule-of-thumb, and if not running tens of metres in a single run, then you can flow up to about 8 l/s at virtually no duct noise as a maximum.  For the boost extract rates you can comfortably double this, as some duct flow noise is usually acceptable for short periods when the MVHR is in boost mode.  I was cautious, and ran double ducts to our kitchen extract to ensure that I could meet the higher BR requirement of 13 l/s and then found I had to restrict the airflow down a great deal to the point where it was clear that a single run of ducting would have been fine.  In practice, it's best to design for the required whole house ventilation rate from the equation in building regs, as that tends to dominate and determine the quiet duct flow requirement.

 

I should add that it's not a good idea to use flexible ducting for anything other than the short, vibration-isolating, connections to the MVHR unit, as flexible ducting has a higher loss than either semi-rigid or rigid ducting (the semi-rigid ducting, which comes as 75mm OD round or 51mm x 114mm oval, is almost the same loss as rigid ducting).

[PeterW]

I’ve done a spreadsheet that shows you how the different duct sizes affect flow, and takes your MVHR and does a very crude balancing and validation of BRegs values. I’ll post it up

[lizzie]

17 hours ago, PeterStarck said:

Can't be that difficult as I did all mine . I've got layout drawings you could use as a basis for yours if you want.

 

I found the building regs. extraction rates too high and after the house was signed off I reduced them to PH levels. Again I've got my tables of PH ventilation rates if they would be of any use to you.

I have a man coming in a few weeks to install enthalpy unit and recalibrate air flow as I am using F7 high filtration filters and manual says it needs to be reset for that.  I think the air flow is quite high as it is so I would be really interested to see your tables @PeterStarck I certainly don't want any higher and I would like it reduced if possible so if I had something to show him would be good.  This is not the original installer who is still saying he is going to come but never fixes a date so after much searching I found a new company, have to wait a month though but its all booked in so fingers crossed.

[Pete]

1 hour ago, Triassic said:

Just a thought, from what I’ve read the flexible ducting comes in two sizes 75mm and 125mm. I assume as I’ve got a large footprint house 310m2 I’ll need to use the larger stuff?

I have similar size house and BPC have just doubled up the ducts to each terminal. I suppose it comes down to cost of materials and how much room you have to run things. As an aside I was pretty naive when I ordered the materials and bought everything from BPC. I have since found out I am possibly two rolls short of ducting and rather than pay for two rolls from BPC I have bought them from Plastics Express which do 75mm ducting. The outside diameter is the same but inside it is 2mm narrower. So I have just bought some couplers and will use a short length (50mm) of the BPC ducting from the plenum box then a coupler and then use the new ducting from there. The difference in the cost of the rolls is over £100!!

(Sorry if this is off topic)

[Triassic]

41 minutes ago, Pete said:

The difference in the cost of the rolls is over £100!!

(Sorry if this is off topic)

The cost is bang on topic. I've got three MVHR quotes and all of them inflate some element of the quote. The quotes are kept obscure and you only get a design with an order, hence my desire to DIY the design and save money.

[Jeremy Harris]

I've just checked and I managed to get away with a two 40m rolls of 75/63mm semi-rigid duct with only about 3 or 4m leftover, for a 130m² floor area house, with pretty short duct runs.  The duct cost just under £300, the terminals, restrictor rings, plenum chambers and 160mm flexible ducting to connect to the MVHR, together with the external terminals, bends etc, cost another £700.

[Triassic]

I've got a high volume kitchen family room in the new build (177m3).

 

Has anyone come across any guidance on how to deal with such high volume spaces when designing a MVHR system?

 

 

Edited February 14, 2019 by Triassic

[Jeremy Harris]

At the building regs kitchen extract rate of 13l/s, it would take around 3.78 hours to get one air change, which is too low.  I'd work on aiming to get around 1 air change every 2 to 2.5 hours; we run at around 1 air change every 2.4 hours and that seems to be OK.  For a room with a volume of 177,000 litres that means an air change rate of about 20.4l/s.  This is too much flow for a single extract terminal I think, especially as it needs to be quiet, so it looks as if you will need two extracts and probably two fresh air terminals at the opposite side of the room, in order to get the required performance at a low noise level (each terminal would normally be operating at around 10l/s).  Doubling up semi-rigid duct runs to each terminal would probably be a good idea, the standard HB+ type 75mm duct terminal connections accept two ducts anyway, one of the duct connections is just plugged if only a single duct is used.

[Triassic]

Is there a list of the typical air change rates for each type of room? The building regs only talk of continuous extract and ventilation rates in l/s.

[Jeremy Harris]

57 minutes ago, Triassic said:

Is there a list of the typical air change rates for each type of room? The building regs only talk of continuous extract and ventilation rates in l/s.

 

Not in building regs specifically, although they do give a total house ventilation rate, which is 0.3l/s per m² of floor area.  IIRC, PHPP normally gives a ventilation rate that's around 0.3 air changes per hour, which is a bit lower than I found to be comfortable (we run ours at just over 0.4 ACH).  I think if you aim to be able to run the background ventilation at around 0.4 ACH, with the capacity to be able to boost this to around 1 ACH or more, then you should be around the right ball park.  It ends up being a matter of choosing an MVHR from any given range that's the best fit, anyway, and we ended up with a bit more ventilation capacity than needed just because the next model down was really a bit too small.

[Triassic]

Can someone sanity check my calculations? Where do I go from here?

 

I've split the kitchen, so it has 2 Extract and 2 supply inlets/outlets.

 

 

 

Edited February 14, 2019 by Triassic

[Jeremy Harris]

Two points, AFAIK you cannot get 125mm semi-rigid duct, it only comes as 75mm OD, or 51mm x 114mm oval.  Probably best to use double runs of either for the kitchen, just to be sure of getting enough flow on boost extract.

 

The second point is that you need to meet the 0.3l/s per m² floor area requirement in building regs, which means that the total extract (or supply) needs to be 177 x 0.3 = 53.1l/s as the background ventilation level.  The sum of all the extracts, or the sum of all the supplies (should be exactly the same number for a balanced system) needs to be at least 53.1l/s.  Also, ideally you should aim to keep the flow velocity below 2.5m/s in the ducts if you can, as that's about the threshold where duct flow noise starts to be heard (in practice I've found you can go a bit above this without any noticeable noise, though).  For those runs where you're getting up to around 2.8m/s - 3.2m/s I'd be inclined to double up the 75mm duct runs, to reduce the velocity a bit.