Green Building Forum - DIY MVHR design and calculations Tue, 19 Dec 2023 08:26:36 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=254549#Comment_254549 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=254549#Comment_254549 Fri, 17 Feb 2017 22:10:13 +0000 djh Posted By: ringi
Posted By: djhUsing multiple boxes would mean getting back into the bad old days of balancing calculations

Not if the ducts to them where oversized so they had no real effect on air flow rates.
It isn't to do with the size of the ducts, it's the relative resistance of the multiple distribution boxes which depends on how many ducts are plugged into them and what restrictors are fitted.

Posted By: djhas well as taking yet more space

That depends on the layout, I expect that for a lot of homes, having separate boxes on each floor may save space as some of the boxes will fit between floor joists.

For normal two-storey homes, the usual plan is to route all the ducts in the first floor joist space. Under the ground floor or above the first floor are outside the thermal envelope and so not suitable without yet more faff. Plus if the ground floor is a slab - a lot more faff! In MarkyP's case he has a specific place in the eaves that he has in mind.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258689#Comment_258689 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258689#Comment_258689 Wed, 07 Jun 2017 17:33:30 +0100 lineweight
did you get any further with your design?

djh -

earlier in the thread you mention a spreadsheet that helps with calculations for the Ubink system you describe. Where is that? I can't fine it on the Ubink website anywhere.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258691#Comment_258691 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258691#Comment_258691 Wed, 07 Jun 2017 20:03:36 +0100 djh Posted By: lineweightearlier in the thread you mention a spreadsheet that helps with calculations for the Ubink system you describe. Where is that? I can't fine it on the Ubink website anywhere.
It's used by the suppliers; perhaps it's not publically available?]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258708#Comment_258708 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=258708#Comment_258708 Thu, 08 Jun 2017 11:19:21 +0100 MarkyP DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261796#Comment_261796 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261796#Comment_261796 Sun, 01 Oct 2017 09:05:15 +0100 morsing
I have tried filling this in for my property, before buying the MVHR system, but I'm actually not sure what I am meant to do with these numbers? Is it to tell me what size ducts to use? If the actual system has 125mm connectors, is it not best just to stay with that?

Thanks]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261836#Comment_261836 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261836#Comment_261836 Mon, 02 Oct 2017 14:26:38 +0100 morsing
Just to follow up on this, I can get a Vent-Axia Kinetic at a good price. It says it's for up to 4 bedroom houses I think, and the number kinda add up, but am I supposed to look at 30% of the 190m3/hr flow-rate?

We're only two people in a 3 bedroom bungalow - 120m2 in size. I'm guessing the standard kinetic and 125mm ducts would be ok, but don't want to end up with a noisy system?

Thanks]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261839#Comment_261839 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261839#Comment_261839 Mon, 02 Oct 2017 16:10:55 +0100 Greenfish
As the beginning of this thread advices see Table 5.1a and 5.1b of Part F, especially the footnotes, and then 5.2d.

The number of actual occupants is also irrelevent. Once over 100sqm total floor area then it is the 0.3 l/s per sqm requirement that dominates. That is 0.3 x 120 = 36 l/s, which is just under 130m3/hr. You want a unit that can deliver that with ease into the resistence of your ducting, and then deliver even more for boost mode.

The 190m3/hr quoted will be max flow into open plenum (no resistence), will it be enough? Well make an estimate for how convuluted and long that ducting will be, and look at the performance/efficiency graphs of the MVHR unit. This is a set of contours showing power consumption to deliver a flow against a back pressure. That should give a better idea of if it is a good choice of unit for you.

I would also recommend doing a basic flow design, that will show if any rooms need more than one inlet/outlet vent to handle the flow in that room.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261847#Comment_261847 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261847#Comment_261847 Tue, 03 Oct 2017 08:44:24 +0100 morsing
I mis-typed, it's rated at 290m3/h - 30% of that is around 90m3/h.

" and look at the performance/efficiency graphs of the MVHR unit. This is a set of contours showing power consumption to deliver a flow against a back pressure. That should give a better idea of if it is a good choice of unit for you."

I have looked at those and running at 36l/s into 75 pressure would be at 30% of max.

"I would also recommend doing a basic flow design, that will show if any rooms need more than one inlet/outlet vent to handle the flow in that room. "

I have seen people talk about this but not sure how to do it. Is it just the 0.3l/s/m2 number and look at the ducting static pressure and flow of the unit and divide it up between the rooms? Is that what the spreadsheet at the beginning of this thread does? I have filled all that in.

I don't have it with me, but the rooms varied between 2.5l/s and 0.8l/s from memory, do I need to add more ducts to certain rooms or can the valves adjust enough to make up for that?

Thanks]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261851#Comment_261851 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261851#Comment_261851 Tue, 03 Oct 2017 11:30:43 +0100 djh Posted By: morsingI mis-typed, it's rated at 290m3/h
That'll be fine for a 120m2 bungalow, IMHO.

I have seen people talk about this but not sure how to do it. Is it just the 0.3l/s/m2 number and look at the ducting static pressure and flow of the unit and divide it up between the rooms? Is that what the spreadsheet at the beginning of this thread does? I have filled all that in.

I don't have it with me, but the rooms varied between 2.5l/s and 0.8l/s from memory, do I need to add more ducts to certain rooms or can the valves adjust enough to make up for that?

Personally, I like the semi-rigid systems rather than branched systems, so the balancing is done at the design stage rather than by tweaking valves. The key thing is to keep the flow rates in the ducts low - below 2.5 m/s I think - in order to minimise the noise.

I would suggest choosing your preferred system and supplier and getting them to design the system. You will still need to double-check it for accuracy and for the practicality of the suggested duct runs.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261852#Comment_261852 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261852#Comment_261852 Tue, 03 Oct 2017 13:01:18 +0100 RobL Place unit in loft away from where people might sleep.
Use large diameter pipes to keep the airspeed down. We used 150mm diameter generally, and 200mm for a long run in the loft feeding 2 valves. I guess if you have loads of valves they could be smaller diameter, but this sounds like more work to fit!
Use some flexible sound absorbing pipe at the MVHR unit itself, as the unit will make some noise.
Use humidity and CO2 control - this way most of the time the unit will be on a low setting automatically. I think you can get these included, or aftermarket add them to the unit.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261854#Comment_261854 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261854#Comment_261854 Tue, 03 Oct 2017 13:47:55 +0100 CX23882
My Nuaire PIV unit came with foam "antivibration mounts" and found them to be completely useless. They worked reasonably well for a few weeks (at normal fan speed) but over time the weight of the unit would compress them down, after which it was not much better than if it was just hard mounted to the ceiling joists. I think they supply the same kit with the smaller Drimaster unit as with my larger Drimaster 365, and they just weren't up to the job. I bought Sorbothane pads in the end, after weighing the unit and finding the optimal properties. No vibration through then ceiling joists even on boost.

I did consider mounting on a bracket on the blockwork gable wall, but it isn't optimal for duct runs, plus although I'm sure a block wall has less chance of transmitting vibration than timber ceiling joists, it's not a given.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261856#Comment_261856 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261856#Comment_261856 Tue, 03 Oct 2017 14:54:33 +0100 djh
Mine is mounted on a timber stud wall, using the rubber-buffered metal bracket that came with it. I did use multiple layers of cement board on the studs to increase the weight and rigidity. It works well enough even in the bedroom directly below. Depending on the setting, I can hear a little duct air noise but nothing seemingly from the unit.]]> DIY MVHR design and calculations http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261857#Comment_261857 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912&Focus=261857#Comment_261857 Tue, 03 Oct 2017 15:19:09 +0100 CX23882
I was surprised by how well those perforated duct "silencers" work at reducing noise from the unit.

I fitted one (1.2 metre length, I think) between the Drimaster fan and the ceiling diffuser. With just regular ducting, the motor noise and resonance of the metal enclosure (on max boost speed) was very audible at the diffuser, along with the expected whooshing of air. With the silencer, there is absolutely no unit noise other than the air movement. I only appreciate the difference when going up into the loft and hearing it when it's on boost.

I'd be interested in measuring what effect they have on flow through the ducting. I imagine it's minimal and easily compensated for if needed.]]>