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.

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?

Hi,

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

The "up to 4 bedrooms" blurb is pretty meaningless. You need to fit a system that when commissioned complies with the building regs, and the suppliers may happily sell you one that does not (at least it nearly happend to me).

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.

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.

For loft mounting, how do you typically mount to avoid vibration?

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.

I like the idea of suspension. As you say, you'd need to include replacement in your maintenance schedule. I'd probably want a fail safe platform below the unit in the worst case scenario of it crashing through the ceiling. Line that with noise damping material and you hopefully prevent any reverberation effects.

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.