The room vents do a good job mixing the air so def not worth any hassle getting right to optimum position - but I mean vents that are specific for supply and extract not the generic cheap as chips 'a vent is a vent' ones. But in any even when talking retrofit I wholeheartedly agree with ringi - just getting one in the room is more than 80% of the battle

I used these and they have a very different design. Def recommend!

http://www.i-sells.co.uk/extract-valves-ku

http://www.i-sells.co.uk/supply-valves-ki

I also bought some for under a fiver I think - def not recommended! I binned mine.

Ringi, checking out MVHRs on ebay I noticed your Itho HRU ECO4 is badged as a Heatrae Sadia on ebay. I have installed the same at home.
Can anyone recommend a MVHR for a small space of 45m2?

Cheers ringi, I wasn't aware of that. I bought the unit/system 5 years ago when they had their offices in Staffordshire. Do you get your filters from Heatrae now?

Anyone with a suggestion for a MVHR unit for a small space ca 50m2?

OK. So I have spent way longer than I intended understanding reynolds numbers, laminar/turbulent flow, major and minor friction coefficients and so on, and have made a spreadsheet for doing the calcs, as I failed to find one online that did this, and the various engineering calculators were all far _too_ flexible, and you needed to know your kinematic velocity and reynolds number before you could calculate anything, with a lot of looking up magic numbers in clever diagrams, or it was all American in stupid units and designed for heating so with epic flow rates.

The spreadsheet is here:
opendoc (libreoffice/gnumeric): http://wookware.org/files/MVHR.ods
ISO29500 (excel): http://wookware.org/files/MVHR.xlsx

I've made my head hurt in the process, so some reality checking would be good at this point:

1) Do I need to allow for pressure drop due to height changes (e.g. dropping up/down 1 floor)?
This seems to be the dominant pressure-loss, and it's mostly not mentioned in other docs. It also produces a negative drop for a short leg that mostly just drops down. I'm not sure what to make of that in terms of balancing.

When a pipe goes up and back down again it clearly doesn't matter, but one that just drops a floor gets a pressure diff (Pascals) of (I think): density x g x height difference. so 1.18 (Kg/m3) x 9.81 (m/s2) x -2.4m = -28 Pa. Is that right - it seems high in comparison to the other components?

For major friction loss I get numbers like 0.14Pa for 70m3/h flow in 4m of 150mm dia rigid galv pipe (@1.1m/s). i.e bugger-all.

For minor friction (bends, outlets etc), I get 8Pa for the same pipe with 2x90 and 2x45 degree bends. i.e some, but still dwarfed by dropping one floor.

Does that sound plausible? i.e. a total pressure drop for a run of major friction (pipe) , plus minor friction (fittings) plus(minus) height-difference pressure? typically 0.15+6+/-30.

2) How much does it matter if my extract piping is much shorter than my supply piping? In the best spot the machine is within 1m of both kitchen and bathroom so there is approx 1m of duct each. But the supply ducting needs to go across the loft for bedrooms so they are all 8-11m. Is that going to make it hard to balance?

3) What am I actually trying to optimise? So now I have a number for pressure drop to/from each outlet/inlet (in the range -17 to 41 Pa for the nominal 70m3/h which is 0.3 ach). Do I want to make the worst one as small as possible, or all of them similar, or all as small as reasonable?

4) Is there such a thing as too-slow airflow? Some of my flow (to the smaller rooms) is laminar even in 100mm duct (<0.4m/s). Is this just good because that's efficient and quiet or should it be avoided for some reason?

5) With the machine in a cupboard I can only get the external exhaust/supply cowls about 1m apart. Is that enough?

I may have forgotten some things but that'll do for now.


The sheet, for anyone that cares:

You can all laugh at my low-tech solution to the problem of iterating the Colbrook-white equation. This made the spreadsheet hard, because you can't just put in a function in the normal way: the thing you want is on both sides so you have to iterate it. Turns out that Gnumeric doesn't do recursive solutions the way excel can, and if I stuck in a python solution it wouldn't work anywhere else. So, I just did it with a 6-step array iteration on a second sheet, which will work anywhere even if it's not at all pleasing to the programmer's eye :-)

The sheet isn't perfect yet by any means (e.g. it uses hydraulic diameter where it should, but that's still set to just 'diameter' for now - it needs to look up whether square pipe is specified and if so use the correct Dh, and it currently assumes that your air is somewhere near 20C, and that your house is HxWxD). But it is probably a useful design tool, or would be with a bit more work to make it less cryptic to use. A neater way of describing the tree structure of ducts would be nice, and the multitude of ways of picking ventilation rates could do with rationalising. It should work correctly with round or square ducts in a range of materials, and of any size, and correctly deals with laminar/transitional/turbulent flow.

Posted By: wookey1) Do I need to allow for pressure drop due to height changes (e.g. dropping up/down 1 floor)?

I'd think not if the air was all at the same temperature. E.g., if you take air in via a vent into a ground floor cupboard then push it upstairs out of a vent in a bedroom then you wouldn't need to worry about the height change because the syphon would be completed by the rest of the house to wherever you extract whether that's in an upstairs bathroom or downstairs kitchen.

However, the point of the whole thing is that the air is not all at the same temperature (otherwise you'd just open a window) so, at least in principle, you ought to care that the pressure differences will be affected by density. Rearranging PV/T = k with ρ = 1/V for 1 kg of air:

ρ₁ = ρ₀·T₀/T₁

If outdoor air is just about freezing (273 K) with a density of 1.2 kg/m³ and indoor air is around 20 °C (293 K) then indoor air will have a density of a tad over 1.118 kg/m³. If that's near floor level downstairs then the pressure difference between a ceiling vent in the bedroom 4 metres above and outside will be (ρ₀ - ρ₁)·g·h = 0.082 × 9.81 × 4 ~= 3.22 Pa which sounds like it's in the ballpark of what's significant for these purposes.

But that's an extreme case as the house as a whole should be well enough sealed to act as a syphon. Still, I think being a bit careful about the temperatures of the air might help, particularly if you're doing PassivHaus-style duct heating where the incoming air is significantly warmer than the exhaust air.

I expect you'll want to tweak the fan speed settings to cope with that. Mine does it automatically so the supply and extract volume rates are the same whatever the duct resistance is, but I believe some other MVHR units require you to adjust the flow rates manually.

OK. took a while to find, but there is a (manual) supply/extract relative speed pot to adjust on the Itoh HRU4, so that should be fine.

you then need to restrict the flow down/up the other ducts to increase their resistance to roughly the same point so you can fine tune the balance with the vents. You don't want to be using the vents to provide most of the resistance because that is one way to make it noisy.

.

Sound sensible. So one should put dampers back up the pipes for 'main balancing'? Do we know any rules of thumb for how much difference there should be between runs before it's worth adding a damper? e.g (Assuming Ed is right about it being right to ignore height) then I have pressure losses at nominal rate of:
Inlet: 2.8, 3.7, 2.5, 2.3 and outlet: 1, 1.1, 1.3 [edited from bigger numbers]

So that's a worst case of 1.4 extra pascals needed to equalise runs (assuming use of supply/exhaust ratio adjuster). Does that make a noisy vent? (I assume I'm dealing in absolute values here - i.e 1.4 Pa not 60%)

Posted By: wookeyNow I have a (non-room-sealed) small woodburner, so this definitely isn't actually true here. Not sure how to allow for that.

I'm definitely not an expert on wood burners so hopefully somebody who knows will come along, but I don't think wood burners and MVHR mix well. The problem is the need to ensure there is always greater pressure outside the stove than inside it, to prevent blowback of nasty gases and particulates. I have this horrible fear that the regs say you need a huge hole in the wall of the room. What does your BCO say?

Where is the wood burner? If there's an inlet in the room, wouldn't that mean the room is positively pressurised?

Posted By: wookeyOn the matter of pipe selection. Everyone rates the galv metal piping, but it's very expensive. What is wrong with PVC ducting? That wouldn't suffer from rusting ever and appears to cost about half as much.

Dunno - static attracting dust comes to mind but I've no idea. You could ask somebody like CVC?

Professionals seem enthusiastic about the semi-rigid pipe with plenum

I'm not a professional and I like it too. It is way easier to fit, IMHO.

Also what do people do about insulating round pipe. In the loft just putting it all on the warm side and making sure there is no gappage should be good enough.

I'd have thought so.

But the bit that goes out of the wall is interesting. One is advised to continue the insulation through the wall so just wrapping a bit of fluff round it isn't going to work there. Some layers of karrimat-type material? What thickness is sensible? 20mm? That would require a 190mm hole in the wall for 150mm pipe which is bigger than commonly-available core-drills. I guess I just have to make a mess....

Whatever you use needs to be vapour impermeable, otherwise you will get condensation on the outside of the pipe under the insulation. I have some of the foam insulated duct left if you want it? Apparently in an ideal world it would have thicker insulation but if your runs are short or you can add extra.

http://www.ubbink.co.uk/ubbinkcouk/media/Ubbink-UK/Literature/UBBINK-Mass-Flow-Ductwork-listing.pdf?ext=.pdf

I got my plastic ducts from https://www.bes.co.uk/products/180.asp, (drain pipe are not the same size, so will not directly fit onto the outlets etc).

For insulation, I use some silver bobble wrap from b&q, the sort that is often used behind radiators.

Posted By: GotanewlifeThe stuff behind rads is in my experience pretty thin 3-5mm really all about being reflective - is it enough?

I got a role of it, and wrapped it round the ducts a few times. I only needed it for the air intake duct to stop condensation on it within the "cold" loft space, I sealed it with "sliver" tape. The rest of the ducts are under my loft insulation.

I diy installed MVHR, in a 4 bed similar house to Wookeys. Have to say, I didn't even try to do anywhere near the number of calcs he's referring to. I think there are so many difficult factors to take into account, and ultimately it's a sliding scale between noisy & thin pipes/ducts, and quiet & fat pipes. I choose fat pipes!

I did try experiments with a CO2 meter in different occupied rooms, while the adjascent hallway window was open - to try to work out if a duct was needed in the room. My rule of thumb after this was only to put ducts in rooms that were wet, or expected more than one occupant. Is the door open/closed, what is the gap under it - all make a difference here. My now-teenage daughter now closes her door by default - so I'm thinking of adding a duct in her room - or at least remeasuring the CO2 level. Definitely recommend a CO2 meter.

As for what ducting - I tried to use metal where I could, thinking it more likely to be biocidal than plastic. For a couple of long loft runs, the pipes are 200mm diameter. The majority of pipework is 150mm diameter, and the room ducts are all 150mm diameter. The system is quiet - I can't hear it at night, even in our quiet neighbourhood. I think the noisiest duct is the bathroom one - it's closest to the MVHR unit, so there's less attenuation maybe.

It's difficult to get ducts where you want in a retrofit, without making a mess. So there was compromise in the livingroom inlet duct location - it's too close to the hallway door. It probably discourages heat from the stove circulating around our house a bit? Not that we light it that often, but something you'd care more about Wookey.

I religiously went around our house measuring flow rates and adding them up, intending to adjust the supply/extract main fan speeds - but they were balanced anyway. I didn't bother adjusting individual room ducts with dampers or closing off ducts - it all mixes in the house anyway I think, and closing off ducts makes the whole thing noisier & less efficient.

Posted By: wookeyOn balancing it turns out that the current manual (http://www.heatraesadia.com/docs/HRU_ECO_4_-_Installer_Manual_-_36006182_issue_1.pdf , p17-p18) has 3 pots, one for in/out balancing. But mine only has 2 pots, not including the balancing pot. This is dull.

Sounds like you need to balance the resistance of the ducts then unless the nice folks at heatraesadia can suggest an alternative. Maybe there's some way to hack the circuit to do what you want, but the warranty may be worth something.

Posted By: wookey'speed 2', and after the early models has been factory set at 'speed1+20%'

The table in section 2.6 of the manual you posted seems nothing like that?

One could put a noggin back in above and a steel strap below to put back some bracing.

That sounds reasonable to me.