MVHR design for a small house

[Crofter]

Looking for some pointers/feedback on my MVHR system. My house is unusually small and, as I mentioned in the other thread, I'm struggling to find units of the appropriate capacity which aren't also very basic in performance and features.

 

See attached image for the floorplan. This was an early draft- the kitchen/lounge has become slightly bigger, and the sofa will sit back into a slight recess, stealing a couple of feet from the bedroom, but otherwise it remains as drawn.

 

The kitchen/lounge has a vaulted ceiling, the bedroom is vaulted to a false apex, and the vestibule and bathroom are normal height. The house is warm roof construction.

 

The plan:

- MVHR unit sited above the vestibule ceiling

- Intake/outlet grilles in the soffit of the east wall (at bottom of sketch)

- Two supply vents, one in bedroom and one above sofa

- Two extract vents, one in bathroom ceiling and one in kitchen wall near the door from the vestibule

 

Questions:

- how far apart should the intake and outlet be?

- Could I put the bedroom supply vent inside a walk in wardrobe, that will be on the dividing wall (i.e. through the wall from the sofa)?

- At what height should vents be placed if the room has a vaulted ceiling?

- Other than higher purchase and running costs, will an oversized MVHR unit cause any problems? Just asking in case something comes up on eBay.

- Do I *really* need summer bypass?

 

 

Thanks for any suggestions.

[ToughButterCup]

@Crofter, if there's one thing that BH has taught me, it's that , in the words of DH Lawrence no less,

'...each flow is related to every other flow...' I seem to remember he was talking about money, and so are you in terms of heat (is 'heat' the right term?)

 

8 hours ago, Crofter said:

 

Questions:

- how far apart should the intake and outlet be?

- Could I put the bedroom supply vent inside a walk in wardrobe, that will be on the dividing wall (i.e. through the wall from the sofa)?

- At what height should vents be placed if the room has a vaulted ceiling?

- Other than higher purchase and running costs, will an oversized MVHR unit cause any problems? Just asking in case something comes up on eBay.

- Do I *really* need summer bypass?

 

- how far apart should the intake and outlet be?

there's lots of guidance about this issue all over Tinternet. Bottom line: try to make sure you don't suck up what you blow. How you do that, up to you. Local winds? Micro-climate? Rotor? Wind shadow?

- Could I put the bedroom supply vent inside a walk in wardrobe, that will be on the dividing wall (i.e. through the wall from the sofa)?

 Yes

Other than higher purchase and running costs, will an oversized MVHR unit cause any problems? Just asking in case something comes up on eBay.

There's no evidence that I have read (and Hell I've worn out several pairs of Poundland glasses) that buying a Ferrari causes you not to arrive at your destination when you could just as easily have driven there in a 2CV

- At what height should [...intake...?] vents be placed if the room has a vaulted ceiling?

I don't know. But I do know you need to ensure a simple cross-flow of air.  Bottom Line Diagonally opposite (or as near to that as you can)  the exit point of  the out-flowing air 

- Do I *really* need  a summer bypass?

Bottom Line: I don't know, and neither will anyone else until a program like PHPP (or JSH's  simple spreadsheet) has been run. 

You, I seem to remember are in an SMZ (Scottish Mozzi Zone), and so will be unlikely to want to open  your house to the local hoards on a long summer's evening just after a lovely warm, wet downpour. 

[ProDave]

My gut feeling says 2 inlet and 2 outlet vents in each room. i.e. twice as many as you said.

 

I wouldn't put the vents in the wardrtobe but in the actual ceiling of the bedroom.

 

Intake / outlet at least 1.5 metres apart.

 

My Kingspan unit on it's slowest setting is almost inaudible even when right next to it and uses very low power on that setting, so I wold have no hesitation to use something like that.

 

Summer bypass might help if you overheat with the stove.
 

[MikeSharp01]

8 hours ago, Crofter said:

Questions:

- how far apart should the intake and outlet be?

- Could I put the bedroom supply vent inside a walk in wardrobe, that will be on the dividing wall (i.e. through the wall from the sofa)?

- At what height should vents be placed if the room has a vaulted ceiling?

- Other than higher purchase and running costs, will an oversized MVHR unit cause any problems? Just asking in case something comes up on eBay.

- Do I *really* need summer bypass?

 

 

That's amazingly compact!

 

Answers from my research - have not actually installed one yet.

 

1. The most you can manage but the general feeling looks like being at least a meter. One, exhaust, could be above the other but some horizontal distance will help, you want to avoid exhaust air returning via the inlet.

2. In theory but you would need plenty of gaps at the bottom / top of the door to ensure circulation but this may interfere with Q3.

3. Higher the better but somewhat depends on whether you are putting heat in via the MVHR with an air battery in the feed duct because hot air rises and you may want to induce gentle, very gentle, mixing of the air in the space using this flow in sympathy with that generated by the airflow. Extracts IMO want to be as high as possible because they go in 'warm' / 'wet' spaces.  In your Living room / Kitchen you will need both inlet and extract and this in itself will cause useful circulation in the room to help handle the cooking vapours. The extract(s) will be above the kitchen and inlet(s) at the other side of the room. If this has a vaulted ceiling you will want to think hard about where the cooking smells will be going, assuming you are using a recirculating cooker hood.

4. Oversize has the advantage of having loads in reserve so should do your job very quietly in normal mode because it will be at a low setting. It can then boost loads of air in the summer if you need it. (see below)

5. Depends on what other cooling you have in the house - windows, induced circulation etc and can then turn off the MVHR. In air tight buildings I would be concerned about turning it off if I could not leave secure openings elsewhere in the house. In the case of loo you will need another extractor if you do intend turning off the MVHR. If it is running without summer bypass it will return exhaust heat. So, again IMO, you probably do need summer by pass.

 

Hope that helps.

[SteamyTea]

Are your internal walls stud walls?

If they are, can you pipe the room inlet down them towards the floor?  For that matter, can you take the extract out of the top of the wall.

Saves having holes in the ceiling.

 

If you go oversize on the MVHR unit (and like an ASHP, you should as you get better efficiency) you could put in quite large vents in each room.  That will keep the noise down and get rid of the risk of drafts.

 

With a house your size (not so different from the downstairs of mine), have you though of using an oversize MVHR in conjunction with two small Air to Air heat pumps.  They do heating and cooling usually and cost very little.

A couple of these would easily heat and cool your place.

https://www.debenhamsplus.com/p/865337/1000-btu-panasonic-powered-quick-connector-wall-mounted-inverter-air-conditioner-with-4-metres-pipe-kit-5-years-warranty?refsource=Deadwords&gclid=CMDDoOr78NACFUa6GwodrG0G3w

[Crofter]

Thanks for all the replies! A lot to take in.

I've read that the intake/outlet should be on the same wall of the building, so as not to unbalance the pressure. I could put one at each end of the east elevation if that helps, but presumably at some point the extra ducting causes more problems than it solves.

Also I had read about the diagonal thing, which makes sense, but tricky to implement in rooms with vaulted ceilings. Do I extract at eaves level, i.e. in a corner, or at apex level, which is less 'diagonal' across the room?

 

The main constraint on duct placement is that I am only planning a 25mm service void, and the slimmest flat ducting I have seen is 29mm. I can't imagine such contrived ducting is particularly efficient anyway, compared to nice smooth cylindrical stuff of equivalent cross sectional area. Thinking a bit outside the box (literally!) I could carry an insulated duct outside the thermal envelope, inside the sofft/fascia space, and route it to the gable end of the building that way. A lot of hassle, surely not worth it?

 

The interior walls are stud, though, so I could run ducts down to floor level that way. Also the cupboards (and the walk in wardrobe, not shown on that plan) will easily allow this.

 

On sizing, my only real concern about oversizing is that the minimum air change rate might be too high and cause draughts.

 

 One final thing- whilst googling all this I turned up some mention on GBF about using decent quality single room MVHR units to improve air quality in a whole house. It might not be the ideal solution but I am on a tight budget and this is looking quite appealing! Especially as a conventional unit with multiple vents in each room does seem rather overkill for a 43m2 house.

[SteamyTea]

3 minutes ago, Crofter said:

On sizing, my only real concern about oversizing is that the minimum air change rate might be too high and cause draughts.

If you double the diameter of a round pipe, you quadruple the area, so you can run air though at a quarter of the speed for the same amount of air shifted.

 

I would be tempted to make up my own MVHR (see GBF about that a few years back).  They are really pretty basic things.  There is no reason why you cannot make one with a polystyrene box, some silicone paper and a couple of fans.

 

[Crofter]

18 minutes ago, SteamyTea said:

If you double the diameter of a round pipe, you quadruple the area, so you can run air though at a quarter of the speed for the same amount of air shifted.

 

I would be tempted to make up my own MVHR (see GBF about that a few years back).  They are really pretty basic things.  There is no reason why you cannot make one with a polystyrene box, some silicone paper and a couple of fans.

 

 

Ooh now that it tempting! Especially as I do not have to meet any regs. Sounds like a fun winter project

 

On duct size, is it really feasible to run ducts inside stud partitions, that will be formed from 95mm timber? 

[SteamyTea]

To a certain extend, a duct is just a box with two open ends.  It does not have to be round.

There is a lot of wasted space inside a stud wall, see if you can use it.

It is easy to make filters, that material is available online or from a manufacturer (I used to make air filters).

The big issue is noise, both through the wall and via ductwork.  Filters in the ducts help reduce this.

 

I have a stud wall I would like to move one day to make the kitchen a bit larger and only loose a book shelf in the other room (I have a Kindle for books now).

I thought I could put my storage heater in that wall and vent out either side.  One 35 kWh storage heater is enough to heat my place.  I could also pipe a bit up the stud wall, though the floor to each bedroom.