Green Building Forum - MVHR upstairs only - retrofit

MVHR upstairs only - retrofit

enerk — Thu, 16 Nov 2023 16:37:04 +0000

Hi
My house is late 80s build, no cavity insulation and renovation upstairs is almost completed. I've recently decided (based on cold bathroom in winter and growing kids closing doors in small bedrooms) that I would want to install MVHR in my house. I like the idea of not needing to open windows during window, I also lack of fresh air quite often but don't like the noise from outside.

MVHR unit will be installed in cold loft
Ideally inlet and outlet through the roof
Needs to be very quiet!
Option to install better filters
Due to long process of renovation it may for at least a year or two be used upstairs only (2 bathrooms, 3 bedrooms, 1 office, so 3inlets 4 outlets).
As mentioned before the house has poor insulation and we are not planning to add anything apart from dealing with some bigger leaks (windows etc).

My main issues questions are:
1) Can I use a large unit (suitable for 120m2 house) use partially or would that cause some issues?
2) Does providing exhaust through the roof make sense? Im worried about positioning ducting upwards.
3) What unit to choose?
4) Ideally I would want a unit I can control over wifi but do I actually need it?
5) Either the unit would be supplied with good filters or have an option to upgrade it depending on needs.
6) Noise - needs to be quiet!

MVHR upstairs only - retrofit

fostertom — Thu, 16 Nov 2023 17:13:56 +0000

Don't have inlet to the bathrom - will feel like a cold draught, when wet.

MVHR upstairs only - retrofit

enerk — Thu, 16 Nov 2023 17:16:44 +0000

Posted By: fostertomDon't have inlet to the bathrom - will feel like a cold draught, when wet.

I meant the other way around: 2 outlets from bathrooms + 1 from landing, inlets to bedrooms + office. Although not sure how would one bathroom be supplied if the 3rd outlet is in landing.

MVHR upstairs only - retrofit

djh — Thu, 16 Nov 2023 18:31:56 +0000

Posted By: enerk1) Can I use a large unit (suitable for 120m2 house) use partially or would that cause some issues?

As well as area, its usual to deal with MVHR in terms of internal volume, since they output m³/hr and adequacy is rated in terms of ACH. But in general turning and MVHR down doesn't cause issues, in fact it's usually beneficial. Ours is nominally 300 m³/hr but we don't normally run it above 125 m³/hr and never above 165 m³/hr.

2) Does providing exhaust through the roof make sense? Im worried about positioning ducting upwards.

It can be done. The no-no is having the intake and exhaust on different walls, or one on a wall and the other on the roof. They need to experience similar external pressures to each other regardless of wind direction.

3) What unit to choose?

Horses for courses I suppose. I live in a PH so we have a PH-certified Brink unit. The benefits are independent 3rd-party certification of performance. Good airtightness; quiet; well-built. etc. But PH cost more than some others.

4) Ideally I would want a unit I can control over wifi but do I actually need it?

Well mine doesn't have it, so no you don't need it. But it might be convenient sometimes.

5) Either the unit would be supplied with good filters or have an option to upgrade it depending on needs.

Look for washable filters. The slots in mine are thick enough that I can put either a G3 or a G4 or both together (which is what I do on the intake) or an F7 filter.

6) Noise - needs to be quiet!

As mentioned; PH tests for noise level.

Look at https://www.ncm-pcdb.org.uk/sap/pcdbsearch.jsp?pid=34 or https://database.passivehouse.com/en/components/list/ventilation_small for specs. Talk to suppliers about system design, and if possible get a professional system design.

About your plans in particular: 1) do try to plan to add some insulation! It is really important for comfort. Why no cavity insulation? 2) You won't need a terminal on the landing: airflows from the inlet terminal through the room it is in, then around the door (remember to leave sufficient gaps), through the landing and through another door into the wet rooms. 3) Consider semi-rigid ducting with a smooth built-in lining. It's smaller and easier to fit than rigid ducting. We have Ubbink.

MVHR upstairs only - retrofit

enerk — Thu, 16 Nov 2023 19:38:02 +0000

I'm considering PH certified unit on a basis that, as you said, it's certified. I got a quote from one company to install MVHR in my house for £15k so I guess if I DIY i'm still have over £10-12k of savings.

Position of my house is such that I can't put the exhaust and intake on a wall hence thinking about roof. If I run it through sofits (which would probably accommodate max 100-125mm shall I then double it?
If i go with roof how to make sure it doesn't cause mould? I know that with bathroom extractors it does cause issue often but would it be different here because it's always on?

Thanks for the link! I hate them tbh :). Lots of systems but I lack a proper shop selling more than 1-2 brands.

MVHR upstairs only - retrofit

djh — Thu, 16 Nov 2023 22:56:58 +0000

Posted By: enerkPosition of my house is such that I can't put the exhaust and intake on a wall hence thinking about roof. If I run it through sofits (which would probably accommodate max 100-125mm shall I then double it?
If i go with roof how to make sure it doesn't cause mould? I know that with bathroom extractors it does cause issue often but would it be different here because it's always on?

Our MVHR unit uses 160 mm ducts. We're 145 m²-ish so a bit bigger than you and we have some fairly high ceilings (approx 440 m³ IIRC) but those are PHPP values so perhaps measured differently. But I'd think 100-125 mm was too small, yes. Low velocity is important for noise.

Dunno about your mould question. Generally the idea is not to have condensation in the external ducts, I think. And the continuous air movement helps prevent mould. But I have no idea what the issues are with vertical ducts.

MVHR upstairs only - retrofit

tony — Fri, 17 Nov 2023 10:24:19 +0000

Suspend unit from roof rafters, add sound attenuators to main duct to house

I quite like ‘Boulder’ units, a variety of sizes and I generally slightly undersize, can always open windows for purge

I have a Genvex sat on polystyrene, virtually silent on moderate flow speeds

MVHR upstairs only - retrofit

djh — Fri, 17 Nov 2023 10:36:41 +0000

Oh, forgot to mention. If you're planning to mount the MVHR in the loft do remember you need to change/wash the filters regularly (every 3 months where we are) so plan for access. And remember it and ductwork will need insulating if in cold/hot areas.

MVHR upstairs only - retrofit

RobL — Fri, 17 Nov 2023 14:03:02 +0000

From bitter experience the condensate drain will freeze in cold weather... our unit backed up and caused a minor leak. It took a few years before we had a suitably long cold period, so I was super confused by it at first. The solution was to plumb in the condensate indoors to something - we went to the bath drain, which is internal.

MVHR upstairs only - retrofit

enerk — Fri, 17 Nov 2023 14:31:17 +0000

How much water can be produced over week? In southwest I don't think freezing should be a big issue. long term I can probably plumb it indoor. Short term I could use to collect it. The biggest issue at the moment is planning exhaust and intake ducting to outside.

MVHR upstairs only - retrofit

jms452 — Mon, 20 Nov 2023 20:00:05 +0000

We have a vent axia sentinel kinetic installed above about bed mounted in the loft directly above our bedroom. It's has 400mm rockwool between it and us and is mounted as tony suggested - We can only hear it after a shower.

+1 What Rob said about condensate drain even in the arm bits of the uk

+1 What dgh said about not needing a vent in the landing.

MVHR upstairs only - retrofit

djh — Mon, 20 Nov 2023 20:33:30 +0000

The condensate drain can go to any convenient indoor waste pipe. Be sure to use a dry trap, not a U-bend wet one.

MVHR upstairs only - retrofit

Peter_in_Hungary — Mon, 20 Nov 2023 21:41:33 +0000

Posted By: djhBe sure to use a dry trap, not a U-bend wet one.

Why ? Danger of drying out ? Or can it go in front of a wet trap already on, say, a bathroom sink ?

MVHR upstairs only - retrofit

djh — Tue, 21 Nov 2023 12:20:19 +0000

Yes, wet traps dry out unless used regularly (i.e. water flows through them). In a house with MVHR operating this is more of a problem than condensation itself, if our experience is anything to go by. We make sure we tip some water down all showers and basins and the bath once a month or so. An MVHR unit won't be producing any significant condensation much of the time so would dry out.

I'm not sure what you're proposing in your last sentence? You mean omitting the trap and connecting it directly to a sink? I believe the trap is specificed by the MVHR manufacturer in the installation instructions.

MVHR upstairs only - retrofit

Peter_in_Hungary — Tue, 21 Nov 2023 14:41:25 +0000

Posted By: djhI'm not sure what you're proposing in your last sentence? You mean omitting the trap and connecting it directly to a sink? I believe the trap is specificed by the MVHR manufacturer in the installation instructions.

Connecting the MVHR drain pipe to a sink outlet above the trap or to a washing machine up-stand drain again before the trap thus giving a wet trap that is well used

MVHR upstairs only - retrofit

djh — Tue, 21 Nov 2023 15:51:52 +0000

Posted By: Peter_in_HungaryConnecting the MVHR drain pipe to a sink outlet above the trap or to a washing machine up-stand drain

I suspect in most case it would be difficult to do that whilst keeping the condensate drain flowing downhill.

MVHR upstairs only - retrofit

enerk — Tue, 21 Nov 2023 23:32:52 +0000

How to position exhaust pipe to tile vent to avoid mould?

MVHR upstairs only - retrofit

tony — Wed, 22 Nov 2023 08:31:41 +0000

Downhill! And having mould in the exhaust vent won’t be a hazard and may not occur as warm air is blowing out all the time

MVHR upstairs only - retrofit

enerk — Wed, 22 Nov 2023 09:49:26 +0000

I wouldn't mind (actually preferred) downhill but can't find a tile vent which is positioned so that the ducting never goes up.
No mould or no hazard of it is something I also consider but ideally , given i have time to do it properly, I would want to avoid it.

MVHR upstairs only - retrofit

Artiglio — Wed, 22 Nov 2023 12:28:54 +0000

Is there not a bit of confusion here between the air ducts and condensate drain when it comes to talking of downhill?
If not i’ve broken the down hill rule on the ducts between the unit and intake and exctract ducts through the roof, along with several others ( inlet and out let are on different faces, some of the distribution ducts within the house are compromised in that I couldn’t run twin ducts to the furthest inlet, the duct runs are pretty convoluted) but these were all pretty much unavoidable if i was to stick with the buildings layout and construction.

As a result i’ve no doubt the system doesn’t operate as efficently as it could have, however it controls humidity levels very well, the comfort and “sense” of fresh air in the house has been remarked upon by several people.
The system is effectively inaudible at normal levels, barely noticeable at boost and can’t be considered a nuisance on boost ( but can be heard).

The house still needs finishing and i expect things to improve once a couple of obvious ( and large) air pathways are dealt with ( not that the house will ever be close to being considered air tight by GBF standards).

I decided on MVHR primarily to deal with any potential moisture/ humidity problems resulting from attempting to insulate an old listed building that has been much altered and a used in its lifetime, my view being that it was sensible to remove moisture rather than have to deal with any problems it may cause later.

The only thing that i’d have done differently and which may get done once everything else is finished , is to have fitted direct extraction to the outside from the cooker hood rather than relying on the filter/recirculation and mvhr when cooking.

MVHR upstairs only - retrofit

djh — Wed, 22 Nov 2023 13:47:29 +0000

Posted By: ArtiglioIs there not a bit of confusion here between the air ducts and condensate drain when it comes to talking of downhill?

I was wondering that. I think the condensate drain should go downhill and I think that the intake and exhaust ducts should basically go downhill if they run through a wall. Obviously the ducts can't if they're going up through the roof. And they can't go uniformly down if they emerge from the top of the MVHR unit as mine do. I think the idea is to deal with any rain that enters through the outside grille. The condensate drain deals with anything that occurs within or above the MVHR unit.

I don't think there are any drainage rules about the internal ducting. They shouldn't have very humid air in them and they have to go where they can.

The only thing that i’d have done differently and which may get done once everything else is finished , is to have fitted direct extraction to the outside from the cooker hood rather than relying on the filter/recirculation and mvhr when cooking.

I suppose it depends on the style and quantity of cooking you do, as well as the arrangement of the extract terminals, but FWIW we haven't missed having an externally-vented cooker hood. In fact we hardly use the cooker hood at all.

MVHR upstairs only - retrofit

Artiglio — Wed, 22 Nov 2023 14:43:56 +0000

Djh - 80%+ of cooking is no problem especially if I use purge function and most of that will be done without using the hood, it’s the remainder that can be problematic , mainly smell from stir fries and fish, but i’m still tinkering with the inlet/ outlet settings and am hopeful ,when the last couple of jobs that’ll greatly reduce the air coming into the house via the structure are dealt with, airflows will alter enough to improve the mvhr extraction efficiency in the kitchen.

MVHR upstairs only - retrofit

djh — Wed, 22 Nov 2023 15:35:20 +0000

The main problem we have is on the occasions my wife fries belachan (a fermented shrimp paste). I feel like I'm choking when she does it.

Anticipating that problem we fitted double sliding pocket doors between the kitchen and dining room, and we find that closing those means the smell is contained in the kitchen for long enough to be extracted by the MVHR.

MVHR upstairs only - retrofit

Mike1 — Wed, 22 Nov 2023 20:53:12 +0000

Posted By: djhI think that the intake and exhaust ducts should basically go downhill if they run through a wall

The terminal should have a rain screen, so rain entry shouldn't normally be a big issue, though an outward slope is still a good idea.

However disposing of any condensation in in exhaust duct is a concern. Most condensation is likely to occur within the MVHR unit itself, but some could occur in the duct. The recommendation is therefore that the exhaust duct should either fall towards the unit (where any condensation will be drained via the condensation pipe), or fall towards the outside, or one then the other.