Mhrv for upstairs only- possible?

Cooled

Hi, I am looking to replace wall vents as I live in quite a noisy park, and with working from home I am finding it difficult to concentrate. At night it can get quite stuffy too in the bedrooms so think this is really the area where I most need better ventilation. Dont particularly want to do the downstairs as I think it would be too disruptive trying to get the ducting into place for those rooms, whereas for the upstairs rooms the system and ducting can all go in the attic. Also downstairs I have a gas stove which I heard you cant use mhrv with.

So is it possible to only have mhrv upstairs and for the wall vents to remain in the downstairs rooms, or are there any issues with this plan? Thanks in advance.

Calahonda52

You might look at it on a room by room basis or maybe two room, with well sealed doors and have a small positive pressure in the room when commissioned, I am looking at it right now as we have a big mould problem.
I am looking at this one
https://www.envirovent.com/products/heat-recovery-ventilation-mvhr/slimline-300/
the flow rate is vey high, I think there is a 200 version but my thinking is that 300 for a bedroom will mean a very low fan speed at night.

FitzElla

I'd be interested in knowing this as well. We have a central extract in the attic from wetrooms and the hole in the wall vents for supply, but they let in a lot of noise and over ventilate cold air when it is windy in the winter. Would love to swap out the vents upstairs and have the fresh air from a MHRV. For the same reason as the OP doing downstairs would be too messy with the ducting required.

ElderWanderer

MHRV relies partly on airtightness to be effective, so having open vents down stairs might be an issue- you're fighting a losing battle trying to recover heat upstairs when there's cold air coming in through a hole in the wall downstairs.

That said, I'm looking at doing upstairs only MHRV in my house as well, for exactly the same reasons: extending it downstairs at all would be a massive pain, and getting it to the right location downstairs would be near impossible.

Unfortunately, sometimes there's no good options, only different kinds of bad. I reckon the least-bad option in my house is upstairs only centralised MHRV, followed by separate tbd solutions for the kitchen & sitting room in due course.

Calahonda52

Lets be clear about what we are about here.
Traditional MVHR has supplies in some rooms and extracts in others and the whole thing is balanced.
The design usually starts with the fresh air required and then the extract falls out of that math.

So doing a partial house system is fraught..

What I have in mind is just two bedrooms upstairs, each with own supply and extract, with a little + pressure to avoid any risk of down draughts in the chimney down stairs.
I need to source a small in-duct heater so as the incoming air is not too cold in winter.

I have not thought it out fully yet but it is doable, its 2021 after all

FitzElla

ElderWanderer wrote: »

MHRV relies partly on airtightness to be effective, so having open vents down stairs might be an issue- you're fighting a losing battle trying to recover heat upstairs when there's cold air coming in through a hole in the wall downstairs.

Does that not mean in the worst case you are getting the same cold air through the MHRV as the current vents? But in a controlled way and with the noise/wind protection. I would still consider that a win

Cooled

FitzElla wrote: »

Does that not mean in the worst case you are getting the same cold air through the MHRV as the current vents? But in a controlled way and with the noise/wind protection. I would still consider that a win

Same here, I dont care about the heat recovery aspect. I just want to get rid of the holes in the wall to reduce noise and have adequate ventilation in those rooms. I just want to ensure its safe to do this and wouldnt create any other issues.

Biker1

Look into Demand Controlled Ventilation.

Calahonda52

Biker1 wrote: »

Look into Demand Controlled Ventilation.

Yes as a option but the lack of air filtration makes it a non runner for me.

MVHR , with the right filters give amazing air quality

Cooled

Biker1 wrote: »

Look into Demand Controlled Ventilation.

How effective are dcv wall vents at blocking outside noise getting in?

Also I heard less ducting is required but not sure how as from what i have read so far its a fairly similar-ish system.

Would also need to run wires for the sensors in each room so not sure that system would be any less hassle than mhrv for a retrofit?

Penn

Cooled wrote: »

How effective are dcv wall vents at blocking outside noise getting in?

Also I heard less ducting is required but not sure how as from what i have read so far its a fairly similar-ish system.

Would also need to run wires for the sensors in each room so not sure that system would be any less hassle than mhrv for a retrofit?

You can get DCV wall vents which are independent for each room (eg. Not interconnected, no ductwork to a centralised system). Some come with grills in the wall vent which can reduce noise too.

http://greenoakenergy.ie/ventilation/aereco-dcv/wall-window-inlets/

I don't know how good they are, but for what you need them for they're a much cheaper and easier solution and go above and beyond a normal wall vent. An MVHR for just the first floor in a house which likely has a higher air permeability than current regs, to me that makes no sense in terms of cost and work involved.

ElderWanderer

Penn wrote: »

An MVHR for just the first floor in a house which likely has a higher air permeability than current regs, to me that makes no sense in terms of cost and work involved.

Like I said, sometimes there's no good options, just different kinds of bad.

Someone considering MHRV should have a reasonably airtight house before they consider installing MHRV. If the house isn't already well insulated and fairly airtight, then it's fairly pointless alright- a leaky house doesn't need ventilation, and heat recovery will do feck all for heating efficiency if there's a gale coming through from outside.

But if the house is fairly airtight (ideally including between floors & rooms) and well insulated, then installing it upstairs only means the bedrooms will be ventilated, and warm moist air will be removed from the bathrooms. It's not as good as having centralised MHRV for the entire home, but you're still getting fresh filtered air into relatively small rooms which people spend probably a third of their lives in without sacrificing the investment in insulation & airtightness by opening windows.

I would consider that worthwhile— but there's plenty of other options out there, and it depends on the building and owner preferences.

Penn

ElderWanderer wrote: »

Like I said, sometimes there's no good options, just different kinds of bad.

Someone considering MHRV should have a reasonably airtight house before they consider installing MHRV. If the house isn't already well insulated and fairly airtight, then it's fairly pointless alright- a leaky house doesn't need ventilation, and heat recovery will do feck all for heating efficiency if there's a gale coming through from outside.

But if the house is fairly airtight (ideally including between floors & rooms) and well insulated, then installing it upstairs only means the bedrooms will be ventilated, and warm moist air will be removed from the bathrooms. It's not as good as having centralised MHRV for the entire home, but you're still getting fresh filtered air into relatively small rooms which people spend probably a third of their lives in without sacrificing the investment in insulation & airtightness by opening windows.

I would consider that worthwhile— but there's plenty of other options out there, and it depends on the building and owner preferences.

I would have thought though that the MVHR would be completely inefficient even if the exterior of the building was relatively airtight because the rooms themselves internally are not. MVHR would work best when the entire house is part of the system. If you only had MVHR upstairs and had background ventilation downstairs, your MVHR is going to be battling the permeability inside (eg. doors) as well as outside because it's not balanced throughout the house.

I think you could undertake all that work to not have a wall vent and noise from outside, and end up being annoyed by the constant noise from the MVHR trying to compensate for the ground floor & stairwell being served by background ventilation. Then probably increased running costs of the MVHR as time goes on too.

ElderWanderer

Penn wrote: »

I would have thought though that the MVHR would be completely inefficient even if the exterior of the building was relatively airtight because the rooms themselves internally are not. MVHR would work best when the entire house is part of the system. If you only had MVHR upstairs and had background ventilation downstairs, your MVHR is going to be battling the permeability inside (eg. doors) as well as outside because it's not balanced throughout the house.

I think you could undertake all that work to not have a wall vent and noise from outside, and end up being annoyed by the constant noise from the MVHR trying to compensate for the ground floor & stairwell being served by background ventilation. Then probably increased running costs of the MVHR as time goes on too.

Do you know of a perfect solution for retrofitting efficient ventilation in a reasonably airtight and well insulated 2 story semi-d? Because as far as I'm aware there's pros and cons to all potential solution, including the solution of doing nothing.

Penn

ElderWanderer wrote: »

Do you know of a perfect solution for retrofitting efficient ventilation in a reasonably airtight and well insulated 2 story semi-d? Because as far as I'm aware there's pros and cons to all potential solution, including the solution of doing nothing.

There's never a perfect solution, and many different ways to skin a cat as they say. I would just be of the opinion that you kind of have to go for one or the other for best results, unless it's properly designed with a full assessment. How airtight is "reasonably airtight"? What's the current heating system? What's the current typical temperature and relative humidity in the rooms? It'd also need an assessment by the MVHR supplier to ensure the system operates efficiently as its not doing the full dwelling.

Ultimately it all comes down to cost. Capital costs and operation/maintenance costs. If the costs are worth the benefits, fair enough. But I think there are many factors which need to be taken into consideration and it's above an Internet forum's paygrade.

MicktheMan

In retrofit scenarios, it would seem to me that too much emphasis can be put on the HR part and less thought put into the MV part.
What I generally recommend in a standard say 3 bed SD is a simple enough centralised MEV system extracting silently 24/7 from the family bathroom and ensuite upstairs via the attic (and if possible dropping a duct via the hp to the kitchen ceiling). The supply air is through any existing vents (window / wall) and the many gaps / cracks & building leakages.

This setup will greatly improve the internal air quality of the whole house and significantly reduce the risk of condensation / mould etc by continually getting rid of the moisture (water vapour).

With this setup , there is zero risk of introducing any type of interstitial condensation in the structure which, imo, is a risk with PIV systems.

burdburd

"an assessment by the MVHR supplier"

Prob with this is many suppliers routinely tell you, house does not need to be airtight, unit can go in attic, half house solution is no problem, noise won't be an issue even with unit over bedroom and duct run <2m and having chimneys won't be an issue.

Obvs, they are in sales and I get that.

OP makes very rational case for partial MHRV.

I am considering same partial deployment in top and middle floors but not ground floor. This will address 4 bedrooms and two bathrooms. Kitchen and utility are unreachble 😔

In short, why does one consider Full Mhrv is a fully reg. compliant airtight new build?

1: To assist the occupants to breath
2: As a by product of 1 above, to recover heat from extract air


Why is one considering partial Mhrv in a 110yr old victorian mid terrace ?

1. Cuz dormer rooms are/will be renovated to good airtight std..... and no trickle vents in windows.
2. Not wanting to punch holes in red brick for bathrooms for astethics, time and cost
3. Sash windows are and will be refurbed but never perfect so secondary glazing fitted to reduce road noise... Makes using windows for ventilation a pain in the ass. Mhrv will supply fresh air 24x7
4.mitigating against any excess moisture in house from habitation with little ventilation upstairs and full occupancy.
5. Accepting the house full of girls not likely to crack windows for fresh air and I don't want to be air-police and this will autonomously address that issue 24x7 in all beds and baths.
6. Alternative solutions do encur a real cost which cumulatively eat into total cost of implementing a Mhrv thus making 'business case' more attractive.


My stairwell will be open, ground floor although refurbed, never going to be perfect but I still see many reasons to push on with partial Mhrv.

Having said all that, I have reservations...

- Will the system be whirring like a dinger due to imbalance? Can this be managed by settings?

- With unit in cold attic, although easily accessible will noise transfer to 3 very nearby bedrooms below.
- am I now into pre heaters, post heaters at additional cost and complexity.

Lastly, for now, I am laying 75mm duct to the rooms above, room by room as I refurb... I will decide later to use, cap or install Mhrv or Central extract system only.


Good thread, so thanks to OP for starting.

Regards,

Soon to be full time air leak detective

burdburd

Update to above:

I have installed a Vent Axia Sentinel BH in the cold attic - only live for 48hrs but here are some initial observations/follow-ups:

• All ducting is well insulated
• Exhaust ( via 2 x 75mm) from 2 x WC/shower rooms & a kitchen
• Supply to 3 x Beds, Office and Sitting Room

No idea if it is balanced - need to rent/buy a hotwire probe or barometer - i will in the new year.

However even using common sense estimates for settings for now of Low = 12%, Normal = 20% and Boost = 40%. the difference was practically immediate to our mid terrace house.

We had done a lot of work in 2023 with insulating top floor beds/baths, inc. VPL and new windows. Achieved a BER of C3 in Nov '23 ( dragged down due to old extension to rear) - but C3 bodes well for a 1900 redbrick house.

When the Gas Central heating went back on two days ago, I knew I done good with my 'fabric first' renovation project; the house very quickly got too hot. No problem I said, and then completed the final MHRV duct insulation in the attic, and flicked on the Vent Axia.

Air became notably fresher, and after two nights condensation on the Kitchen single glazing as almost disappeared. I accept Central heating helping here but the fact that there is constant extract of moisture is satisfying.

Also when one shower running , normal mode (20%) was automatically boosted to 30% and with a 2nd shower running , incremented again to 40%; I also noted a scaling back of fan speed 35%, 28% etc until humidity target of 70% was reached again and Normal Mode of 20% maintained. Great to see humidity detection at work with no 4" wall vents.

On the above post, I had reservations...

- Will the system be whirring like a dinger due to imbalance? Can this be managed by settings?

- With unit in cold attic, although easily accessible will noise transfer to 3 very nearby bedrooms below.

Update: Nope its very silent unless you are sitting in a quiet room and listening for it. Admittedly in the Shower Rooms it can be heard (similar to a hotel Bathroom) - these rooms have double ducts; Also at night the whole thing backs of to low mode of 12% fan speed making any noise minimal even in the bathrooms.

it is early days for me, and the weather is not presently at the coldest levels, but I will post again on my DIY, Partial house MHRV - for now i say do it, I am very pleased with results.

Yellow_Fern

https://www.boards.ie/discussion/comment/121521681#Comment_121521681

What was the situation downstairs? Did you skip it as it was hard to run ducts down there? Is a lath& plaster plaster? That might account for quietness?

eiritated

I’m considering same project - upstairs only - ours is a 6yo house. Would love an update and any more details @burdburd hope you are still as happy with it!