New build - 2 bedroom house - High voids - Do we need MVHR?

[Paurav]

Hello,

 

We are building an end-terrace house.

 

Its a 2 bed house with living space and kitchen on the ground, one bedroom on the first floor and one on second (loft conversion). The house has 2 x high voids from ground floor to the 2nd floor ceiling (sloping roof in the front and flat roof at the back). It was struggle to get our fire strategy approved, but we finally have it approved too. At present, we are building the concrete slabs and beams and bricks are on site. We have decided to hire Stroma for environmental checks who will initiate their works soon.

 

Being a tech guy, I am too keen on having an eco-friendly house and feel fitting an MVHR will bring lots of benefits in terms of heat recovery, fresh air, more oxygen, etc. But am concerned on the requirements of having too many ducts and vents. Please feel free to answer on these questions for me to move more towards a firm decision:

 

1. To avoid having too many ducts running across the floor, can we fit the entire system on a single side-wall with all wall vents in the bathrooms and rooms at all levels? i.e. can we have a complete system without a ceiling vent? In worst case, we can have vents in the roof, but feel it will need a lot of ducts.

2. Would be be feasible to fit the main unit on ground floor?

3. What will be vastest cost effective small sized system which can send in oxygenated/healthy air and also have cooling functionality?

 

Thanks,

 

P

 

 

[Oz07]

I reckon you could have all wall vents, also ground floor sounds ok. Don't know q3. 

 

[Bitpipe]

How airtight are you aiming to make your house?

 

While you need a strategy to meet Building Regs ventilation standards, air that is free to leave your home through extractor vans, trickle vents, leaky door and window seals and small gaps throughout the fabric will also take heat with it. 

 

MVHR replaces passive ventilation with a ducted fan driven system and in additional to providing a constant stream of fresh air to each room, and removing stale damp air from kitchens and bathrooms it has a heat exchanger to recover the majority (90%) of heat from the extracted air and use it to warm incoming fresh air. It runs in a background mode (usually 30% power) and a boost mode (50%). Boost needs to be triggered either manually with a pull cord/push switch or automatically by PIR sensors, co2 detection, extractor hood activation etc.

 

Building regs requires a max 10ACH, passive standard is 0.6ACH and it's commonly accepted that the heat recovery aspect of MVHR is only effective when you have ACH of 2 or below - that said, the ventilation will always be useful but heat recovery will be minimal.

 

It's also a common misconception that MVHR can significantly contribute to space heating or cooling - given MVHR air movement is very low (by design) and air is a very poor carrier of heat (or coolth) the effect will be negligible in most houses but can act as a trim in a very low energy home (passive standard).

 

To meet BR, you need an extract in each bathroom/toilet and kitchen and they need to meet specified min extract rates. You then need to balance this with equivalent supply to your living and sleeping areas. Typically this is a ceiling vent per bedroom and one per enclosed living area. You need to aim for cross flow from supply to extract and ensure that there is the regulation 7600mm2 gap under each door where there is a supply or extract.

 

Another common misconception is that you are not allowed to open doors or windows if you have MVHR. You can do what you want, and many of us do, especially in summer. Just be aware that creating natural cross draughts in your house will take warm (or cool in summer) air with it and may undo the good work your MVHR has been doing. Although one unique aspect of an airtight house is that just having one open door does not create a draught as there is nowhere for the air to go - open a second door and you'll know all about it.

 

You can put the main unit where you want, ideally you want all internal ducting (usually 75mm flex duct, some use rigid) to be as equidistant as possible to make balancing easier but ours is in the basement on one side of the house and we got it to work ok. You'll also need large bore ducting from outside to the unit for the supply and extract. These need to be insulated so can get a bit bulky. You'll also need manifolds to collect / distribute the flexi ducts and access to the main unit to service and change filters is needed. The unit will also generate condensate so that needs to be piped away as waste water.

 

Vents are usually in celling but you can wall mount (as we have in our rooms in roof) and even floor mount but just be aware of the cross ventilation requirement, no good air scooting across the floor and out the door.

 

Many here have DIY installed their systems, you can also self calibrate and 'sign off' - there is no professional qualifications required. 

 

I think that's it!

 

My suggestion is to approach someone like BPC with your floor plans and let them spec a system for you and see if you can make it work with your design constraints.

 

 

 

Edited December 3, 2020 by Bitpipe

[Oz07]

Good post @Bitpipe just noticed though build regs air target minimum isn't ACH I forget how the difference is worked out. 

 

Also nowadays I think it's hard to have air target of 10 and pass SAP most I think aim for around 5

[Bitpipe]

4 minutes ago, Oz07 said:

Good post @Bitpipe just noticed though build regs air target minimum isn't ACH I forget how the difference is worked out. 

 

Also nowadays I think it's hard to have air target of 10 and pass SAP most I think aim for around 5

 

Good catch  - found some details here http://www.icopal.co.uk/Products/Pitched_Roofing_Underlay/Products/Vapour_Control_Layers/domestic-use-vcl/design-guide-airtightness/building-regulations.aspx

 

I guess the take away is that if you are aiming for good airtightness then MVHR will become a core part of that strategy (as you'll need it not to suffocate), If you're not being as aggressive then it's a nice to have but be realistic about the cost/benefit, return on investment etc. 

 

Being Eco-friendly is a laudable objective but can take you down some less productive avenues. Investing in a good design, well insulated and airtight fabric, etc is often the best return for your money - shoehorning 'green' tech into a poorly designed and built house will never be that effective.

[Paurav]

Thanks for your responses and suggestions folks! I feel comfortable of fitting wall vents now and may go for 2nd floor ceiling vents above the high open internal voids to take in most hot rising air for recycling. My thought is, with heigh voids across of 3 floors, an air recycling system between ground to ceiling will be highly beneficial. Haven't found any MVHR system with cooling yet, but would be good to get fresh cold air during few hot and humid days we get in London. Please send your best unit/system suggestions.

 

As we are in initial stages of the build, we have designed and are aiming for max air tightness or <5 m3/hr/m2 with no leaky door and window seals. The kitchen extractor has air seals too. Trickle vents can all be replaced with MVHR ducts and should not be needed anymore. Only opening for a few times a day would be our front door to enter/exit the house. Although, its hard to measure what figure we could get at this stage and I need to form a decision for MVHR immediatly. We got Stroma involved and I will be speaking to their ventilation experts during next week. Thanks for suggesting BPC, I will send our plans for their advise too.

 

 

Edited December 6, 2020 by Paurav
spelling correction

[A_L]

25 minutes ago, Paurav said:

Haven't found any MVHR system with cooling yet,

 

Any except the 'combis' have a heat pump that provides cooling - https://www.genvex.com/en/products/air-ventilation---air-heat-pump  just do not expect them to cool an 'oven',