MVHR system in new build

[JamesP]

I used BPC who supplied a very good service.

However the MVHR system is not adequate to meet the building regs even running at 100% so had to be creative with figures for BC. Fortunately you can self certify.

They recommend a specific kit based on the floor area though I think the internal volume should be taken into consideration as we have many high ceilings, 3 and 4 metres in the majority of the house.

Just something to consider.

[Rishard]

@JamesP- so does your system struggle? Seems a shame to install something which isn’t up to its intend job. So you think they missed out on the volume area? 

[ProDave]

2 minutes ago, Rishard said:

@JamesP- so does your system struggle? Seems a shame to install something which isn’t up to its intend job. So you think they missed out on the volume area? 

Many think the BC ventilation requirements are too big, and after sign off turn the unit down to a lower ventilation rate. Passive house does not require such a high ventilation rate.

[JohnMo]

Had MVHR flow rates set per Scottish planning the other day.  As soon as the guy left I put mine on to setback (mid way between min speed and current set point).  Considerably lower flow rates, closer to passivhaus and no noticeable increase in CO2 levels, in living space and bedrooms.

[Rishard]

Ah I see so it’s more a case of yet again inadequate understanding from bc rather than a fault of a supplier selling you an under spec system. Gottcha 

[TerryE]

41 minutes ago, JamesP said:

However the MVHR system is not adequate to meet the building regs even running at 100% so had to be creative with figures for BC. Fortunately you can self certify.  They recommend a specific kit based on the floor area though I think the internal volume should be taken into consideration as we have many high ceilings, 3 and 4 metres in the majority of the house.

 

If you want an installer to take design authority and prepare the building control documentation and certification, then this is one valid route but I feel you should expect to pay ~2-3K for this service plus typical manpower rates for installation. On the other hand if you want to DIY, then I suggest that you need to take design authority and do your own calcs.  IMO, BPC provide a useful design validation, but it's essentially a "free" service funded on their supplier margins.  Having 4m high ceilings is the sort of thing that they might miss when working from plans.

 

20 minutes ago, JohnMo said:

Had MVHR flow rates set per Scottish planning the other day.  As soon as the guy left I put mine on to setback (mid way between min speed and current set point).  Considerably lower flow rates, closer to passivhaus and no noticeable increase in CO2 levels, in living space and bedrooms.

 

Again, this is a common theme discussed in the past.  The BReg ventilation reqs are rather high in terms of volume exchange.  A lot of us cranked the flow-rates up high to do the certification report, and then dialled the rate back (say to 30%@ normal; 50% @ boost) for living in.      

[SteamyTea]

28 minutes ago, Rishard said:

Ah I see so it’s more a case of yet again inadequate understanding from bc rather than a fault of a supplier selling you an under spec system. Gottcha 

Except Building Control are the ones that sign things off, not the suppliers.

It is not that hard to calculate the individual room flow rates, add them all up, then look for a suitable unit.

[Rishard]

@SteamyTea- is there a simple calculation I can do? Happy to send of my plans to several suppliers to see what their workings look like. One thing I did think was, is the mvhr system ducting something which needs to go in to first floor during the build, if say I was to use rigid ducting. I appreciate this opens up another can of worms regarding duct design. From reading peoples experiences on here a combination of rigid spiral duct twinned with semi rigid /flexible is popular with plenty of silencers along the way. 

[JamesP]

1 hour ago, Rishard said:

@JamesP- so does your system struggle? Seems a shame to install something which isn’t up to its intend job. So you think they missed out on the volume area? 

No blame on BPC, it works fine. We run it at 30% all the time though have a manual boost option and the humidity sensor kicks in when having showers. The machine was specified for the area supplied in the drawings though was at its maximum area.

Would not be without it.

[ProDave]

The easiest for DIY is the semi rigid ducting and a radial system to a plenum box.

 

 

 

[JohnMo]

Easy to work out read building regs for your region.  In Scotland it's basically 0.5 ACH.  So calculate the internal volume, divide by 2.  That's the basic normal flow, add 25% for boost.

 

Choose a unit that flows your boost figure at about 70% or lower it's rated capacity.

 

Extract in wet rooms and kitchen, supply everywhere else.  

 

As Dave say semi rigid is easy to install, just plan ahead.

 

 

 

[Rishard]

Ok great. Semi rigid looks straightforward enough. What’s the difference between radial and other designs? I’ve not read into this part much. 

[TerryE]

19 minutes ago, ProDave said:

The easiest for DIY is the semi rigid ducting and a radial system to a plenum box.

Pretty much identical to mine except that I put the manifolds in a cupboard in the loft so the pipe go down into the floor rather than up into a wall

[TerryE]

9 minutes ago, Rishard said:

What’s the difference between radial and other designs?

IMO, they are easier to balance and a lot easier to run: no pipe Ts or joints; just a 1-1 from the central manifold to the individual vent,

[Marvin]

7 hours ago, Tom said:

 

Eh? I didn't say anything about energy saving!

Sorry @Tom I miss understood.

 

But whilst on the subject, for anyone else interested in MVHR's:

 

I think the average temperature here, over the whole year is 7 C. The average temperature in our home is 21C day 19C night. Taking into consideration that when the temp is above 21C the place has to be cooled and energy will be used to cool the internal air and the MVHR will be used to recycle the cool, I wonder how much energy/ money it saves a year. 

 

Say three-quarters of the year of 10 degree temp change (some days about the same temp), based on @TerryEs figures, reduced to only 10C change of temperature would be a saving of about 17 kWh a day. Say 25p a kWh that’s about £4.25 a day, for half a year that's about £1,100 a year saving.

 

The box was say 1200, the pipes and insulation say 1800 labour would have been about 1500 so about £4.5k

 

If it was trickle vents on a cold windy winter's day...

 

Ours is a 100m2 bungalow. Trickle vents on a 300m2 one would need more trickle vents so...

 

 

M

[Rishard]

So from outside to inside it’s. Atmosphere-mvhr box- manifold- smaller ducting to plenums in each room. One manifold serving exhaust one manifold serving supply? In a 2 story building is the main unit better on ground or first floor. We plan to have vaulted ceilings so probably won’t have attic space. A small space could be created above a bedroom ceiling if this is a preferable location. 

[ProDave]

39 minutes ago, TerryE said:

Pretty much identical to mine except that I put the manifolds in a cupboard in the loft so the pipe go down into the floor rather than up into a wall

Mine is on the first floor, so some go up into the loft for the upstairs rooms, and some go down into the floor void for the downstairs rooms.  Every install is individual, there is no right or wrong.

[TerryE]

@Marvin, I just cranked the maths on the figures that you provided, not on my figures.  For my own house the numbers are a lot more compelling.  I have a 3 storey house with ~ 250m² floor-space with UFH on the ground-floor slab only.  The inflow on the MHVR inlets is around 19°C, but we keep our internals at around 22½ °C.  We don't have any heating on the top two floors (except a couple of small oil filled rads which my HA system turns on for a few hours every night during the coldest three months). 

 

If the 0.5 ACH was coming in at 0-5°C overnight instead of 19°C, then we would have needed some form of decent CH system for the top two floors. 

 

You just can't look at one component and try to optimise it in isolation (not unless your name is Boris 🤣).  You have to treat the house as a whole.  You also can't just use averages, because if you want to avoid freezing your bollocks off, then your system still has to cope with that Dec-Feb window when the night temperatures can hover not too far above 0°C. 

 

We did our MVHR for ~£2K, labour DIY.  We avoid maybe £4-5K of CH costs and needing to install gas, etc.  Ignoring the avoidance of replacement CH solutions, the payback period was under 3 years.  As well as living in a fresh damp-free environment.

 

IMO, this one was an  absolute no-brainer.

 

48 minutes ago, Rishard said:

A small space could be created above a bedroom ceiling

 

You do need reasonable access for maintenance / cleaning, say once a year or so.

Edited March 13, 2022 by TerryE

[SteamyTea]

9 hours ago, Rishard said:

is there a simple calculation I can do?

I think all the relevant rates are in the building regulation documents.

Have you downloaded them?

[Nickfromwales]

8 hours ago, Rishard said:

So from outside to inside it’s. Atmosphere-mvhr box- manifold- smaller ducting to plenums in each room. One manifold serving exhaust one manifold serving supply? In a 2 story building is the main unit better on ground or first floor. We plan to have vaulted ceilings so probably won’t have attic space. A small space could be created above a bedroom ceiling if this is a preferable location. 

Yes, a manifold for each. The only thing missing from your line up would be the attenuators. The silver boxes in the image.

I never do an MVHR install for a client without them tbh, as my focus is always on having a system which is inaudible on trickle rate and that starts with a design where such things are considered and factored in early on in the build process ( eg so the required plant space can be realised, as that is often under-estimated ). One such current project has needed the plant space increased to almost twice the original size allowed by the architect!! Nearly half the utility room had to be sacrificed or we wouldn't have got everything in. It still surprises me that self-builders ( and their architects ) will apply an allowance that they deem is 'sufficient' for plant space, and not engage earlier with an M&E consultant to get this fully understood before finalising plans.

Having a mechanical system which is running 24/7/365 says it should have your undivided attention from the get-go, even more so when it is a system with the second largest bore distribution ducting after the soil pipework and may require penetrations through steels etc.

 

 

I've also now started installing the Brink Air Comfort units too, which are supplied with either warm or cold water from the ASHP to give active post heating / cooling. These take up quit a bit of space though, but are a great companion to the Brink MVHR units.  They're basically an intelligent FCU where the fans speeds get matched ( the Air Comfort has aits own integral fan to alleviate any stress on the MVHR unit fan which balances itself to the correct pressure / flow rates ) Nice bits of kit tbh. Attenuators on that install also.

 

 

These installs really do need some thought and planning, but the results, as @TerryE's calcs demonstrate, can be very well worth the capital investment of such a system, purely in the knock-on costs they may negate eg not needing heating on 1st & 2nd floors because of the heat recovery element factored into the MVHR design. I'd rather spend money on a system that will save me buying fuel for heating, plus gives me an option to integrate cooling ( where an ASHP is present ), plus any money saved can be deducted from the supply and fit costs of the MVHR. Add to that the constant fresh air supply to all the rooms, 24/7, plus the ability to go mad with airtightness to vastly minimise ventilation heat loss, the biggest issue any dwelling would have, ( more so than having great levels of insulation ), as also stated above is a complete no-brainer. 

[Marvin]

9 hours ago, Marvin said:

based on @TerryEs figures

.... using @TerryEs way of calculating the energy required to heat air.....

[deuce22]

14 hours ago, JohnMo said:

Out of interest what airtightness figure was used to get an 88 from SAP?

 

 

I haven't had an airtightness test done yet.

 

13 hours ago, Nickfromwales said:

Brink Flair 400 would be suitable. Change of £2.5k. PH certified and very quiet / robust. I've had great feedback from clients we've fitted these for, and the guys at CVC Ltd have provided the units, associated materials and design support with excellent results every single time. Very quiet units, but we always fit silencers ( aka attenuators ) in every instance for near inaudible operation, often inaudible at trickle rate and only slightly notable during boost.

I'll have a look into these today, thanks.

 

13 hours ago, Rishard said:

Another learner venturing into mvhr systems. I’m also very early in our project, planning passed and working towards building control application stage/demolition of existing structure.. we are 200m2. Any other recommendations for supplier designs to quote for our system? I will be installing as I’ll be doing most of the build (builder) along with good plumbing/electrician friends. None with much mvhr experience on low energy housing, yet! 

I'm in a similar situation to you. A builder with some decent tradesmen friends to help out.

 

 

[deuce22]

There's a lot to take in, but I'll start speaking to some companies this week.

 

Thanks all for the help and advice.

[dpmiller]

19 minutes ago, deuce22 said:

14 hours ago, JohnMo said:

Out of interest what airtightness figure was used to get an 88 from SAP?

 

 

I haven't had an airtightness test done yet.

 

But there needs to be a number typed into your design SAP- what was it?

[deuce22]

5 minutes ago, dpmiller said:

 

But there needs to be a number typed into your design SAP- what was it?

Is this it?