MVHR Design Help - Calculations In Layman's Terms

[Johnny Jekyll]

Hi Everyone,

 

I've spent ages trying to understand MVHR and what's best for my house, I could really do with some help please? Below is my design and some calculations, would you mind casting your eyes over it and help with any of my questions at the bottom? Thanks very much.

 

OVERVIEW

• Planning accepted August 2020 so falls under building regulations at that time.
• Small 3 bed detached house, total of 126.5 square metres across two floors.
• Includes a family bathroom, master en-suite and downstairs utility containing toilet.
• Cold loft with insulation roll at ceiling level (200mm between 9by2 joists and 200mm crossways above), and Intello+ airtight membrane.
• Loft legs will support a loft deck over the insulation.
• Posi-joist midfloor so plenty of room for ducting although only 75mm gaps around steels in certain parts.
• We are not officially going for Passivhaus but there has been good attention to air tightness and good results are expected.
• House is up and watertight, starting first fix now.
• Plan is to install and commission the MVHR system myself.

 

UNIT & DUCTING

• The current spec (from BPC) is a Vent-Axia Sentinel Kinetic BH with 75mm Anti Static & Anti Bacterial radial ducting.
• The most feasible and spacious location for the MVHR unit is in the cold loft space (above en-suite) to the front left of the house (North gable wall).
• The intake/extract through the gable wall will be nice and short, insulated, spaced minimum 1.5m apart to avoid cross-contamination.
• It’s also far away from the boiler flue on the other side of the house.
• Being a cold loft I will attempt to insulate the unit in a purpose made box to help.
• The radial ducting for upstairs rooms will run between the loft mineral wool insulation.
• The radial ducting for downstairs rooms will drop down into the mid-floor behind a stud wall in the en-suite.

 

DRAWING

• The attached drawing shows the house layout, with beds, baths, showers, toilets, wardrobes all in what should be their final positions.
• The purple MVHR is the unit location in the cold loft to the North gable wall (above en-suite).
• I've worked out and planned all the MVHR duct runs myself.
• The green arrows / circles are supply ducting / room valves.
• The red arrows / circles are extract ducting / room valves.
• The numbers show either one or two duct runs to that valve (specified by BPC).

 

BUILDING REGS NOTES

 

• Document F minimum supply of air to habitable rooms should meet both the following conditions (basically whichever is higher):            
• Minimum rate of 0.3 litres per second per m2 of internal floor area - 126.5 * 0.3 = 37.95 litres per second (l/s)
• Minimum rate determined by the number of bedrooms (3) = 31 litres per second (l/s)

 

• Building Regs minimum high extract rates (l/s) by room type:
• Kitchen = 13
• Utility = 8
• Main bathroom = 8
• En-suite = 8
• Total = 37    

 

• Building Regs minimum low extract rates (l/s):
• 0.3

 

• Space under closed doors - A 760mm wide door needs a 10mm gap to give required 7600mm2 of space. Wider doors need proportionately less.

 

CALCULATIONS

• House Floor area = 126.5m2
• Room heights = 2.45m
• House Volume = 310m3
• The table shows my actual calculations when measuring the rooms in the house (slightly reduced floor area and volume due to stud walls etc).

BPC

• BPC have designed / specified / quoted a full system. I'm happy with their pre-order service and help but it's of course limited at this stage.
• It's quite a basic specification so I'd like to check if I need to make any affordable improvements, for example making my own acoustic manifolds (as some have done on here) instead of the basic stainless steel manifolds in their spec or the very expensive acoustic ones you can buy.

QUESTIONS

• I'm trying to better understand my calculations in the attached table (copied how others did it on the forum). What am I looking for with litres per second? Would anyone be able to explain in simple layman's terms?
• Are there any building regs on air tightness? I'm planning a test soon and results should be good. Do I need to add air tightness into my MVHR calculations?
• Any other calculations I need to look at / understand?
• BPC specified a Vent Axia Sentinel Kinetic BH, which is suitable for houses up to 150 m2. Should I be looking at a bigger / better unit, considering my duct lengths (see table) and the fact that my wife wants the system very quiet? Am I daft looking at a high quality passive unit like the Zehnder ComfoAir Q350 (much more expensive as you know)?
• Are there any specific makes of plenums or room valves (perhaps acoustic versions) that will help with noise reduction?
• Am I missing anything else obvious?

 

Sorry for the big post and thanks very much.

[Conor]

1. Building regs min flows to rooms is based on an average for the whole floor area, not per room basis. So you don't need 11lps for the living room. Increase the flow rates to other rooms and reduce this rooms rates.

 

2. Can you straighten the duct runs to the kitchen extract? This will be your highest flow rate run and you need it to be straight and short 

 

3. Bedroom plenums are in the wrong location, esp master. Need to be right in the far corner to generate the right flow paths. With that current design, you won't be refreshing the air at all above the bed. It'll go straight to the ES extract point.

 

4. I'd always reccomend a bigger unit than needed, but saying that I've had to dial my unit back so much that it wouldn't make any difference if if got the model below, other than saving myself £200.

 

5. Running the ducts will compromise your overall insulation but probably not enough to worry about. How are you making the boundary between the living space and loft airtight? 

 

6. BC airtightness requirement are a joke. Even the now one here of 5ac/hr is poor. You want a design assumption of 3ac/hr and hope to get 1ac/hr.

 

7. Don't forget you'll need a condensate trap and drain for the unit. But you're right above a bathroom so 5ha5 should be easy.

 

8. For runs over 7-10m you'll need to double them up if using 75mm ducting.

 

 

[JohnMo]

Could you locate the plenums more centrally this would limit duct lengths.  Reduce system pressure drop, less noise as the fan.

 

Just buy the plenums there's normally enough other stuff going on and life's too short.

 

I would use passivhaus flows as a good starting point up them slightly to get br rates if required.

[Johnny Jekyll]

Thanks very much @Conor and @JohnMo for the replies and info, which I'll digest over the next few days, and no doubt post further questions.

[Johnny Jekyll]

On 04/12/2022 at 08:16, Conor said:

How are you making the boundary between the living space and loft airtight?

 

Mixture of parge coat and liquid membrane paint on the walls.

Intello+ airtight membrane with Tescon Vana tape and Tescon Naidec tape to ceilings.

Mixture of Orcon F, Airtight foam and Tescon Vana tape to all other openings e.g. around windows.

I’m sealing everything.

 

[TerryE]

Any penetrations in your inner air-tightness barrier will compromise the integrity and need work to address.  Eg. how are you hanging your joists?  Sitting them on your inner blockwork leaf will involve a lot of taping to mitigate leak paths; joist hangers make this all a lot simpler.  This is just one example: most air tightness issues are simplest addressed / mitigated by design before build rather than post-build repair.

Edited January 21, 2023 by TerryE

[Johnny Jekyll]

1 minute ago, TerryE said:

Any penetrations in your inner air-tightness barrier will compromise the integrity and need work to address.  Eg. how are you hanging your joists?  Sitting them on your inner blockwork leaf will involve a lot of taping to mitigate leak paths; joist hangers make this all a lot simpler.  Most air tightness issues are simplest addressed / mitigated by design before build rather than post-build repair.

 

Hi @TerryE , thanks for comment. The house is actually up and watertight.

Airtightness was planned in from the start. For example regarding the mid floor, we took the walls straight to the top and put the roof on (7n blocks both skins) then windows went in (used a temporary scaffold birdcage mid floor on the inside to do this). Then parge coated the inside blockwork, then installed 9by2 ledger board / joist hangers / posi joists mid-floor (all specified with structural engineer). Doing most of the overall work myself, and every bit of the airtightness. Project is taking ages but standard is very high. I'm changing my mind on all my MVHR stuff, it's the next big to-do as I'm starting first fix now.

[Johnny Jekyll]

Hi Everyone,

 

Had to finish another job in the house over Xmas but now full steam ahead on my MVHR stuff.

 

I had a good chat with a tech guy at Zehnder. He said locating the MVHR unit in the cold loft (which was my original plan at the top of this post) will vastly reduce MVHR performance, both in winter and summer. So, I’m trying to re-design it into the building envelope. Haven't started first fix yet so can do this.

 

Looking at different options. Hopefully a simple-ish question to start.

 

Question - If I were to run my radial ducts for upstairs rooms within the stud walls and locate the room valves high up in those stud walls, rather than in the ceilings (this could avoid ducts going up into the loft altogether), would that reduce performance? Especially in terms of extract valves in the bathroom and en-suite?

If the answer is as simple as all extract valves should be in ceilings then I'll ditch this idea and move on?

 

Thanks in advance for any valued opinions.

[TerryE]

16 minutes ago, Johnny Jekyll said:

... the cold loft ...

 

What was your reasoning for this design decision?  The estate agent that was selling our old house got chatting about our new-build plans and gave us an incredibly useful piece of advice: if you think that you might ever convert your loft then by far the cheapest option is to do this within your initial build.  We had to change the pitch from 40° to 45°, but now my some has his own bedsit on the 2^nd floor.  It also simplified doing a lot of services.

 

Whilst it is too late for you now, a similar argument applies: if you ever want a warm loft ....

 

There is nothing wrong with having the MVHR on the upper floor, but running the ducting for the upper floor in the loft space; you just need to be very particular about how you insulate your cross-rafter runs. Alternatively you can opportunistically drop these into the warmspace, e.g. if you are using a double stud wall for stepped back-to-back walk-in closets then you might run them along the hidden ceiling, or if you can lose the extra 110mm or so headroom in a hallway, then run then in a false ceiling.  

 

This all being said, with a typical flow rate of ~ 1ms^-1,  the air will only spend seconds in this run of ducting, so heat losses with be negligible even with a 10cm insulation cover.

[Johnny Jekyll]

Hi @TerryE Unfortunately loft space is not high enough for warm loft, in plans it needed to be subservient to neighbouring property. Long story.

False ceilings are not an option, 2.4m room heights as it is and upstairs ceiling plasterboards already up. Happy to go into loft with ducting if needed as 400mm cross ways insulation going up there so ducting can fit well inbetween.

But would love opinions on my last question at this stage then will move onto others - If I were to run my radial ducts for upstairs rooms within the stud walls and locate the room valves high up in those stud walls, rather than in the ceilings (this could avoid ducts going up into the loft altogether), would that reduce performance? Especially in terms of extract valves in the bathroom and en-suite?

Again thanks in advance.

[ruggers]

On 21/01/2023 at 16:26, Johnny Jekyll said:

had a good chat with a tech guy at Zehnder. He said locating the MVHR unit in the cold loft (which was my original plan at the top of this post) will vastly reduce MVHR performance, both in winter and summer. 

Hi, interested to know, was the loft option a bad idea for the MVHR unit, the ducting or both? 

 

I plan to have mine in the loft but the pipes will mainly be short runs in the loft then right into the thermal envelope. I might try making a little insulated room within the loft to house the unit and the manifolds and monitor the temperatures within as an experiment. I won't know the results of this until built and too late but hoping it might help.

[Johnny Jekyll]

2 minutes ago, ruggers said:

Hi, interested to know, was the loft option a bad idea for the MVHR unit, the ducting or both? 

 

I plan to have mine in the loft but the pipes will mainly be short runs in the loft then right into the thermal envelope. I might try making a little insulated room within the loft to house the unit and the manifolds and monitor the temperatures within as an experiment. I won't know the results of this until built and too late but hoping it might help.

 

Hi @ruggersI've spoken with a tech guy at Zehnder and he strongly advised not installing the MVHR in a cold loft (mine is a cold loft), as it will vastly reduce performance of the unit. Both in winter (cold) and summer (hot).

I explained I was trying to do what you're doing (insulated room around it), and he still said the same.

He said to try and locate it within the building envelope.

So I'm now burning time trying to find a way to do that.

 

[ruggers]

On 21/01/2023 at 17:02, TerryE said:

This all being said, with a typical flow rate of ~ 1ms^-1,  the air will only spend seconds in this run of ducting, so heat losses with be negligible even with a 10cm insulation cover.

I agree with what your saying but I think in a cold loft, well insulated ducting would manage ok in winter especially with 400mm of mineral wool over it as Johnny is is doing, but summer would become the issue as the tiles/slate heats up from solar gain, it then warms everything with it and if the heat can reach the ducting, your going to get hot air blowing into the rooms even in summer bypass mode. 

If the unit manifolds and ducts can't all be within the thermal envelope, there has to be a trade off, so to minimise that It's important to keep loft duct runs as short as possible, and individually lag them. Some companies say the loft insulation over the ducts is enough and it certainly isn't.

Adding some roof vents and gable end vents will help in summer to add cross ventilation but it's probably never going to be enough. It will certainly help move some of that hot air though.

On 21/01/2023 at 17:15, Johnny Jekyll said:

But would love opinions on my last question at this stage then will move onto others - If I were to run my radial ducts for upstairs rooms within the stud walls and locate the room valves high up in those stud walls, rather than in the ceilings (this could avoid ducts going up into the loft altogether), would that reduce performance? Especially in terms of extract valves in the bathroom and en-suite?

Again thanks in advance.

Have you tried contacting some reputable installer companies and asking this question, the two I've spoken to will answer this for you. Even if you add them to the walls but have to do the ceiling vents for the bathroom and ensuite if it's best, it's a small trade off in not doing all of them this way?

[ruggers]

@Johnny Jekyll thanks for the info. I don't see an insulated room built around the MVHR unit in a cold loft being much different to a warm loft if the roofs insulated too, so I can only assume the ducts must be affected the greatest from this unless, it's the surrounding temperature of the remaining cold loft design would affect the room built within it.

 

[Johnny Jekyll]

7 hours ago, ruggers said:

was the loft option a bad idea for the MVHR unit, the ducting or both?

 

It was the MVHR unit, cold will affect performance in winter, hot will affect performance in summer.

Edit - Sorry think I already answered this above. Been a long day.

Edited January 29, 2023 by Johnny Jekyll

[Johnny Jekyll]

Progress update - After Zehnder tech guy strongly advising not installing the MVHR in our cold loft, we are now going to re-design it into the ground floor utility room.

 

We've had two supply quotes (new build 127m2, other details about the property at the top of this thread). Installing ourselves.

Quote 1 - BPC (free design) using a Vent-Axia Sentinel Kinetic BH system (up to 150m), with 75mm semi-rigid ducting.

Quote 2 - Green Building Store (paid design) using a Zehnder ComfoAir Q350 Pre-Heater, with 90mm semi-rigid ducting.

Our experience so far with both companies has been great. Green Building Store is twice the price, due to the design fee and of course much more expensive Zehnder unit.

 

I'd like your opinions as to whether the paid design and bigger Zehnder passive unit are worth it - what would you do if this was your house?

-- Is the Zehnder unit overkill for a 127m2 3-bed property like this?

-- How would the paid design help me? I pretty much know where the ducts must run (not many options), noise from so called pressure drops etc may be unavoidable (my understanding is pressure drops = noise)?

-- Fellow self-builders on here have built their own acoustic enclosures (I could also do that).

-- Did you just install the damn thing with a design and accept the final outcome after commissioning? How much science is behind the design?

 

Thanks everyone. So many decisions to make on your own, friendly opinions would really help.

[crispy_wafer]

I put a vent axia sentinel s in, twin 75mm ducts to all rooms, apart the master, where I split to two single 75mm ducts feeding low level vents. 

 

I ran the system up last weekend, as boys with toys just want to play.  Bear in mind the build has no plasterboard up, and is pretty much open plan at the moment.  The unit is located in the loft (insulated between and under the rafters).  Tried it at its pre-set normal and it's boost setting.  I was happy with the noise levels, which made no real discernible difference to me.  Noise levels at the Supply and extract plenums, nah, nothing that I could really tell unless I stuck my ears at the vent.  I reckon I'll be happy with my choice based on cost/performance.

 

I got BPC to quote, I then had a good guess at the layout plan, based on their number of supplies and extracts, and reading some of the best practice guides.  Plenty of plans have been discussed on here too, which helps with design decision.  I shopped about, ventilationland, isells, blauberg, and bpc, for components.  I did add a couple of metal duct silencers on the building supply and extract.

 

 

[Susie]

I like the vent axia S for my build also, 135m2 .

I was advised against the xs as the set up is a bit different I can’t remember why but the Wi-Fi bit can be purchased separately for the s model and it works out cheaper than the sx. 
not purchased yet but BPC have done some plans already for free enough to get through new SAP and Part O hopefully. 
you need to compare the SAP test results for your size of dwelling kitchen+n the thermal efficiency and SFP note BPC and some other sellers had the out of date SAP test 2013 is the old one I think it’s 2016 but you should check it’s been a few months since I looked at this.  The test changed and you need to check your comparing apples with apples. 

 

[Johnny Jekyll]

Thanks @crispy_wafer and @Susiefor the feedback. Looks like the Vent-Axia Advance S you mention use 125mm ducts instead of the bigger 160mm ducts that the Zehnder unit uses. Cool, hadn't looked at that unit. So did you simply get simple layout plans and equipment lists? Then just get on with it? Feel like I just want to do that rather than keep worrying about all the exacting design theory. I'm pretty good at following best practice and my work is very neat. I just wish someone would grab me by the scruff and say get on with it!

 

I've also started reading about the filters, which seem to make a big difference. Seems tests can produce much better (manipulated) results if coarse G4 filters are used, rather than the much better F filters. I guess that's where the (more expensive) Passive Zehnder type units come into their own. Still nice and quiet when using better filters. This stuff is never easy!

 

Edit: It's also ridiculous that Vent Axia units come with 5 year warranty and Zehnder units just a 2 year guarantee (talk about a put off). I take that back, Zehnder seems to have an extendable warranty as long as you replace filters as per their terms.

Edited February 18, 2023 by Johnny Jekyll

[Mike]

On 17/02/2023 at 19:24, Johnny Jekyll said:

Is the Zehnder unit overkill for a 127m2 3-bed property like this?

It depends on your objectives and in what way are you think it may be overkill.

 

FWIW I'm planning to install a Zehnder ComfoAir200 in a 40m² apartment, which you could call overkill. However it's suitable for several reasons: I can minimise noise by keeping the fans pretty slow under all normal conditions, it's one of the best performing units that I can fit in the limited space available, and I need a good intelligent summer bypass. Plus, by shopping around, I can get it for an acceptable price. I'll be feeding it with 125mm ducts - you don't have to use 160mm with Zehnder if you don't need the extra airflow.

 

You can find independently evaluated performance figures for the Zehnder unit in the Passihaus List. You can't do a you can't do a direct comparison with the Vent-Axia unit; they haven't had units evaluated so as you're reliant on their own figures.

[crispy_wafer]

 

1 hour ago, Johnny Jekyll said:

So did you simply get simple layout plans and equipment lists?

 

Just the quote/equipment list, then thought thanks for the unit suggestion, bet I can do it better and cheaper.  So I did.

[ruggers]

Hi @Johnny Jekyll I'll post my experience so far and see if you can take anything from it.

My supply rooms are: 4 x Bedrooms, 1 x living room, 1 x dining room.
Extracts are: 2 x bathrooms, 1 x ensuite - Toilet sink no shower, 1 x boiler room which houses the boiler & cylinder & will double up as a drying room with UFH passing through it, 1 x kitchen, 1 x utility room.
 
So for my build I contacted 4 companies, BPC was one of them, the other two i met at the home show and both were very similar in their quotes for spec and price but I found one of them willing to discuss every option in detail much easier. BPC only offered design and supply, not commission being in Ireland.

 

Their prices were good and they also mentioned something up that i need to look into in more detail which was how they recommend sizing the unit for my build. Something i will need to add a post for. The other two companies have told me that a Zehnder 350 is fine and BPC said that I'd need a 450Q or a Vent axia plus B. I need to know if the volume is calculated by ground floor and first floor total volume based on 2.4m ceilings, or by including the 250mm joist void into the calculation like my SAP did saying ground floor was 2.7m (250mm between intermediate floor). My floor space is 200m2. BPC didn't think a 350Q could run under 70% of full power on boost hence the need for a different or larger unit. Taking these comments back to the other designer, they provided an explanation are maintain that the 350Q is fine leaving me slightly confused.

 

When i first made contact, I supplied my preferred locations for MVHR unit, the room plenum locations and my proposed duct runs. the companies seemed to be happy with my design & said it's pretty much where they'd put things too, but I may require an extra vent or two in some of my rooms and this would be calculated in a detailed design. It was due to me having more extracts than supplies. They discussed options of 125 or 160 vents, having 75mm or 90mm pipes. Because i want a very quiet running system I immediately opted for 90mm semi rigid ducting.  BPC then said they'd always go with twin 75mm which was different again. A detailed design is going to cost around £700, i had 6 extracts and 5 supplies, I've now been told I will require 7 extracts and 7 supplies, for my large 24m2 living room they will look at the option of either 2 supplies or 2 ducts to one plenum. I'd like to use 90mm ducting.
 

The kitchen diner requires a supply where as I'd only included the extract at the kitchen end, & my larger main bathroom might require further extraction. I think it's possible with good self research to get a 90% efficient design, but as everything's fully enclosed once complete, I only get one chance, So i'm going to pay for the design to ensure everything's been considered by professionals, I'll do the full install to which they've listened to my suggestions and also highlights any bad ideas, then I'll pay for the £450 commissioning service to balance everything & complete the final set up.

[Johnny Jekyll]

Thanks very much for taking the time to reply everyone. I'll digest everything over the coming days. 👍