MVHR - one unit or two?

[SBMS]

Currently looking at MVHR options for our build. Around 390m2 Over three floors (room in roof) so we have been given two options from BPC for MVHR:

 

2x Zehnder Q450 (total price 7901)

1x Komfovent r700 (total price 6000)

 

same ducting for both options. 

 

The zehnder is 95% heat recovery but the komfovent is 83%. However I would prefer to only have a single unit and there is a price differential of 2k which I probably wouldn’t make up in heat recovery costs vs the equipment cost difference over lifetime of the unit. I also don’t know if electricity running costs would be greater with 2 units. 
 

Anyone been in a similar situation or have any advice as to which route to proceed?

[SBMS]

Also - can anyone help with a very simple way to size a unit based on the size of the house? I see figures on the units like 600m3/h airflow and 108l/s requirement for building regs - how can I calculate the requirements for the house?

[Conor]

I went for two 400m³ units for our 350m² (gross, 275m² net). Largely driven by shape of house and needing one unit on each side of the building. Other than duct run savings, other advantage is both units are only running at 25% capacity, so dead silent.

 

You really want to oversize you mvhr so it's running on low rpm. Don't get a unit that scrapes the flow rates for your floor area 

[JohnMo]

Go to building guidance notes, for MVHR and the rules are there. Plenty of posts on here also. Your trouble with a big house is going to be over ventilation. Big house needs big flow rates, a few very cold days will drive humidity down low, if you are not careful.

 

Those prices are daft, but you are building a mansion so you get what you get. Hope you end up saving £350 in heating costs a year, because thats what you are paying just to buy the MVHR over 20 years.

 

Two units (what we have) can make install easy with way shorter runs, but two sets of filters to change each time.

[SBMS]

10 minutes ago, JohnMo said:

Your trouble with a big house is going to be over ventilation.

Not sure what you mean by over ventilation? 

 

Re your comment on price over 20 years, it’s not really the motivation for the MVHR. More because we are aiming for high air tightness so need MVHR and the comfort element. 

[JohnMo]

1 hour ago, SBMS said:

Not sure what you mean by over ventilation? 

The more air you take into the house that is cold and heated up (MVHR heat exchanger, heaters) the dryer (low humidity) it gets. This normally balanced by humidity generators (people, showers, general life) the bigger the house the few the people per m² the bigger the issue. We are down to about 40% humidity after a few cold days. Got down to 35% last year.

 

My first quote for MVHR was £10k, ended spending closer to £2k

[Russdl]

I did a lot of research into the Komfovent when I was looking for our MVHR, sadly because I’m a goldfish I can’t remember the specifics but I do recall concluding that the Komfovent was not the way to go for us. Something to do with the rotary wheel system and its efficiency - or lack of, in a domestic setting.
 

It may be worth researching rotary wheel MVHR pros and cons before you head in that direction. Sorry I can’t be any more specific. Probably an age thing. 
 

[Nick Laslett]

2 hours ago, SBMS said:

Currently looking at MVHR options for our build. Around 390m2 Over three floors (room in roof).

I’ve not looked at this for a long time, so please take this comment with a degree of skepticism. 
 

The Zehnder ComfoAir Q600 is suitable for a house up to 350m2. Not sure how accurate your 390m2 measurement, but maybe 40m2 is not enough to really discount the Q600. My understanding from other posts on BuildHub, is that the Building Regs requirement for air supply is too high and after building controls signoff they dial back the MVHR settings. 

[bmj1]

I'm going to be controversial - but if you're doing x2 units - why do you need the same ducting requirement ? Could you reposition one of the units - and reduce your ducting run length ?

 

Perhaps it's marginal - but asking the question

[Iceverge]

A complete ballpark to sizing is to have a unit that can do 1 ACH at its max setting. 

 

Take a 165m² house with average ceiling heights of 2.7m you get 450m³ ish. 

 

A single Zehnder Q450 would be fine. 

 

In reality the sizing depends far more on the amount of people in the house than anything else. 

 

Make sure you can easily and economically service the machine. I'm about to do bearings on our PROAIR 600pli after 4 years. Filters are not particularly cheap at €22 each( I think) I've never taken out the heat exchanger and I'm not sure how easy it is yet. Will report back. 

 

 

 

[Iceverge]

Commercial units appear on eBay a lot cheaper.

 

This one will do 1000m³/HR..

 

 

Beware through it does use 250mm ducting so will take up a big chunk of space. 

[SBMS]

On 13/12/2024 at 00:32, Iceverge said:

A complete ballpark to sizing is to have a unit that can do 1 ACH at its max setting. 

 

Take a 165m² house with average ceiling heights of 2.7m you get 450m³ ish. 

 

A single Zehnder Q450 would be fine. 

 

In reality the sizing depends far more on the amount of people in the house than anything else. 

 

Make sure you can easily and economically service the machine. I'm about to do bearings on our PROAIR 600pli after 4 years. Filters are not particularly cheap at €22 each( I think) I've never taken out the heat exchanger and I'm not sure how easy it is yet. Will report back. 

 

 

 

So if the house volume was, say, 900m3 and I was aiming for 0.5ACH the unit would need to do 450m3? (Obviously would want to run it around 70% so probably would need 645m3?)

 

when they do their sizing do they account for target ACH?

Edited Saturday at 20:37 by SBMS

[JohnMo]

10 minutes ago, SBMS said:

aiming for 0.5ACH

Why so high, only Scotland demands that, and they are stupid, may make sense in shoe box, holding 4 people, but most other properties it's way too much.  You are wasting energy, dragging down humidity in the heating season, for no good reason.  Even Passivhaus only want 0.3ACH, which way more sensible. The fresh air requirements per person from PH is 30m³/h per person, while English BR is about 20m³/h. So 450m³/h would be fine, if your building for 15 to 20 people, otherwise...

[SBMS]

5 minutes ago, JohnMo said:

Why so high, only Scotland demands that, and they are stupid, may make sense in shoe box, holding 4 people, but most other properties it's way too much.  You are wasting energy, dragging down humidity in the heating season, for no good reason.  Even Passivhaus only want 0.3ACH, which way more sensible. The fresh air requirements per person from PH is 30m³/h per person, while English BR is about 20m³/h. So 450m³/h would be fine, if your building for 15 to 20 people, otherwise...

What would you aim for? I did do my heat loss calcs targeting 0.3ACH as it was a ‘healthy minimum’.  I suppose this reduced the need for a larger unit even more?

Edited Saturday at 21:00 by SBMS

[JohnMo]

13 minutes ago, SBMS said:

What would you aim for? I did do my heat loss calcs targeting 0.3ACH as it was a ‘healthy minimum’.  I suppose this reduced the need for a larger unit even more?

Really depends on how many you have in the house. We have 194m² and average room height around 3.2m, so 620m³.  We are set at about 140m³/h or 0.24 ACH (2 people plus dog) and looking to reduce during winter based on humidity. Certainly when we have had visitors, so it became 6 plus dog, no issues then either.

[SBMS]

3 minutes ago, JohnMo said:

Really depends on how many you have in the house. We have 194m² and average room height around 3.2m, so 620m³.  We are set at about 140m³/h or 0.24 ACH (2 people plus dog) and looking to reduce during winter based on humidity. Certainly when we have had visitors, so it became 6 plus dog, no issues then either.

Thanks. Never really thought about from perspective of number of people in house. Family of four with 2 dogs so my gut is that although it’s a large house, something like the q600 would suffice. 
 

@JohnMo why did you go for two units out of interest (and how on earth did you get that for 2k!)

[JohnMo]

8 minutes ago, SBMS said:

why did you go for two units out of interest (and how on earth did you get that for 2k!)

Two units made the install easy. House is 25m long and would have resulted in a few long double runs, these became short single runs.

 

2k, the 2x Titon units came from eBay, new unused for £250 and £320. Rest of the costs for duct etc etc.

[Iceverge]

1 hour ago, SBMS said:

So if the house volume was, say, 900m3 and I was aiming for 0.5ACH the unit would need to do 450m3? (Obviously would want to run it around 70% so probably would need 645m3?)

 

when they do their sizing do they account for target ACH?

 

Try to size the unit to run at about 30%. 

Using 30m³/head/hr it conveniently works out at sizing a unit at 100m³ max output per person. 

 

 

For example this unit at 600m³/hr would be perfect for 6 people. 

 

Remember there's no point in sizing your MVHR for 10 people of they're only going to be there very occasionally. 

 

In the event of visitors just crank up the unit a little and tolerate the lower efficiency for a day or two. Or open a window. 

 

 

 

 

 

[SBMS]

39 minutes ago, Iceverge said:

 

Try to size the unit to run at about 30%. 

Using 30m³/head/hr it conveniently works out at sizing a unit at 100m³ max output per person. 

 

 

For example this unit at 600m³/hr would be perfect for 6 people. 

 

Remember there's no point in sizing your MVHR for 10 people of they're only going to be there very occasionally. 

 

In the event of visitors just crank up the unit a little and tolerate the lower efficiency for a day or two. Or open a window. 

 

 

 

 

 

How come all MVHr designers size off the volume of the house, not the number of occupants?

[Iceverge]

15 minutes ago, SBMS said:

How come all MVHr designers size off the volume of the house, not the number of occupants?

 

I think the regs push them that way. 

 

In fairness if you have a v large house it is theoretically possible that it might be occupied to the rafters in future. 

 

Regs must cater for worst case scenario.