MVHR specification

[Rich97941]

First Post and a perhaps rather stupid question regarding MVHR specifications. I've done the calcs for our 150m2 bungalow and get to a requirement of 190m3/h. So when specifying a MVHR unit do I need it to be 190m3/h as that's the total flow through the system on maximum or do I need 380m3/h as the sum of supply and exhaust flows. (I guess I'm just a bit suspicious of how manufacturers might choose to describe their units.) My feeling is that it should be 190m3/h + an allowance for duct loses (I'll probably end up with 70mm semi rigid ducting) so something in the 250-300m3/h range should be good, but don't want to fall into a newbie pooh trap by either under or over specing the unit.

[tuftythesquirrel]

No, not stupid and I’m going through the same exercise. Our useable floor area is around 360m2 and I’ve had quotes for a single Stiebel Eltron LWZ 280 Plus MVHR Unit (Rated at 350m3/h) through to twin Vent Axia Plus B (Rated at 980m3/h). So I’ve gone back to first principles.  

The Passivehaus Trust Good Practice Guide: MVHR for single dwellings September 2018, Rev 1.2, says use a minimum of 1m3/h per 1m2 of floor area, so based on that we need 360m3/h.

However, when using the Building Regulations 2010 Part F1 - Ventilation Dwellings, 2022 (for Wales – sorry if the Table references are different), you have to use Table 1.2 for extract and Table 1.3 for supply.

But the following also applies (for supply only)…. 1.24 The minimum whole dwelling ventilation rate for the supply of air to the habitable rooms in a dwelling should meet both of the following conditions:
a. A minimum rate of 0.3 litres per second per m2 of internal floor area (this includes all floors, e.g. for a two-storey building, add the ground-floor and first-floor areas).
b. A minimum rate determined by the number of bedrooms, as specified in Table 1.3.

So in practice, because I have MVHR supply and extract terminals in nearly all rooms, for extract if I use the 0.3 multiplier my kitchen is 9.9 l/s, however table 1.2 says it should be 13 l/s. Again, for supply if I use the 0.3 multiplier my bedroom is 6.5 l/s, however table 1.2 says it should be 10.3 l/s (3 bedrooms is 31 l/s).

So the BC tables effectively add flow for specific rooms to the overall flow rate. I’ve attached my spreadsheet to hopefully demonstrate this.  Doing the maths, I end up with 281 m3/h for supply and 289 m3/h extract. I have the same issue, regarding do you sum the supply and extract or take the highest value. I also want to get though building control, so I was going to fill in the Appendix C - Completion checklist and commissioning sheet myself.

Jeremy Harris on this forum gave a good method for commissioning the system, but other people seem to agree that you set it up to get through using the building control values, then throttle it down again.

Any input appreciated.

MVHR Flow Rates v0.04 (1).xlsx

[JohnMo]

You need to add boost also, if you haven't done so already.

 

You need to look at min and max flow rates and size bigger than you think you need, so next size up ideally.  Otherwise the system will have too much fan noise.  You want the fan running as slow as possible while meeting your flow requirements.  

 

Flow is the same for supply and extract, the manufacturer figure is either the extract or supply rate, as they are both the same.

[Conor]

Fyi when picking a unit don't pick one that can barely meet the flow rates, you want a good bit of head room. We needed 600m3hr but the combined capacity of my two units is 800m3hr.

 

For a 380m2 house you'll struggle with a single unit, that's why we went for two.

Edited August 1, 2022 by Conor

[JohnMo]

2 hours ago, tuftythesquirrel said:

 

MVHR Flow Rates v0.04 (1).xlsx 82.62 kB · 2 downloads

 

 

It's not normal to extracts in hallways, this would normally be a pass through area.  So you supply a dry room the air flows through that room through corridor and across a wet room to the extract.

 

Master bedroom flow looks low, I would bump up to 40-45m3h, also bump up the lounge.  Wet room flows look a little low also.

 

You need to get the flow and return rates the same, so the system is balanced.

 

I set mine as follows

Passivhaus house asks for 0.3ACH, Scottish building regs 0.5ACH. worked out the room flows at passivhaus flow rates, prorated the flows up to Scottish building regs flow rates, to give an overall 0.5ACH.  After sign off, I can easily slow the fan speed down to get 0.3ACH.

 

I would delete the extracts from the hallways, only extract from the workshop and plantroom. An airing cupboard if you do anything should be extract not supply, as its for drying damp things out.

 

Once you have sign off, you will find that with a big house in cold weather your relative humidity may be low, unless you have a house full of people.  As you are really over ventilating the house.  Passivhaus say on a large low occupancy house 0.25 ACH may be the correct figure.

[Gone West]

This is the table I calculated back in 2010 for my PH build. I can't remember much about how I did it but I commissioned the setup myself using a Testo 405i and the BCO wasn't interested in any aspect of the system.

 

[tuftythesquirrel]

Thanks for the replies.

I’ve included the hallways since that’s where we will be storing our coats, boots and umbrellas, so a source of moisture. It also has the side effect of balancing the supply and extract rates. Our airing cupboard is a misnomer, since it is actually a walk in wardrobe, some I’ve allocated it as a supply point. I know I’ve also included a plant room and a workshop, which normally is done, but we may convert these to bedrooms in the future so I would rather include these now. So this makes my unit larger.

According to my calculations the maximum rate I need is 78 l/s for supply and 72 l/s for extract to meet building regulations, but also in the rooms that people don’t normally include, hallways, workshop. So this is 281 m3/h supply and 260 m3/h respectively. So if I round it up to say 290 m3/h and pick for example, the Vent Axia Plus B, rated at 490 m3/h. Even when running at the flows recommended by the regulations, I will still only be running at just under 60% of the units capacity. Does this not give me acceptable headroom? I believe most people say the regulations flow rates are over the top, hence giving me even more slack.

Approaching it from another angle, using ACH, if I take the Vent Axia Plus B unit and run it at 60%, it will generate 294 m3/h. My building volume is 749m3. So around 0.4 ACH is possible.

Unless I’ve made a mistake in the maths, the unit I’m looking still seems a contender.

[tuftythesquirrel]

6 hours ago, Conor said:

Fyi when picking a unit don't pick one that can barely meet the flow rates, you want a good bit of head room. We needed 600m3hr but the combined capacity of my two units is 800m3hr.

 

For a 380m2 house you'll struggle with a single unit, that's why we went for two.

If you needed 600m3/h and you can generate 800m3/h, that means you are running at 75% of maximum capacity, which I was led to believe by some companies that this is too high and it should be around 60%. Or do you not run at 600m3/h but lower, which I have referred to in my response in the main thread. Thanks.

[tuftythesquirrel]

43 minutes ago, Gone West said:

This is the table I calculated back in 2010 for my PH build. I can't remember much about how I did it but I commissioned the setup myself using a Testo 405i and the BCO wasn't interested in any aspect of the system.

 

 

Ahh. good to know I get away with virtually anything, but I just need to get it straight in my own mind. The above suggests you needed 100m3/h, so what size unit did you fit and do you run at 0.4 ACH on a day to day setting or lower and boost when required? Thanks.

 

[Conor]

24 minutes ago, tuftythesquirrel said:

If you needed 600m3/h and you can generate 800m3/h, that means you are running at 75% of maximum capacity, which I was led to believe by some companies that this is too high and it should be around 60%. Or do you not run at 600m3/h but lower, which I have referred to in my response in the main thread. Thanks.

I'm running at half that, it only runs at 600 when on boost. I've more work to do getting everything fully working and properly setup. 

[Gone West]

1 hour ago, tuftythesquirrel said:

 

Ahh. good to know I get away with virtually anything, but I just need to get it straight in my own mind. The above suggests you needed 100m3/h, so what size unit did you fit and do you run at 0.4 ACH on a day to day setting or lower and boost when required? Thanks.

 

We used a compact unit which was a Genvex Combi 185LS. It's a combined MVHR, hot water cylinder and EASHP. IIRC, we've since moved house, I set the fan levels at around 30% and we never changed that. We never used boost and we didn't have any condensation in the kitchen or bathrooms.