Has anyone got feedback good/bad on using several small MHRVs systems instead of one big one?

I am looking at having 3 units each of about 100m3/h max capacity (30m3/h trickle), located in different parts of the house (1 upstairs front, 1 upstairs back, 1 downstairs). Each would extract from one room and supply one room, so no manifolds etc.

Pros that I see:
-much shorter/simpler duct runs (a big plus in this retrofit)
- can add them one at a time during piecemeal retrofit
- can add more/fewer units if ventilation rates need to be more/less than anticipated
- can boost the units individually, eg just the upstairs one if the shower is steamy, or the downstairs one during cooking
- small units are £250 each, bigger units are £1500+ plus duct

Cons
- small units claim to be less efficient 70% instead of 90%
- small units don't have summer bypass (we open windows instead)

Has anyone tried this and found anything different?

Downstairs loo is a leanto extension on the back stone wall with no viable duct route through to connect to a MHRV. It does have an extractor fan for short periods of dire need. However for 23.75 hours each day its door is open to the utility, which is always open to the kitchen which will have continuous MHRV extract. Main family bathroom is upstairs.

Not quite what you are asking but I am happy with 2 x single room MHRVs and would add one or even two more if I could...

Damon

I'm looking at 'two room' units that have ducted extract from one room and supply into another (VA HR100R or equivalent). Freshr is well out of budget.

Damon, iirc from your blog earth.org.uk, you have two 'single room' (small) wall-mounted units with relatively low flow 10-20m3/h each on trickle for your family's house. You mentioned wanting to have more. If you could start your project over again, would you still go for several wall mounted units, or one central system?

For what it is worth, the architect doing the PHPP calcs considered a Freshr for our small house & extension. After requesting ventilation plans from Freshr, they advised to use 2 units, one upstairs and one downstairs.
Putting this in PHPP, a central MVHR with rigid ducting won hands down, both on initial cost as well as efficiency/running cost.

For refurbs, since the airtightness is probably not super high, the MVHR efficiency (kW/m3) is a really important parameter, dominating the system efficiency.

Although I am not 100% sure as I did not do it myself, I think the PH accreditation data was used.

fostertom quoth: "That's odd, as one of Freshr's claims is super-efficiency"

https://passivehouse.com/01_passivehouseinstitute/02_expertise/01_researchinbuildingphysics/03_Ventilationsystems/03_Ventilationsystems.html

https://database.passivehouse.com/en/components/list/ventilation_small

It's always worth asking suppliers for the PHI certificate for the actual equipment/component that they're supplying.

If I was looking at single-room units (I'm not) - most of them are around the 70%-80% 'headline' thermal recovery range. You have to go to a centralised ducted system to get up to 90%.

For an airflow of 30m3/h and annual average 10degC difference btw inside-outside, the heat loss in the air is 30 x 1.2kg/m3 x 1.5kJ/kgC x 10C x 24x365 / 3600 = 1300kWh/a (this includes latent heat of condensation)

Assuming gas heating @5p/unit and 20y lifetime, this heat costs £1300 over the lifetime of the unit

So it is worth paying £130 extra to get a unit that is 10% more efficient. But not £1000s extra, which is what the ducted PH certified ones cost.

Obvs the heat costs more/less for different fuels, or passive gains.


The electricity usage is around 0.3 Wh/m3 which costs 30 x24x365 x 0.3/1000 x 20y x £0.15/unit = £230 over the lifetime of the unit.

Wot a shame - groundbreaking potential/principles much anticipated on this forum, long in development - so back to clunky old square one.

Posted By: WillInAberdeenIf I was looking at single-room units (I'm not) - most of them are around the 70%-80% 'headline' thermal recovery range. You have to go to a centralised ducted system to get up to 90%.

For an airflow of 30m3/h and annual average 10degC difference btw inside-outside, the heat loss in the air is 30 x 1.2kg/m3 x 1.5kJ/kgC x 10C x 24x365 / 3600 = 1300kWh/a (this includes latent heat of condensation)

Assuming gas heating @5p/unit and 20y lifetime, this heat costs £1300 over the lifetime of the unit

So it is worth paying £130 extra to get a unit that is 10% more efficient. But not £1000s extra, which is what the ducted PH certified ones cost.

Obvs the heat costs more/less for different fuels, or passive gains.


The electricity usage is around 0.3 Wh/m3 which costs 30 x24x365 x 0.3/1000 x 20y x £0.15/unit = £230 over the lifetime of the unit.

Success in reducing energy use is not measured by £ cost though. The £ cost is relevant when it comes to deciding between options and in terms of absolute affordability.

A new gas boiler in a new build or retrofit now is in a strange position, given that the government is trying to abolish them. Will people who fit them now be regarded as the last of the lucky ones, or unspeakable anti-climate neanderthals in times to come? Who knows?

When comparing against a whole house system, the whole house is probably a better unit of comparison than a single single-room ventilator. If we said 300 m³/hr as a basis, then your numbers work out at £1300 and £2300 for gas and electricity respectively, which is perhaps a more reasonable premium for a 10% efficiency gain.

Posted By: WillInAberdeenAs we discussed on another thread, running at 300m3/h would be serious over-ventilation. I think you mentioned your whole-house system runs at 50m3/h?

Indeed, I wouldn't dream of running my system at 300 m³/hr, but that's what it's sized at which is why that number came to my mind as well as being a convenient multiple of your 30. Sizing is based on maximum occupancy and/or area proxy. But pick your own number by all means.

I believe the recommended minimum air supply is 30 m³/hr per person. Since there are usually two of us in our house I run it at 50 m³/hr (the default trickle rate) some of the time and 125 m³/hr the rest of the time during periods when I care about the efficiency and heat loss/gain (i.e. winter when the heating isn't on and very hot days in summer). At other times I tend to run it at 125 m³/hr or 165 m³/hr.