MVHR for small units

[Rachel Mitchell]

It is is feasible to put a MVHR unit in a building which is about 20m2, there would be no wet rooms, just one air tight room?
if it is feasible, is there a unit small enough for this sort of volume

[Mark Siddall]

In principle there is no problem providing that you achieve suitable standards of airtightness. At 85m2 air barrier area and 50m3 if you went for 0.6 ach/hr that would be 30m3/85m2 = 0.35 m3/m2@50pa. If you relax to 1.5 ach/hr @50pa, in my view the upper threshold for using MVHR, then you would be needing to achieve 0.88 m3/m2@50pa. On this basis, realistically, I think that you could chase 1 ach@50pa providing that you have simple construction and good detailing. (That lands you at about 0.6 m3/m2@50pa which would get you into the right kind of place for addressing moisture transfer/interstitial condensation risks; assuming appropriate air/vapour barriers are being used in the first place.)

What is the puprose of the building? Garden office? How many people will occupy the space? Without wet rooms you could simply expect 30m3/hr per person. This ventilation load would allow you to start seeking suitable units. If one or two people then I think that the biggest challenge will be getting a unit with a low enough fan speed. I'm tempted to ask what others would think about using demand controlled MEV as an alternative. Others will hopefully be able to chip in better than me on this particular aspect.

The cost effectiveness of the investment for MVHR may also be an issue. How important is cost?

Mark

[Rachel Mitchell]

Thanks for your prompt reply Mark, yes it is a garden office, so one or two people at the most and only in the day time
and the challenge has been to find a unit that small enought not to produce over ventuilation
what's your view on single room units such as the vent axia HR 25 - could this do the job?

the client is looking to try for passivhaus certificaiton so at the moment MVHR is the option being considered

[Mark Siddall]

Certification is know to be difficult for small buildings. You will have to achieve the 0.6 ach@50pa - which in your case will require exceptional, though achievable, workmanship. As the vent-axia is not PH certified you will be required to reduce the calculated efficiency by 12 percentage points. You could end up with some very impressive U-values for such a small building regardless of this penalty.

The specific fan power could do with being less than 0.45 W/m3. Also the Vent-Axia unit is pretty noisy - so not really what you want blaring away when two people are in there. PH requires less than 25 dB(A).

[Rachel Mitchell]

thanks for the advice Mark, I did notice the dB of the unit, and was not aware there was a PH requirement for noise, so that is useful to know
I'

[Rachel Mitchell]

Does anyone have any knowledge of the FiWi MVHR system which Viking House in Ireland are offering?
http://www.viking-house.ie/fine-wire-hrv.html

[Nick Grant]

Hi Rachel

I agree with Mark re MEV.

Neighbour is building a tiny 8m2 well insulated guitar workshop that needs constant temp and RH. It has 300mm+ insulation all round, air tight, no thermal bridges, triple glazing etc.

We assumed a small single room HRV but thought we should model it in PHPP.

Attached shows energy balance assuming MEV and 0.37 airchanges average vent rate (higher when in use then turns off or to trickle).

Adding efficient MVHR would only reduce the ventilation by a bit as there is still leakage and door opening etc is more significant in tiny building.

Result really surprised me and I got Alan Clarke to check it.

So model in PHPP and be prepared for a surprise if you think you can achieve 15 kWh.m2.a or 10W/m2!

In practice for small buildings the internal heat gains will be higher per m2 than the assumed domestic level and you can work them out in PHPP, I didn't, my fee was a couple of pints down the pub!

Nick

[Dave Howorth]

Does anyone have any knowledge of the FiWi MVHR system which Viking House in Ireland are offering?
http://www.viking-house.ie/fine-wire-hrv.html

I haven't used one or even seen one. But I believe the principles are correct and have been proved in practice by the designer - Jon Kristinsson - in Dutch greenhouses. google shows some info. I'd have thought it might work well in a single room. My only reservations are with using it in larger buildings without also using circulation systems such as ducting.

[Rachel Mitchell]

thanks Nick and Dave for your replies is there an issue with single room MEV and air tightness?
Rachel

[Mark Siddall]

Rachel,
What is your concern regarding airtightness and MEV? Is it that the building could be to airtight for the MEV to work appropriately? (Or result in excessive fan power demand to achieve the required air flow?)

Mark