MVHR conflicting advise from installer whether one unit or two units are needed

CaraK

Looking for advise on HRV as I am getting conflcting recommendations from different manufacturers/installer

The orginal BER and PHPP were based on 2 storeys with space of 220m2 and a Paul Nibe 300 was recmmended. We have since decided to make the roof space habitable which bring the overall size of house up to 300m2 and hence we need to review the HRV unit. The 2nd floor is adding a lot of volume to the house

So we looked at units capable of moving 375m3 of air. ( 300*2.5*0.5 = 375)

We are building to passive standard and hopefully with much lower thermal bridging than an average A rated house built to building regs. In the vast majority of new builds there is a lot of small thermal bridges at wall/floor/roof/window junctions that are cold enough for water vapour to condense on, and create mould. We have concentrated on these details to avoid any cold surfaces. As a consequence it should follow that we wouldnt need the same volume of air in order to keep the relative humidity around 50%-60% which is why we originally thought a higher specced unit would be what is needed. However this is where we are getting conflicting recommendations

One installer recommended one higher volume HRV unit will suffice as he reckons the airflow requirements in a passive house are a lot lower as there is a lot lower risk of condensation and if two units were installed there is a real risk in a passive house of the air being too dry if it is over ventilated.

The other installer says that a two unit system will definitely be needed in this case because the house is being built to passive standard this means the air changes per hour change of 0.5 ACH means we actually need the 2 units to compensate for the house being so airtight and that if only one unit it would be heavily loaded running at 90% all the time and would be a lot more noisy.

The difference in price between both options is €1800 but at this stage i would much rather make the right decision than let finance dictate i go with cheaper option( one unit).

Any advise would be greatly appreciated

BryanF

Have you done your phpp assessment and issued it to 3 hrv manufactures.

CaraK

BryanF wrote: »

Have you done your phpp assessment and issued it to 3 hrv manufactures.

Yes but original PHPP was based on 2 storeys not three and was done about 3 years ago. I don't fancy getting back in contact with him as i didnt have a good experience with the guy who done it. He assumed a lot of things without actually asking me what i wanted when i queried anything he was quite startled that i was questioning him in the end i got the internet equivalent of a shrug of the shoulder.

MicktheMan

OP, whatever you do, make sure that the unit(s) are not required to run at a high rate (>60 to 70% of capacity) in normal operating mode. Imo, any installer or "expert" suggesting that a passive house needs lower levels of ventilation are best avoided.

Calahonda52

Op, none of this analysis makes sense: MHRV units do not condition the air: you need someone who understand this business.
Designing ACHs based on thermal bridges....new one on me.
MtM is correct, in addition proper ducting layout, correct location of supply / extract as well as correct use of silencers will be required.