MHRV - calculating air requirement

H.E. Pennypacker

I've got a couple of quotes for retrofit heat recovery ventilation. The installers have completely different ideas of what the air requirement of the house is which isn't giving me much confidence in at least one of them.

The house is a 135 sq meter bungalow with air permeability of less than 5m3/(h.m2)

Vendor A says that the house needs 108 m3 air per hour with 146 m3 on boost.

Vendor B says that the house needs 330 m3 per hour with a complete air change every two hours giving 165 m3 air per hour as a minimum requirement with the unit capable of providing 290 m3 air.

My reading of Part F (Section 1.2.3.2) reckons that the house needs a minimum of 145.8 m3/hour (0.3 * 135 = 40.5 l/second = 145.8 m3/hour.

I can't find any reference in Part F regarding the impact of air changes per hour and how that affects the above calcs.

Can anyone tell me which of the above figures is correct?

SemperFidelis

It should be the greater of

a) 5 l/s plus 4 l/s per person, e.g. 25 l/s
for a five person dwelling.
Or
b) 0.3 l/s per m2
internal floor area

The boost rate then depends on the number of wet rooms added to the normal rate

Kitchen 13
Utility room 8
Bathroom 8


For my unit, it was sized to run at about 30% of its maximum rate when at normal speed. This uses less electricity and improves the life of the unit rather than running near max all the time. also generates less noise.

H.E. Pennypacker

Thanks,


So Vendor A is wrong by the look of it. We're a four person (5/s + (4/s * 4) = 21 l/s) 135 sq m (0.3 * 135) = household so minimum rate is 40.5l/s = 145.8 m3/h



Two bathrooms, one kitchen so 13 + 8 + 8 + 40.5 = 69.5l/s at boost = 250 m3/h


I presume that they are the values that apply with the air change rate taken into account?


Did you fit your own system or have it done by contractor? If it was a contractor and you're happy to recommend them would you mind passing on their details by PM? Its surprisingly hard to find someone reliable to fit a ventilation system

sydthebeat

H.E. Pennypacker wrote: »

Thanks,


So Vendor A is wrong by the look of it. We're a four person (5/s + (4/s * 4) = 21 l/s) 135 sq m (0.3 * 135) = household so minimum rate is 40.5l/s = 145.8 m3/h



Two bathrooms, one kitchen so 13 + 8 + 8 + 40.5 = 69.5l/s at boost = 250 m3/h


I presume that they are the values that apply with the air change rate taken into account?


Did you fit your own system or have it done by contractor? If it was a contractor and you're happy to recommend them would you mind passing on their details by PM? Its surprisingly hard to find someone reliable to fit a ventilation system

dont work on the basis of your specific occupancy, but what the possible occupancy is perceive-ably.

ie if the house has 4 double bed rooms the occupancy is 8 persons.

This is the basis of sizing your effluent treatment system as well.

H.E. Pennypacker

sydthebeat wrote: »

dont work on the basis of your specific occupancy, but what the possible occupancy is perceive-ably.

ie if the house has 4 double bed rooms the occupancy is 8 persons.

This is the basis of sizing your effluent treatment system as well.

Thanks - we're a three bedroom house so technically six people but the square footage calc value will still be greater than the occupancy calc (29 vs 40.5). The effluent system was sized for at least six when it was done so we're grand on that front.

SemperFidelis

I fit it myself, as we're in a bungalow it wasn't too difficult to drop the pipes down to each room and run back to the unit. I used semi-rigid ducting from distribution boxes, which were connected to the unit. Unit connections to the atmosphere were thru the gable end of house via spiral steel ducts.

I did up an excel spreadsheet with all of the room sizes and the house overall size and then calculated the volume of air required for each room as a percentage of overall supply. Total extracted air being equal to the total air supplied. The kitchen required two vents as its a big room and I couldn't supply enough air with just one while using the semi-rigid ducting.


Balancing the system took a good bit of time, lots of running around adjusting vents. Before the MHRV the house was stuffy and suffered from mould. much fresher now and no mould so we're happy with it.