I’m currently looking at dMEV, potentially Brookvent for ventilators and Greenwood for extract, but I have no idea how to work out the overall heat losses in order to get an idea of how my ASHP heating bills might be affected.
I appreciate that a lot depends on current situation and how much moisture is generated.
At the last count three rooms have trickle vents, 2 have ventilation into chimney, 1 has a painted over airbrick to the outside, and the only extract is a cooker hood in the kitchen and an intermittent extract fan in loft bedroom.
I don’t use gas for cooking, and most of the time it’s just me in a 4 bed house.
I monitor temperature and humidity in each room of the house, as well as half hourly and have data going back 15 months.
The Greenway brochures refers to running costs in terms of jars of coffee or tins of beans - not a currency I’m familiar with 
!
My ASHP heat loss survey shows 26% of total heat loss (7988W) attributed to ventilation (2063W), as calculated from ‘custom air changes/hour’. An air-test showed an air permeability rate of 15.1 m3/m2.h, and a second test with all intentional vents (trickle, chimney and airbricks) left open indicated an increase only to 15.6 m3/m2.h.
Part of the underfloor insulation work, still on hold, is draughtproofing around the edges of the floors, but unsure how much difference that will make to overall infiltration.
Q: How would I convert the measured m3/m2.h to air changes/hour to see how closely the model matches reality?
Do you know the area and volume of your house?
There is a complication that the permeability test gives results in SAP units of gross area and volume but PHPP ach is based on treated floor area and subsequent conversion to volume.
I would calculate the total volume of air per hour using the permeability readings and then see how many air changes an hour that work out as.
Thanks Tim,
Total floor area from a 2015 Carbon Co-op survey was 138.6m2
EPC total floor area is 155m2
I have room-by-room internal floor area and volume from the ASHP heat loss survey, total internal floor area is 125m2.
I’ve been trying to understand how to ‘calculate the total volume of air per hour using the permeability readings and then see how many air changes an hour that work out as’, but not got there yet.
I have even resorted to AI which suggests that ACH=(Infiltration Rate×Envelope Area)/Volume, and even if my house were a cube (which it of course isn’t) the ACH would be 16.
Is that the right approach?
The complication is that volume differs between SAP/EPC and PHPP. Therefore strictly speaking one of the constants isn’t constant. The other constant is the surface area. I do not know how SAP calculates that but for PHPP it is the external measurements around the outside of the structure, so under the floor, outside the walls and over the loft insulation. I think SAP measures the inside volume including internal walls, but I’m not sure.
Permeability is pretty useless for serious retrofits as it is dependent on surface area, whereas ach is not, so you need to remove surface area from the number. I’m neither a mathematician nor a physicist but converting m3/m2.h to ach must involve 2 sets of calculations. Air is measured in m3 either way.
1 ach = 1 (your house volume)/h
15.1 m3/m2.h uses (volume of air/h)/your house surface area
In seriously hoping, at this stage, that a mathematician reviews this post!
Multiply permeability by surface area as used to calculate the figure, giving the permeability of the whole house rather than an average m² of it. This gives the total volume of air passing through your house. Then divide by the volume of the house to give the number of air changes. Hour remain constant throughout.
To summarise the above, yes,
ACH=(Infiltration Rate×Envelope Area)/Volume
is correct.
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yes thats the sum you need to do.
But becareful for comparative purposes what measurments you take, as previously mentions PH uses external measurments whilst SAP internal, for envelope. Not sure what envelpe was used for your infiltration measurments? Internal?
Seen as your installing decentralised ventilation, have you not considered heat recovary versions? Although not as efficient as centralised systems at least some heat will be recovered?
Is there a reason you are not considering centralised system? if only a small one, say with 2 extract and 2 supply?
Thanks again Tim.
I only have a summary report for the permeability test giving whole house permeability rate, with some recommendations, and the house envelope/surface area isn’t easy to measure due to extensions, room in loft with sloping ceilings etc.
But I might have managed to make some sense from the figures. If I add up the modelled room by room ACH from the heat loss survey I get 10.5 ACH, which is of the same order of magnitude as the the measured permeability rate of 15.1 m3/m2.h.
The permeability test is more likely to have used PHPP methods than SAP, while the heat loss survey is almost definitely SAP, which makes the modelled heat loss infiltration more conservative than measured, rather the more usual less conservative.
Thanks Russell. I only have a whole house figure for permeability/infiltration, and I don’t know whether it was based on internal or external measurements. See my reply to @Tim_Gilbert this morning re possible lightbulb moment.
I was looking at MEV, but the cost and disruption I was quoted was excessive, even with ducts being boxed in rather than running ducts under floors .
I’ve also been thinking about whether one of the extracts having heat recovery in the kitchen/diner, which is the coldest room in the house, would be possible.
I have 4 wet rooms (kitchen, utility, 2 bathrooms), two living rooms, and 4 bedrooms over 3 floors. There are trickle vents in windows in 1 living room and 2 bedrooms, an air brick in a third bedroom, vent into chimney in fourth bedroom, an extract fan in one bathroom, and a cooker hood.
PPR have recently supplied a specification for dMEV, and I have started approaching contractors. Hence trying to arm myself with some understanding of ventilation - having spent the last 15 months getting to grips with ASHP, and insulating the ground floor of a damp house!
The sum of room air changes is not the same as the whole house air changes. By room should be more, this is because some of the air loss from a room will be to adjacent rooms and not lost from the house.
In a similar way the room by room heat loss used to calculate radiators comes to a greater number than the whole house heat loss used to calculate heat pump size.
That’s interesting.
The Heat Geek heat loss calculations assumed show heat loss through intenal walls/floors/ceilings as 0, since whole house assumed to be heated to same temperature, so little movement of air between rooms. Certainly the summary heat loss and design figures are the same.
Apart from understanding whether the permeability tests used internal or external measurements, I have a better understanding overall. Certainly enough to ask the right questions before the next round of decision making!
Ideally this should be a case by case calculation but I expect most installers have a single approach.
I know that there is air movement between my ground floor ceiling and first floor. There are also stud walls, which may have air movement and electrical trunking which can transfer air around the house.
Rooms on the windward side of the house will be cooler, their warmth being moved to the ones on the leeward side, before leaving the building. As wind direction is variable but radiator size is fixed each room needs the heating capacity that being on the windward side on a cold day requires.
The solution is to prevent the air movement but that is a future project, when my flooring needs replacing.