There are lots of assumptions (weather, occupancy, ceiling height, stocking density of humans per m2, whether bedrooms are open to the rest of the house air volume or not etc) rolled into many of these simplified rules of thumb.
Sleeping in a giant open plan box with bedroom door open needs radically different airflow to two adults in what is effectively a sealed box because even with a door undercut there’s no air exchange with the rest of the house. (even with MVHR we have to prop bedroom / bathroom door for this reason - else no return air path for the MVHR)
That’s the same way I see it. Why would I continuously ventilate my upper floor rooms when I’m not using them? I ventilate based on CO2 and humidity, as those two are proxies for human and building health. If you want to get really fancy calculate the dew point and compare that to the known coldest spot in the room (as determined by IR camera). Home Assistant even has an integration to calculate that for you based on indor and outdoor temperature as well as humidity (need to calibrate once).
Most MVHR rates are quoted in m³/hr rather than l/s, which is annoying. Personally I find it easier to deal in m³/hr; l/s doesn’t ‘mean’ anything to me.
It doesn’t matter terribly where you ventilate because the various gases will diffuse anyway. All doors are supposed to have undercuts (or overcuts in our case - we recessed the architraves so the air paths are invisible and the noise transmission is reduced). So with an MVHR there should be an inlet in the bedroom and the air will flow out through the undercut if necessary.
I’ll grant you that there is more flow if doors are left ajar, but things generally sort themselves out, Partial pressures of the various gases see to that.
was just the basic heat and air exchange. yeah humidity can get pretty low in the winter when it cold with air exchange down to 10 to 20% and you are basically a walking ball of static after that with constant nose bleeds.
So I appreciate that when we actually use a house it makes sense to ventilate in relation to demand proxied by e.g CO2 and humidity but we also need to calculate expected ventilation rates at the design stage both for the ventilation system or lack of and for the heat loss calculation for the heat pump.
A 4 bed house with 2 occupants will need less ventilation than when there’s 4 people present but we probably need to design for the 4 person case. Hence why rules of thumb such as 30m3 per person per hour of fresh air are useful?
Perhaps for heat pump heat loss calculation using part F minimum ventilation rates or the PHPP equivalent is a good place to start. If the resulting heat pump does not have enough capacity in windy conditions it indicates that attention is needed to draught proofing. If the ventilation rate is lower due to lower demand as a result of lower occupancy oversizing of the heat pump is unlikely to be a big issue as we’re still using much lower air change rates than pre-2000 MCS guidance e.g 0.6 ACH (output of ventilation calculation starting with L/s or m3/hr) vs 1.7 ACH (average value from MCS/CIBSE guidance)?
mind if I ask what libraries you used for BME680 and perhaps a link to the sketch you used - as bought a couple BME680’s as well to see which I can make work the best
I have that one connected to an ESP32 running ESPhome which takes care of all the libraries and sends the data to Home Assistant. So unfortunately no code I can share besides the (trivial) ESPhome yaml.
okay thank you. i investigated the BME680 a little bit. it uses a closed sourced library for the CO2e calculation. so i will use probable the BSEC library from bosch. and see what happens with that one. as the open source one apparently only work so so . I found some comparison out puts for BME680,ENS160, SDC4X and sun-CO2 . the SDC4x and the SUN-CO2. track identically . as they are CO2 sensors . the ENS160 (CO2e) track well until the 1000ppm then they go a bit whacky after that the BME680 seam to do their own thing. if you were curious but exhibiting the same issue as what yours displayed. during the day it was al over the place at night it was closer… so i will adjust for humidity and see how it tracks then… okay thank you for time and responses
Ah yes, I have another BME680 on an RPi in the basement with that Bosch library. It doesn’t do anything there innthe CO2 sense because there’s no noticable sudden fresh air influx for calibration. I could send you the code I use there…I think it sends the data via MQTT.
Indeed and to meet legal requirements the calculation has to follow the Part F rules, which fortunately work out to be a bit more than is required, so just follow the Part F rules.
Yes, that’s basically what I use in practice.
I don’t know much about the subject. Using PHPP as the design and build rule means that it calculates the building heat loss (by fixing the airtightness limit and knowing how much insulation there is etc) and the result can be trusted in my experience. I haven’t tried anything with an existing building - I expect EnerPHit would work though.
Highlights that “the age of the building may not well indicate the loss of heat due to air passing through the structure” - as noted earlier, that’s great as it’s implying that the pre-2000 CIBSE guidance is not a rule.
It discusses that “it could be argued that the full instantaneous heat loss during a ventilation cycle need not be met as there will be considerable off periods…”“So overall time averaged heat losses are relatively smaller”… “Sizing heat emitters in such spaces to fully compensate for intermittent ventilation related heat losses may not be needed” and that air drawn into areas with extract ventilation will most likely be partially drawn at least from within the heated envelope.
The tip suggests to design for ‘static’ losses and then ensure that there is sufficient capacity elsewhere in adjoining rooms to cope with the heat for the shorter extract periods…
That all seems very sensible and seemingly a fairly clear indication that CIBSE guidance for e.g 3.0 ACH in a pre-2000 bathroom does not need to be followed… there is nothing in the MIS_3005 that states these ventilation rates must or shall be followed to the letter as far as I can see? Must or shall having specific meanings within the MCS standard.
Interestingly the minimum intermittent extract ventilation rate for a bathroom in the part F building regulations is 15 L/s which for our 19 m3 bathroom results in an air change rate of (15 x 3600) / 19,000 = 2.8 ACH. But that air will largely be sourced from the heated envelope of the rest of the house, which as a whole needs to meet a minimum ventilation rate of (31 L/s x 3600) / 186,000 = 0.6 ACH. The time averaged effect of that 15 L/s will be pretty tiny and at least a fair part of it will be coming from or contributing to that whole house ventilation rate…
Part of the issue I think is the way that the room by room heat loss calculation is implemented. Each room has an air change rate and that air change rate is fully with outside air. If you enter 3 ACH for the bathroom the calculator assumes that all of that air change is with the outside air and that no air is sourced from within the heated envelope which is the actual physical reality in the case of extract ventilation in a bathroom, the air coming into that room will at least partly be coming from under the bathroom door from the hall…
I hadn’t spotted the assumption in the MCS spreadsheet that all “make up” air for a room was external.
That’s clearly wrong. This should be calculated as a “whole house” air change rate then apportioned across rooms based on either volume or occupancy. Else you undersize emitters in the main spaces with supply air (the air that actually needs heating) and oversize them in the rooms with extract air (which doens’t need heating).
Agreed! I’m currently working my way through the BS EN 12831 standard itself which is what MCS references (albeit the 2003 version, google search BS EN 12831 free download).
It describes the simplified calculation method that most heat loss calculators use (including the MCS spreadsheet) which assumes external source air and a more complex calculation method that specifically caters for the physical reality of different ventilation methods including a worked example for simple extract ventilation that calculates based on air sourced from adjacent rooms!
All in all I believe this means that as long as the installer follows this more comprehensive method detailed in the BS EN 12831 standard they will still be complying with MCS.
Im just building a little spreadsheet example of this calculation and a worked example for our house to compare the results.
FWIW should you need anything newer the “BS” is largely BS these days. The standard is EN 12831. The BS just means you’re paying BS for it rather than another country. We find the Estonians have the most organised national libraries for accessing these international standards:
You can also browse for 24 hrs for less than the buying it cost.
The standard doesn’t appear to say anything about time averaging extraction rates? and the calculation methods adds the extraction rate to the base infiltration rate which if you use instantaneous extraction rates with all extractors running results in some pretty sizeable ventilation heat losses…
E.g for my house with my blower door test result of n50 = 10.13 ACH which gives an infiltration rate of 0.41.
Adding non time averaged… intermittent extraction rates from part F to the kitchen and bathroom adds 0.89 ACH on top of the base infiltration rate = 1.3 ACH
Adding continuous extraction rates from part F to the kitchen and bathroom adds 0.41 ACH on top of the base infiltration rate = 0.82 ACH
I would say if I time average the extraction rates we are talking at most 1h for the bathroom and 1h for the kitchen extractor this would result in very low equivalent continuous rates around 0.04 ACH which is then too low at 0.41 + 0.04 = 0.45 ACH (lower than part F minimum whole dwelling ventilation rate that is)
I’m looking for a figure closer to 0.6/0.7 ACH which I think is more representative of average conditions in our house and the range that gives the measured heat loss that Im seeing…
Work out steady state heat loss through walls/windows/roof/floor based on 24hr average temperature - reasonable given the averaging effect / decrement delay of a foot of mineral wool - on a room by room basis. Apply per room.
Assume an air change rate on a whole house basis (rather than per occupant per room) - reasonable given that it’s managed by airtightness and mvhr rather than infiltration and luck. Apportion this per m3 to each room that is supplied with fresh air that needs heating; ignoring the rooms from which air is being extracted and doesn’t need heating.
Ignore intermittent extraction entirely. (doesn’t run long enough to move the numbers)
Ignore temperature differences between bedrooms and living spaces. (if anything bedrooms will wind up underheated slightly as they’ll actually be commissioned with a greater proportion of fresh air per m3 than the main living space and that will keep them just the touch cooler; else limit them on TRVs whilst the main space is large enough to keep the heat pump happy)
To any standard? Probably not. Adequate? Hopefully!
It definitely avoids overcooking the bathroom emitters though; and is closer to the observed heat loss whilst it was running on the little 2.5 kW class minisplit AC over the first heated winter.
On the plus side the above EN 12831 standard’s more comprehensive calculation method is certainly a significant improvement on what most standard heat loss calculators are doing. It provides justification and a calculation method for sourcing ventilation requirements for bathrooms and kitchens from adjacent rooms, which is a useful step forwards in my understanding.
If I put my own house through this calculation with the n50 blower door test that I have and use the minimum hygienic air exchange rate I get 0.69 ACH, which is purely in my case the minimum hygienic air change rate…
That’s slightly lower than minimum continuous extract ventilation which for our house would work out to 0.78 ACH (corrected down from 0.82 above).
0.69 ACH is still above Part F minimum ventilation of 0.6 ACH for our house so I still wonder if there’s a way to justify using slightly lower levels for Table D.6 as long as the result complies with Part F… Anyway that’s probably as good as Il get for now…
Credit where credit is due, thanks to Sam McGowan who is a design engineer at Octopus for putting me on to the existence of these more comprehensive EN 12831 calculations. I had assumed EN 12831 just described the simplified method that’s in the CIBSE domestic heating guide, but it actually has a whole lot more in there that’s much more comprehensive!
