MortarThePoint

I'm not seeing the relevance of 15C and having a quick Google suspect you may have used the EngineeringToolbox calculator but the air after heat exchanger temperature won't be constant at 15C. I can imagine the recovery (i.e. heat exchanger) efficiency being lower at smaller temperature differentials, but I can't find data readily available.

Well that's vitally important as it's very rarely -5C around here. It's efficiency at an outside temperature of 6C is probably more representative. Can you share a link to the calculator please

Yes, that is a concern. I know there has been a lot of progress in gas sensors over the past 20 years and hoped that some would have filtered down to consumer grade products. I do see some interesting IR based CO2 sensor modules for around £20 and PM2.5 ones for around £30. That's probably what I would incorporate into a homebrew one. Yes internal AQ. We're not having MVHR but I am looking at PIV and/or dMEV and so there is scope to close the loop on air quality which MVHR would also allow. MVHR is obviously a more energy efficient approach, but we're exploring all of that on another thread.

My budget is probably up to around £175. I don't need professional grade calibrated stuff, but I don't want junk (despite having just bought a £17 one ? ) Looks like I may have to make my own system which is a time sink but interesting. I'd buy something now rather than waiting to make something in a couple of years time.

Your numbers look correct. 100m2 with an average U-value of 0.32W/m2K and temperature difference of 13C will pass 100 * 0.32 * 13 = 416W of heat. An ASHP will consume about 140W of electricity to create that heat (based on COP=300%). 140W average for 6 months with an electricity price of 14p/kWh works out as 0.14kW * (24hrs * 183days) * £0.14/kWh = £86/yr. That's the total cost of all of the 100m2, not just the windows or the difference between the two window types. My previous calculation identified 22.8kWh per year of heat for 1m2 as the difference between DG and TG. Based on 20m2 of windows, an ASHP (COP=300%) and 14p/kWh that would yield a total electricity saving of ((22.8kWh/m2 * 20m2) / 300%) = 152kWh per year, which amounts to 152kWh * £0.14/kWh = £21.28 per year. I worked out on another thread that 1kWpk of solar panel sited well in my area would generate 1000kWh per year. So if I wanted to save that 152kWh, I would need 152kWpk plus whatever inefficiencies, call it 200kWpk needed. Solar costs around £1/Whpk so that's £200. A 350Wpk solar panel is about £350. If you have a solar array you would get more environmental benefit by adding a single panel to your array than by upgrading >30m2 of windows to triple glazing (in the South of England). There are exceptions to this logic though. For example if you don't have an ASHP and want to keep all heat leaks down in order to avoid one. You may live somewhere that is much colder and gets less sun. The perversion of all of this is that I may end up going TG due to the 'brochure value' of it. We're building for ourselves, but I have to be mindful of the impact of decisions. That said, I'm still not convinced we will go TG.

Thanks, I've started looking at various sensor modules so may end up with a bespoke system

I am interested in getting an air quality monitor, to include a CO2 meter. Can anyone recommend one? It would be good if it could measure: CO2 Particulates (e.g. PM2.5) Temperature Humidity Even better if it could log these, failing that Max/Min would be good as well. I have found various on Amazon, but they aren't particularly popular. This one was so cheap I have bought it to take a look, but am very dubious given the low cost: https://www.amazon.co.uk/dp/B085DJQRY1/ I'd be keen to get one I would trust a bit more though.

I'm interested in getting one of these, what model do you have and do you recommend it? Don't know much about MVHR unfortunately so can't really help with your main question

It's not a utilitarian argument though. The real justification for WBS and open fires is emotional/aesthetic. People want a fire. I can see they're bad in cities but in the countryside not so much. We're all wasteful even when being environmentally conscious. I doubt many here have built the smallest house they can bear. We make compromises and balances to fit our own priorities and that's not one size fits all.

Completely agree, the cost of powering the fan (20W?) is small compared to the cost of heating the cold air up (400W of electricity at 60l/s and 20C temperature difference). 200W at a more normal dT=10C. But if trickle vents are/were doing the job they're supposed to then they'd be exchanging the same volume of air and costing 400W too.

In theory it should be no worse than trickle vents, except for the power consumption of the fan itself.

This sounds intriguing. Is that like food chillers? You need someone that wants to keep something warm or does a lot of endothermic reactions. Converting hot air to electricity is pretty inefficient (thermoelectric generators, thermopiles or even Peltier). Heating offices etc another good option.

I don't know how they work but you have presumed a single thermalization chamber that averages the temperature of the incoming and outgoing air. If instead you had three in series, the middle one would be at 15C, the inner at 17.5C and the outer at 12.5C. I'd imagine MVHR works on a succession of thermalization points. I presume the airs don't actually mix however. Another method would be to have a heat pump extracting heat from the outgoing air and putting it into the incoming air. Do you ever get condensation issues with MVHR? There is, wind ? couldn't resist it. Doesn't fit the context as well as a gale though.

Doing a similar calculation for humidity C_eqm = 0.8% by mass based on 50%RH at 21C [1] C_out = 0.64% by mass based on 80%RH at 10C [1] Sources adding water to the air: Human breathing up to 1l/day, four people --> 4kg/day [2] Ignore sweat Evaporation about 3kg/m2day so guess as 3kg/day to cover water left in shower, basins and loos [3] Cooking and kettle, guess 2kg/day (ignores extractor or this amount gets round extractor) Bathing guess 250g/person/day --> 1kg/day (after extractor effects) i = 10kg/day total based on above list which is hopefully an upper bound. I could imagine it being half this, but can't imagine it being much more unless clothes are drying into the air which is a bad idea. q = i / (C_eqm - C_out) = 10kg/day / (0.8% - 0.64%) = 6250kg/day of air Density of air is around 1.2kg/m3 so that flow rate equates to 5200m3/day which is about 220m3/hr or 60l/s. Coincidence? Both calculations assume that the air leaving the house is representative of the house as a whole, i.e. good mixing.

This is interesting. Resting human breathing rate <8 l/min, so 4 people well less than 1l/s. Don't need >10l/s of ventilation for breathing then. Wrong: C_eqm = equilibrium CO2 concentration C_out = background CO2 concentration = 0.04% q = air exchange rate = 60 l/s air i = CO2 influx rate = 8l/min * 4people * 4% CO2 = 0.02 l/s CO2 At equilibrium: i - q.(C_eqm - C_out) = 0 So, C_eqm - C_out = i/q = 0.02l/s / 60l/s = 0.03% C_eqm = 0.07% which is 700ppm or +75% over outside CO2 concentration Thresholds vary by standards organisations, but 1000ppm is often seen as the upper CO2 limit for good air quality. https://en.wikipedia.org/wiki/Indoor_air_quality https://www.velux.com/what-we-do/research-and-knowledge/deic-basic-book/ventilation/indoor-air-quality?consent=none&ref-original=https%3A%2F%2Fwww.google.com%2F

I'm inclined to disagree*. Unless I am missing something, trickle vents are 100% inefficient just like PIV, so for the same amount of ventilation (m3/h) you'll have the same amount of heating cost. However, it's hard to know what flow rate your actually getting from trickle vents (e.g. effects of wind etc) so it is less controllable. If the trickle vents cost your heating less it's because they are exchanging less air with the outside. * the caveat to this is if the air is so still that it is driven by diffusion in which case air will be coming and going in the same vent and so exchanging energy. This feels unlikely however.

That's why I like the look of PIV. It doesn't distribute the air as well as the multiple outlets of an MVHR system would, but it is fresh filtered air coming in. It's not heated by scavenging 90% or more of the outgoing air's energy, but those economics are discussed elsewhere.

The PIV system sucks air out of the ventilated loft space

Makes sense, though I'm looking at not having window trickle vents so that is the only difference to what you say

>> The volume of air in just the hallway is around 50m3 which is something like 10 minutes of the PIV's maximum flow rate I don't fancy delving in to the mathematics of diffusion, but I would expect to be able to smell a fart in the hallway within a number of seconds which is much less than 10 minutes. Consequently, the air mixing due to diffusion should be plenty good enough for the PIV to work effectively.

@Cpd I'm hopeful there would be enough mixing with the air already in the hallway. The volume of air in just the hallway is around 50m3 which is something like 10 minutes of the PIV's maximum flow rate. I could investigate the option of a trickle extract on the kitchen extractor fan which would help draw the air through more of the house.

They both look a bit Noddy (or should that be NooNoo) but the Drimaster and Pozidry both cost around £300 https://www.fastlec.co.uk/ventilation/nuaire-dri-eco-hc-drimaster-eco-positive-input-ventilation-unit https://www.fastlec.co.uk/ventilation-extractor-fans/vent-axia-lo-carbon-pozidry-pro https://www.fastlec.co.uk/vent-axia-lo-carbon-pozidry-pro-fd-with-heater-multi-storey https://teletubbies.fandom.com/wiki/Noo-Noo

You're a maestro at finding a bargain.

Thanks. I think you're right, they miss the human consequences sometimes. I hope you and your parents get there soon

Take a look at the thread below. I don't think they're worth it. Thermally broken lintels probably a better choice https://forum.buildhub.org.uk/topic/15192-dont-forget-thermal-bridging/