No, I disagree, if you change air which has not been heated up, you are not losing heat in the extacted air.
What do you mean by changing air that has not been heated up? Any air coming into the house will be heated up if the incoming air is cooler than the house.
It won’t be heated up much if the house heating is off. And once it is gone it is gone. However the house fabric will still be around and will need reheating at some later date. The air that went thru the house will not.
I suppose what you are saying Dave, is that if the house is going to be unoccupied for a long period, is there any advantage in keeping it heated when unoccupied.
If you will be away for a decade, then there will undoubtedly be no advantage. But if you are away for 8 hours then maybe there will be an advantage. The trick is to find where the crossover point is.
Maybe we could work out what the total heat capacity of the house is in kWh and then work out how many hours it would take for the house temperature to decay to the ambient temperature. And then work out how many hours it would take the installed heat pump to deliver that many kWh back into the building.
The value judgement can then be made whether the time taken to reheat the house to equilibrium is convenient.
It all depends on the overall length of time and the air change rates involved. FWIW, the heating of the air doesn’t depend overmuch on the heating being on. Most of the air is heated by gaining heat from other warmer air. And most of the warmer air (that wasn’t itself heated in that way) got that way by absorbing heat from the floors, walls and ceilings, and furniture etc of the house. When the heating is on some fairly small part of the air gets heated by the heater, but heated quite a lot. With UFH there’s a greater proportion heated to a lower temperature, of course.
So air that is expelled is typically close to the temperature of the interior of the house. It’s not some separate thing.
But as David Wolley says there’s a crossover. It’s generally accepted that it’s not worth heating for example a church continuously, but instead it’s cheaper to heat it just when it’s going to be used. Another factor is that heating systems take time to heat up a building, and heating systems like heat pumps that are more carefully designed typically have lower maximum outputs than traditional boilers and so take longer to heat up a building again.
An airtight house with MVHR loses heat a lot more slowly than ‘normal’ houses.
There’s a great Protons for Breakfast article about that. It also applies to longer off-periods.
To quote the main takeaway message from the article:
“If the heating is 100% efficient, then a setback period will always save energy. It’s generally not a big saving, but it is always a saving.
If the efficiency of heating varies with power, then a setback period may save energy, or may not.”
So this all boils down to how much COP varies with heat pump power and how long you keep the heating off.
What you’re assuming here is a house where the ACH are so high that we can no longer assume a thermodynamic equilibrium between house fabric and air, i e. the air moves so quickly through the house that not all of it is reheated when it leaves the house.
Sure, if you open all windows and the wind rushes through then this might be the case but for all realistic cases internal air is assumed to be at equilibrium with the fabric, and I think that’s a valid assumption. A 60 m3 room at 1 ACH gets around 1000 liters of cold air every minute. We could sure take the step to approximate the time it takes to reheat that volume based on surface temperature of the room and geat transfer coefficients, but essentially this volume mixes with all the air in the room and is reheated via all the rooms surfaces. It is safe to assume that the air at those ACH numbers is in equilibrium with the fabric.
This is also the assumption that goes into all heat loss calculations. Look into any heat loss survey document - there will be an assumed (or measured) ACH value. The stated ventilation heat loss is exactly equal to the energy required to reheat ACH * V_House of air from the outdoor design temperature to desired indoor temperature.
This does not change for the scenario of longer abscence and having the heating off - in fact the heat loss surveys are just that - looking at heat loss at a specific indoor-outdoor temperature and then you pick a heating solution capable of just supplying the lost power back as heat. Instead of the energy being resupplied by the heating when that is off, the house loses heat and cools down. The cooling process is an exponential decay towards the outdoor temperature.
Heating of ventilation air (or infiltration) can account for 20-50% of the total heat pump capacity, which is significant. However, the total heat pump capacity (ventilation + heating + water) is calculated based on the lowest outdoor air temperature, which occurs only 3-10% of the year. This can be leveraged.
For example, when calculating the total heat pump capacity, heat losses can be estimated for a low temperature (-9 °C), while heating for ventilation can be based on a higher temperature (-3 °C), which typically occurs only 10-15 days a year. During these rare days, it may be acceptable to reduce ventilation slightly, which would significantly lower the heat pump capacity, as well as the size of radiators and pipes.
For instance, the attached photo shows the temperature distribution for the city of Prague. As you can see, extremely low temperatures are infrequent.
Just thinking out loud… When considering CO2 data for regulating ventilation air flow, it’s worth mentioning potential drawbacks:
- CO2 sensors are not the cheapest option.
- Most high-quality sensors require calibration.
- Installation is often required near the ceiling (due to rising humid air).
- Inertia of ventilation activation
It’s also worth mentioning an alternative approach: using microwave presence sensors, such as the HLK-LD2410. These are relatively new sensors that detect presence rather than just human movement, as older models did. They are likely to become increasingly common in systems like lighting control and similar applications.
Overall, CO2 sensors are an excellent solution, but I just wanted to point out the existence of these new presence sensors—this information might be useful for someone.
FWIW, I just calculated the total heat lost if all the air in my house, which I assume to be at 20°C, was replaced by air at 0°C. The answer is just less than 3 kWh (2.93 kWh). Not a lot in the grand scheme of things.
But I typically run my MVHR at 125 m³/hr (just under 0.3 ACH) so if I was ventilating without heat recovery I would need to supply 0.83 kW to heat the ventilation air up. That’s over three quarters of the actual heat demand of my house. Of course most houses don’t behave quite like that.
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Hi Andrii,
I have CO2 sensors as my main ventilation indicators since two years and they work nicely. Unless you’re experiencing an electrical glitch that messes with the sensor storag I’ve never had to recalibrate. Even then, recalibration is super simple - I have a button exposed in Home Assistant to recalibrate them. Put the sensor outside for 2 minutes, calibrate, done. My main SCD30 in the living room worked without calibration for more than a year, always stabilizing to 400 ppm. If you regularly have them at 400 ppm, you can also use self calibration, but for me the one-off calibration has been rock solid.
I’m not sure why I would need to install them on the ceiling. Mine are roughly at 30 cm above ground and work nicely. CO2 should not really stratify within the room, or does it? Also they respond very fast to ventilation, just look at the response times for SCD4x sensors in the datasheet.
I also have a presence sensor in the living room. Interesting approach to track persons over time to estimate pollution, but for that it’s too unreliable with plants often causing false positives for me.
I recently bought an Everything Presence Lite – Everything Smart Technology mainly for the optional CO2 sensor but it does have a presence sensor.
The difficulties I see with using presence sensors to control ventilation rates are: (1) you’d need them in every room together with some logic to add up the numbers, whereas a single CO2 or humidity sensor can be placed within the main MVHR extract duct and (2) I’m not sure how well they would cope with things like parties.
Hi Andre_K
I am also a supporter of CO2 and just wanted to share an alternative approach that might be more suitable for someone’s specific conditions. I recently started using the SCD30 for monitoring (built it with Arduino). Warm air, like humid air, rises along with CO2, but I think you are right that in typical small residential spaces, this is not critical. Regarding inertness, I also agree with you.
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The difficulties I see with using presence sensors to control ventilation rates are: (1) you’d need them in every room together with some logic to add up the numbers, whereas a single CO2 or humidity sensor can be placed within the main MVHR extract duct and (2) I’m not sure how well they would cope with things like parties.
CO2 readings in the main air duct represent a mixture of air from all rooms, which would be inaccurate if emissions are concentrated in a single room. For instance, if there are six rooms, but people are present in only one or two, the readings should focus on the room(s) with higher levels, or the ventilation system should use VAV (Variable Air Volume) technology, where airflow is regulated for each room. However, for residential settings, this solution is overly complex and unnecessarily expensive.
In a perfect world, perhaps. In the real world both CO2 and water vapour diffuse quite well, and people are sensitive to a large excess of either and move elsewhere if necessary. So a central measurement is quite normal and adequate in a house.
Apologies, yes, @awjlogan is currently working on integrating this on to the next design of the EmonTH and we will look to getting some initial prototype quantity to you likely before this is available more generally. Not 100% sure on timescale yet.
Trystan, thank you for the update. I look forward to testing this out. @awjlogan, let me know if I can assist in any way.
Best regards,
Gordon
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