I might have raised this before: Does an air-source heat pump (i.e, air to water) emit heat during all 24hrs of the day? How do we prevent overheating at night - do we just have to wind down the upstairs radiator TRVs? We (still) have gas CH ; during my various (and probably irritating, at times) experiments to ‘test’ a hypothetical ASHP, I ran the heating for all 24hrs for a few days(with large radiators and low flow-temp.) My partner said the constant warmth was dehydrating to the skin and eyes, very uncomfortable. Anyone come across this problem? PS our indoor air is not all unusually dry here, RH approx 55-60+%.
Ideally a HP is on 24/7 during the heating season but that is not to say that it is outputting heat all that time. There are many combinations of controls and settings that will influence how much heat and when it is produced.
Taking my system as an example, in the cusp weather it produces a few 100W of heat most of the time but more at night as that is when it is generally cooler. Now it is circulating water at 37°C with an outside temperature of 1°C. The maximum I have preset is 49°C flow at an outside temperature of -10°C.
Due to the electricity consumed by catching up it is not normally worth switching off overnight. Unless perhaps you have a Passivhaus. Instead people set back the temperature by up to 2°C. I tried the opposite to take advantage of overnight cheap rate electricity but gave up as the house got uncomfortably warm for sleeping.
The better your emitters, airtightness and insulation the lower the flow temperature you can choose and subsequently the better the COP.
I do not use a room or house thermostat. I balanced the radiators to give out the correct proportion of the available heat for each room. This method relies only on weather compensation adjusting the flow temperature. It is the most fiddly and tiresome to get right but I recommend the results.
While I have been writing this post the outside temperature has dropped to -0.3°C and the flow temperature has increased to 40°C to compensate.
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The figures above are out of date now but the broad description is still accurate.
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Emitters - good, all large radiators
Airtightness - better than it was, but a way still to go
Insulation - good, cavity wall insulation/ underfloor insulation/ 500mm Rockwool in loft
Flow temp - usually in the high-30s, sometimes (like now) 45deg.
Thanks, v useful. Still unsure how to address the skin-dehydration problem, I’ll
ask around a few more people.
Actually having a 50-60% RH is ideal for a house to prevent condensation. We have that in our house and it feels very comfortable.
You don’t really want much more than that.
However, in winter you often get even lower RH as the absolute moisture in the outside air is lower at lower temperatures. So you might get below 50% RH inside.
My mum in Germany likes to open all doors for a few minutes each day to replace the air in her small flat and I find it becomes very dry air with the radiators on all the time.
What kind of ventilation do you have in the house?
Any MVHR to help circulate and filter the air?
We have a single DMV (ie, extractor without heat recovery) in the bathroom, running all the time ; also, trickle vents on several windows, plus some ingress etc where I haven’t yet tracked down or cured a draught.
Ah yes, Stoßlüften and Querlüften. Excellent practices but can have unexpected impacts on humidity.
Like you I tried to run my gas combi as an ASHP the winter before I had one installed in summmer 2024. And yes, it was sometimes too hot overnight…
I now run my Viessman ASHP in inactive mode (pure weather compensation). At this time of year my WC curve is 0.7, target room temperature is 19.5 and overnight setback temperature between 10pm and 6am is 18. I only need a very thin duvet even when below zero outside, as my bedroom is currently the warmest room in the house (retrofit in progress).
I’ve had dry eyes for years, and my skin is also getting drier, but given that RH is generally still > 50%, I’ve never associated the dryness with a warm bedroom. Food for thought…
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I ran my Vaillant system boiler at 40°C for years, with manual change to 45°C during the coldest periods. This wasn’t a heat pump trial but economics. It was only later that I discovered that my home was therefore heat pump compatible. It then took me a few more years to get the correct heat pump installed.
The warmest rooms in our house are the bedrooms, which I attribute to convection as they are also the only rooms which still have active eTRVs. It is not the heat of the bedroom in itself that dries skin or sensitive membranes but the reduced humidity.
I am about to turn 70 and my skin is definitely drier than in my youth but I do not attribute that to the heat pump as my eyes etc. are not affected. It is the reduced production of sebum with age.
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“I ran my Vaillant system boiler at 40°C for years,…”
Since you mention this Tim, I’ve been running our Vaillant nominally at 55deg - but that setting appears to be a maximum, if I look at the boiler when it’s in full flight, the actual temp is generally showing approx 38- 45deg (confirmed by a thermometer BluTak-ed to the flow pipe.) Is that how it works?
It never occurred to me to verify the flow temperature. The temperature dial went below 40°C but I don’t recall how far. I set the temperature and assumed that the requested temperature was the one provided.
Incidentally, there was only one temperature. I used the immersion heater on cheap rate to top up cylinder temperature beyond 40°C, less system loss. I had no system monitoring so do not know what temperature the hot water reached.
For much of the year the boiler was off as I have solar thermal water heating anyway. I switched off gas at the meter box from late March/April to late September/October, so there was no gas in the house.