Posted by: @benson

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I wonder if a lot of these installs and complex control strategies are based on what the customer demands as well, because most people think that the ability to [try and] independently control each individual rooms temperature and schedules is (a) better and (b) saves money. They have no idea about importance of flow rates, cycling, and thermal dynamics and heat losses of buildings. Ignorant plumbers masquerading as heating engineers don't help either for sure.

Frankly I have no idea how a room thermostat is ever going to work (whether the heat source is a heat pump or a boiler) for controlling UFH in a concrete slab and a fairly low loss house.  Essentially you have a storage heater but without the controllable finned vent.  So you need to modulate in advance how much energy you put in, because (unlike a storage heater) you cant change real time how much energy is released!  So a thermostat, reacting to the current room temperature, is just never, ever, going to work in this circumstance.  The only way is to put energy in based on forecast temperature or, simpler, to use weather compensation which more or less has the same effect because of the lag in response.

That said I guess the fact that low temperature UFH is close to self regulating (because the energy released decreases/increases as the room temperature changes, and its a pretty steep gradient) helps a lot in practice, but its not really the thermostat thats doing the control in this case, its simple physics.

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Posted by: @benson

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We visited an ashp install that we spotted just down the road from us before we had ours put in to ask them about it, and this was the thing the home owner complained about- controls. They had to leave it on all the time in winter they said. They had no traditional thermostat that they could turn right down when they left the house, and turn it back up again when they came home.

I asked them if their bills were ok and they said yes- very good, but we don't want to leave it on all the time. Their house is tailor made as well- completely renovated with good insulation all round, with dedicated plant room.

I noticed a few days ago that they've now taken it out and put a gas boiler back in. It is probably costing them more money but they are no doubt happier just because they can employ their set back strategy. Bizarre. 

Unfortunately the heating controls industry has brainwashed us into thinking that controls=control=cost saving.  Its barely true/probably not true for boilers and definitely not true for ASHPs, but the brainwashing is constantly being reinforced.

The better ASHP controls (eg Vaillant, Nibe and some other principally european manufacturers) let you program a different target temperature at different times of day but then implement this (under the hood) by changing the flow temperature not by on/off (so you can even do it if you have no room influence).  My theory is that they developed this interface when WC was mandated in several EU countries a couple of decades ago, as a way to present a familiar interface to an unfamiliar concept.  My personal view is that this is the way to go.

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Despite having a degree in physics, I too have puzzled over the concept of "just leave it on". It's an ingrained cultural thing.

Its not cultural, its brainwashing by the controls industry.  I also have a degree in physics and it took me a while to get my mind around it, but the explanation is actually very simple.

As a physicist you will recognise that the underlying fundamental is that the heating must supply to the house exactly the same amount of energy as the house loses to the outside world.  You will also recognise that the house continues to lose energy at the (nearly) same rate when the heating is switched off, and that energy lost while it is switched off  has to be replaced when its switched back on again (which of course means that the heating must work harder). 

Only if the house cools materially, which most modern houses don't because they are well insulated and have a heat capacity which is material in relation to the loss, does the reduction in energy lost from the house start to become material.  Its easy to do a sum to work out  very roughly the average house temperature (and thus the total energy loss) over a period of 24hrs if it drops by a few degrees for a few hours due to setback.  Its very difficult to get that sum to show more than 10% reduction in energy which must be supplied to the house. 

Given that heat pumps (and boilers, albeit to a lesser extent) are more efficient if they run at a lower temperature, and of course the heat supplied to the house by the emitters is dependent on their temperature, the savings to be had because the house requires less energy to be supplied to it are soon negated by the reduction in efficiency due to the higher running temperatures required.  Setbacks can work out in your favour, or they can work out against you, overall its generally not worth bothering other than for reasons of comfort or to take advantage of a cheap tarrif.

If you live in a house which has the thermal characteristics of a ten then calculation is different, most of us don't.  The other case is extreme setback, where you heat the house for (say) an hour in the morning and a couple of hours in the evening but it never really warms up (so basically you heat the air only).  In this case on/off heating does use less energy but of course it is never actually comfortable.  Some people have no choice.

Before I got my heat pump I started operating my boiler as close to heat pump like as I could.  Result - lower bills and more comfortable house, presumably because it was condensing most of the time as a result of running at a lower flow temperature, rather than not condensing.  The difference between these states is about 10%.  Sadly most boilers in the UK, whilst they are condensing boilers, dont condense much of the time because thats how our heating industry sets them up.

Posted by: @profzarkov

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I just have one thermostat - an old Hive one. It's set to 19° and I just leave it alone. I've adjusted the Water Law as Samsung call their weather compensation. I'm very happy & my bills are very low. I think that thanks to the 5.2kW solar & Powerwall 3, the biggest part of my annual electricity bill is the standing charge!

L:ike many here I dont even bother with that, just set a weather compensation curve and let it do its thing, no room influence, the physics works, much better than anything I have previously experienced.   The exception is at the end of the season when I do apply a limit because solar gain tends to dominate.