Reducing hysteresis by altering settings.

27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
All house heating.jpg

Red ovals are the radiators, orange square is the thermostat, kitchen has a eQ-3 programmable TRV head, toilet shower is a old wax type and the rest are MiHome Energenie heads, the thermostat is a Nest 3rd generation.

Can't move thermostat boiler in floor below and have to use existing wiring. But by time the wall thermostat has warmed up, there is a lot of heat around and it over shoots, there is a huge difference depending on if doors left open or closed, and my idea of linking the TRV heads to the wall thermostat has failed.
TRV-report5.jpg So I think if I set the TRV heads right it will slow heating as it approaches target and reduce the hysteresis, 51RAAw1+hEL.jpg all but one TRV head is programmable, TRV-times.jpg so with careful setting of times or/and adjusting lock shield valves I should be able to reduce the hysteresis.

But can't get my head around what to adjust first, it seems the boiler is too big, it will run for around half and hour before it starts to cycle, 18 kW three floors, the granny flat I can forget hardly used and has its own pump, zone valve and wall thermostat so that is in real terms independent.

What I want is for the wall thermostat to switch off/on with no more than an hour between each run, this will mean up stairs room TRV heads will have some hot water to work with, but at moment it is switching with around 3 to 4 hours between each run, as it over shoots so much.

Been use to gas before this house with a modulating boiler, now using oil simple off/on boiler, and just can't get head around what I need to do.
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I have no idea about the practicalities of doing this, but it sounds as what are lacking is a buffer - something to store surplus heat once your system nears the desired temperature, a heat store.

Something like a second stat, set a touch lower than your main stat, where once its temperature is achieved it diverts part of the boilers flow into a heat store. When another heating session is required, then it should draw the initial demand from the heat store.
I can see where the problem lies, the radiator is too far away from the thermostat, however between the two are too doors, plus some heat will go up the stairs, so no natural circulation between radiator and thermostat.

So what I am hoping is a way to cheat the system, but what I don't want to do is mess up Nests built in system and stop it learning when to switch off and on. May be forcing it off/on by changing temperature or leading it slowly up in steps so maybe 18, 18.5, 19, 19.5 and 20 degrees at half hour steps, or even hour steps.

But I can't be the first with this problem, so it makes sense to ask, and see how others have got around the problem. Last house had a gas boiler, which modulated, so in the main it was the TRV heads which controlled the temperature, but with an oil boiler it does not modulate, so be it the wall thermostat or the boilers temperature control, it switches off/on but with a rather large gap between each run.
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May be forcing it off/on by changing temperature or leading it slowly up in steps so maybe 18, 18.5, 19, 19.5 and 20 degrees at half hour steps, or even hour steps.

That is what I do anyway, with my system. Absolutely not a matter of having to do that to deal with a problem in my system, rather that's how I feel the need for heat when I get up on a morning, but spaced over the day. The place, even with no heat input, rarely drops below 18C. I just gradually raise it up to twenty in time to be sitting around on an evening.

I do have an open door issue, in as much as the stat is in the hall close to the kitchen door, which is perfect most of the time. When cooking is taking place and the hall - kitchen door is open, the wireless stat/ timer/ main controller sees the heat from the kitchen and shuts the heat for the rest of the house off. Simple solution during those times, if I can be bothered, is to move the stat temporarily to a more representative temperature location.

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