Central heating on/off or up/down control?

Discussion in 'Eco Talk' started by MGW, Feb 16, 2020.

  1. MGW

    MGW Screwfix Select

    Clearly any central heating must at some time be switched off, be it leaving the premises, or reducing temperature as when retiring for the night, so the programmer is still a valid control.

    However modern boilers are designed to modulate, i.e. turn down, and thermostats connected to the ebus and to the coolant water be it opentherm or thermostatic radiator valves are designed to turn the boiler down as the home warms up, and once the minimum limit is reached use a mark/space ratio (on/off) to further reduce output, and in that case when it switches back on it will normally switch on at minimum output, and often it means the circulating water is cooler as it approaches the switch off point, so when it does switch off less heat it lost from the flue.

    OK that is theory, but in practice many boilers which can modulate don't as they have an off/on thermostat which controls temperature that causes the boiler on restart to revert to maximum output so many a modulating boiler does not in real terms modulate, this has been made worse by the zones used in modern homes, so it is hard to split the programmed on/off to the temperature control modulating of the boiler, often a thermostat switches off the motorised valve which in turn turns off the boiler.

    Both modulating and mark/space control works, however what is the difference in real terms using a modulating boiler with a mark/space control rather than modulating control? Are the losses that much less when the circulating water is held at 40°C to when at 60°C when it runs all time at 40°C being 20°C over target temperature and less than 50% of the time at 60°C when 40°C over target temperature?

    I would guess the time scale is a large factor, so a well insulated home a burst of heating for 1/2 hour could keep home at above re-heat threshold for 2 hours, where with poor insulation the time could be 1/4 hour on and 1/2 hour off so nothing really cools down. Radiators remain hot and boiler loosing heat through flue all the time. So likely no one answer, but a scale at which one method or other breaks even.

    But each time asked I always say analogue is best with modulating boiler, modulating TRV, and modulating wall thermostat, giving a very good hysteresis and allowing the boiler to extract the latent heat, and likely that is true, but if two identical homes with occupants of identical life styles are controlled with say the same thermostat, one connected mark/space and one with opentherm what would be the cost difference?

    If a home is zoned into 2 zones, dormitory and living, with a mark/space control for both areas, with a modulating boiler, if you fitted a change over relay so when the living area was selected on the programmer a thermostat in living area connected to boiler ebus, but when living area switched off it was disconnected and one in dormitory area used instead, would it make much difference to using on/off thermostats?
     
  2. Jimbo

    Jimbo Screwfix Select

    Total consumption between the two systems is influenced by average temperature difference between inside and out over the test period and the burner efficiency.

    As a traditional mechanical on-off thermostat will overshoot more, digital thermostats with TPI were hoping to reduce this overshoot and hence reduce consumption somewhat. However in practice studies have shown that this achieved basically nothing in terms of savings, though it may have improved comfort perhaps.

    Boilers will modulate burners to regulate the flow temperature. So if the thermostat is calling and the boiler has a warm return line, it will still be modulating based on the load required to sustain the set flow. However, a thermostat communicating with the boiler may be varying the flow temperature, i.e. reducing flow temperature as set point is approached, so ensuring higher efficiency. Weather compensators aim to do this based on outside temperature.
     
  3. MGW

    MGW Screwfix Select

    If I am reading your reply correct, you are saying if the circulating water is kept as cool as possible, the system is more efficient, it does not really matter if weather compensation or modulating thermostat, keeping the circulating water as cool as possible is the key?

    Turning burner on/off or up/down does not matter as long as circulating water is kept as cool as possible, if the boiler turns the burner off, or down, when it turns it on again it can still keep the water cool.

    But if a programmer or thermostat turns it off, then until the water has circulated the boiler does not know what is the lowest water temperature that will keep the home warm, so it will have to heat the water far higher than required which must mean not running as efficient.

    But if the result is 85% instead of 87% efficient, then hardly matters, but if 85% instead of 95% then clearly it must be avoided, and that is the question is it worth doing all that is needed to improve it, or is the saving not worth the effort?
     
  4. Jimbo

    Jimbo Screwfix Select

    Yes, of course keeping the return sufficiently below the condensing temperature of the fuel in question is key to extracting the latent heat as that condensation occurs, but in general the lower the flow the greater the efficiency as the lower the exhaust gas temperature. Provided it is actually condensing I suppose it will be quickly diminishing returns, as it will be already >90% most likely. Probably better to spend on smart controls that enable a reduction of heating in spaces when unused, improving insulation, and maybe heat recovery ventilation.
     
  5. Dot99

    Dot99 Active Member

    Is a water temperature sensor/monitor required
     
  6. MGW

    MGW Screwfix Select

    Always has been, although we call them boilers, we in fact don't want them to boil, so there has always been a output water temperature control, and now with modulating boilers also an input water temperature control, with a boiler it is more to do with pressure, although with super heated steam I suppose temperature is also important, next time I am at work I will have to look at what gauges there are in the cab, but that is the fireman's job, to anticipate when more steam will be required and shovel on the coal, and to stop shovelling on before reaching the station so steam is not wasted. If anything the fireman's job needs more skill than the driver, however can't become a driver unless been a fireman so driver must have the greater skill. But as said for central heating in homes the idea is not to make steam, heating with steam is normally total loss, the steam is feed to carriages where it condenses and it then allowed to run onto the floor outside. There should not be any steam coming from the carriages, but it adds to the ambience, so the drivers often send too much steam to carriages, specially in the station better going to carriages than blowing off the safety valve.
     
  7. MGW

    MGW Screwfix Select

    Sorry I should have not talked about a real boiler, I know the condensing of the water can produce more power, in fact in early days all the power was got from condensing the steam, but that is not what is meant by a condensing boiler, it is the gaining of latent heat which is the whole idea.

    And to gain that latent heat the boiler should only be turned off externally when heating is not required for a few hours. I no longer have a condensing boiler, but I see so many reports on how heating has been set up, and it does not seem to make sense.

    I see the problem, if the boiler modulates then at some point it will reach the minimum setting and so the boiler will turn off, but then to turn on again really it would need some thermostat as without it only way boiler can find out if required is to turn on and circulate water. If the system is all TRV then pressure would be enough, but if using fan assisted radiators or any radiators which do not stop the flow when off, pressure alone would not work, so it has to heat water and circulate it to test if required, I don't know how much water is in a central heating system, but there needs to be some time involved to heat up all the pipes and matrix's even if no fans switch on to heat the rooms, so the boiler has to run for up to 15 minutes just to see if required.

    So in real terms we need some system to tell the boiler when it needs to start again other than simple time. As far as I am aware we don't have anything, so only way to tell boiler to run again also will tell boiler to stop.

    Up to date we do have electronic TRV heads which will tell some central hub/thermostat when the boiler is required. But nothing to for example turn off boiler when by-pass valve lifts, and on again when required again as will simply don't have a latching system that could do that.
     

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