by ericmark »
Mon Apr 04, 2022 1:34 am
Most combi boilers should not normally be used when there is no water in the heat exchanger. However clearly the boiler has been modulating before it should.
So a little insight to how a boiler works may help.
The return water temperature is used to control output, the boiler assumes if return water is hot, then the home is satisfied heat wise.
So we have three things which control the temperature of the return water.
1) The lock shield valve on the radiator, this is really a simple tap, is should be set so there is around 15 degrees temperature drop incoming to outgoing water, it ensure every radiator gets it's far share of water, if open too wide it allows hot water to return too soon to boiler, and causes a hysteresis as the TRV can't open/close fast enough.
2) The TRV (thermostatic radiator valve) once room gets warm this should close, and when all have closed the by-pass valve will open.
3) The by-pass valve, some times inside the boiler, and some times external, as the TRV's close the differential pressure raises and the by-pass allows water to return without going around the system, when the hot water returns to boiler as this lifts, then the boiler turns down.
So a restriction to flow will open by-pass and turn down boiler, or faulty by-pass, but also any single radiator wide open can also cause hot water to return turning down boiler, old systems did not have a by-pass valve they had one radiator often towel rail which has no TRV to return hot water.
The theory is the TRV can't turn the boiler on, so in a room normally kept cool, on the ground floor, with no outside doors, and no alternative heating the radiator has no TRV and you have a wall thermostat, in practice the room rarely exists, so TRV in room with wall thermostat has to be carefully set.
A on/off wall thermostat is only there to switch off boiler when summer arrives, however we also have modulating thermostats which connect to the boiler e-bus and turn boiler up/down not on/off.
