Boiler behaviour

ARainyDay

Morning all- sorry another boiler post! Is it usual for a boiler to turn on/off when the thermostat is calling for heat? I have a 28i junior Worcester Bosch boiler, flow temp 2.5, which on average (depending on what rooms we are using) heats 3 (out of the 5) rads and a towel rail. The 3 rad TRVS are set to 2.5-3, and the rads not in use set to 0. The thermostat is in a good position and temp it's showing is correct as I measure it with another thermometer and boiler pressure is fine. In the morning, when no other activity is taking place and the thermostat is still calling to meet the target heat, I've noticed that the boiler stops for periods of time. On average it is 8 mins on, 2 mins off, but even this morning it was on for 5 mins. off for 5 mins at times. I'm pretty sure in the past (with one exception) the boiler has just been firing continuously to meet the target heat. Is this usual? Or is this what is known as 'short cycling'? Or is it because the 3 rads do not need the full power of the boiler? Thanks in advance

MattMattMattUK

ARainyDay said:

Morning all- sorry another boiler post! Is it usual for a boiler to turn on/off when the thermostat is calling for heat? I have a 28i junior Worcester Bosch boiler, flow temp 2.5, which on average (depending on what rooms we are using) heats 3 (out of the 5) rads and a towel rail. The 3 rad TRVS are set to 2.5-3, and the rads not in use set to 0. The thermostat is in a good position and temp it's showing is correct as I measure it with another thermometer and boiler pressure is fine. In the morning, when no other activity is taking place and the thermostat is still calling to meet the target heat, I've noticed that the boiler stops for periods of time. On average it is 8 mins on, 2 mins off, but even this morning it was on for 5 mins. off for 5 mins at times. I'm pretty sure in the past (with one exception) the boiler has just been firing continuously to meet the target heat. Is this usual? Or is this what is known as 'short cycling'? Or is it because the 3 rads do not need the full power of the boiler? Thanks in advance

The boiler will stop firing if the return temperature is too high, could it be that the water is flowing through only the pipes (and potentially the towel rail?) as the room temperature for the TRV has already been reached and so the water is not losing enough heat. 

For TRVs 2 is 15c and 3 is 20c, what is your thermostat set to?

[Deleted User]

What you are experiencing is perfectly normal. Most gas boilers are set up for 6 cycles per hour. Running a 28kW boiler on a system comprising of 3 radiators will result in high temperature return flows. The boiler will modulate down based on the return flow temperature but even if it modulates to 7kW, you will end up with the boiler turning on and off. What boiler flow temperature have you set? 

I have a 24kW WB boiler that heats 14 radiators/towel rails. This was my usage pattern for yesterday:

oldagetraveller1

As 3 x Matt writes, the boiler will flame out when the water returning has reached the temperature set on the boiler thermostat.

I have a WB Ri15 non combi and in spite of the room 'stat set temp. not being satisfied it shuts down until the return flow drops sufficiently to fire up again. It does modulate but not enough to reduce the flow temps..

I only have 4 of 10 radiators in use which means the flow temperature soon reaches the temperature set at the boiler, not the room 'stat.

If I increase the boiler 'stat or open more radiators the room 'stat is satisfied rather than the boiler cycling.

It was exacly the same with my previous Gloworm boiler - wish I had not replaced that with WB!

InvertedVee

It's perfectly normal. I have a Worcester Bosch 30i and mine does the same.

Just be careful that you don't set your TRVs to temperatures below the thermostat. That would cause a constant demand for heat but the system would be pumping water that can't give away its energy and you would get fast cycling.

I prefer to keep my TRVs up at 4 to 4.5 and set the thermostat between 19C and 20C.

I have restricted the maximum boiler output to 7.4kW and set a flow temperature of 50C. I have experimented with adjusting the anti-fast cycle flow hysteresis setting from 10C to 15C, which has extended the run times and I'm happy with that.

I also tried reducing the pump speed which had been set to maximum but with less success. It prevented the system from maintaining the room set temperature in this cold weather, probably because the micro-bore pipework restricts the flow. I may try it again with a higher flow temperature, monitoring the return temperature to ensure that I'm still getting good condensing.

If anyone has any other suggestions, I'm always interested. But I'm quite happy that in the depths of winter, the system was only using 48kWh per day.

ARainyDay

Thank you all, this has put my mind in rest that it's not a fault. My stat was set to 15 degrees, so with the TRVS at 2.5 it wasn't that the rooms had reached the temperature, but good thing to check thanks Matt x 3. Dolor, my flow temp is at 2.5 which I think is around 57 degrees (according to manual 2 is 53 degrees and 3 is 61 degrees). I think I will turn my TRVS up to 3, as I know all TRVS are not made equal and on some 2 is not always exactly equal to 15.

I do have a question though, if say my thermostat was set to 15 degrees, and for argument's sake setting a TRV to 2 did equate exactly to 15 degrees, and 3 to 20, would there be any difference in gas used between setting it to 2.5 and setting it to 3? I would assume not, as the stat target temperature of 15 is below both settings, but perhaps I may not have understood the workings of them correctly.

InvertedVee

ARainyDay said:

Thank you all, this has put my mind in rest that it's not a fault. My stat was set to 15 degrees, so with the TRVS at 2.5 it wasn't that the rooms had reached the temperature, but good thing to check thanks Matt x 3. Dolor, my flow temp is at 2.5 which I think is around 57 degrees (according to manual 2 is 53 degrees and 3 is 61 degrees). I think I will turn my TRVS up to 3, as I know all TRVS are not made equal and on some 2 is not always exactly equal to 15.

I do have a question though, if say my thermostat was set to 15 degrees, and for argument's sake setting a TRV to 2 did equate exactly to 15 degrees, and 3 to 20, would there be any difference in gas used between setting it to 2.5 and setting it to 3? I would assume not, as the stat target temperature of 15 is below both settings, but perhaps I may not have understood the workings of them correctly.

Very little difference, assuming that the thermostat is in a 'good' position (unaffected by draughts, sunshine etc.).

But if doors to the heated rooms are closed, local temperatures may rise to the higher TRV temperature rather than to the thermostat set temperature.

BUFF

btw as the 28i Junior is a non-condensing boiler there is no real advantage to be accrued by turning the flow temp. down.

InvertedVee

BUFF said:

btw as the 28i Junior is a non-condensing boiler there is no real advantage to be accrued by turning the flow temp. down.

The WB Greenstar 28i Junior is a condensing boiler https://www.boilerguide.co.uk/productinfo/worcester-bosch/greenstar-28i-junior. ARainyDay - is this what you have?

marlot

ARainyDay said:

Morning all- sorry another boiler post! Is it usual for a boiler to turn on/off when the thermostat is calling for heat? I have a 28i junior Worcester Bosch boiler, flow temp 2.5, which on average (depending on what rooms we are using) heats 3 (out of the 5) rads and a towel rail. The 3 rad TRVS are set to 2.5-3, and the rads not in use set to 0. The thermostat is in a good position and temp it's showing is correct as I measure it with another thermometer and boiler pressure is fine. In the morning, when no other activity is taking place and the thermostat is still calling to meet the target heat, I've noticed that the boiler stops for periods of time. On average it is 8 mins on, 2 mins off, but even this morning it was on for 5 mins. off for 5 mins at times. I'm pretty sure in the past (with one exception) the boiler has just been firing continuously to meet the target heat. Is this usual? Or is this what is known as 'short cycling'? Or is it because the 3 rads do not need the full power of the boiler? Thanks in advance

Wow.  That's a huge boiler for 5 radiators.

I have an 18kw boiler supplying 16 radiators in a 4 bed detached house - and it manages just fine.  (The house is a modern build, well insulated).

FreeBear

marlot said:

ARainyDay said:

Morning all- sorry another boiler post! Is it usual for a boiler to turn on/off when the thermostat is calling for heat? I have a 28i junior Worcester Bosch boiler, flow temp 2.5, which on average (depending on what rooms we are using) heats 3 (out of the 5) rads and a towel rail. The 3 rad TRVS are set to 2.5-3, and the rads not in use set to 0. The thermostat is in a good position and temp it's showing is correct as I measure it with another thermometer and boiler pressure is fine. In the morning, when no other activity is taking place and the thermostat is still calling to meet the target heat, I've noticed that the boiler stops for periods of time. On average it is 8 mins on, 2 mins off, but even this morning it was on for 5 mins. off for 5 mins at times. I'm pretty sure in the past (with one exception) the boiler has just been firing continuously to meet the target heat. Is this usual? Or is this what is known as 'short cycling'? Or is it because the 3 rads do not need the full power of the boiler? Thanks in advance

Wow.  That's a huge boiler for 5 radiators.

I have an 18kw boiler supplying 16 radiators in a 4 bed detached house - and it manages just fine.  (The house is a modern build, well insulated).

If it is a combi boiler, it will (or rather, should) be rated for the flow rate of the incoming cold water.

Been looking at new combi boilers myself. Whilst I only have 8 radiators and need just 8KW to run them, to heat the cold water to a decent temperature means having a 28-30KW boiler. If I were to go for a system (heat only) boiler, I could get away with a ~10KW jobbie.