MrPotts

Things have somewhat improved thanks to the advice on this thread. I am now reasonably happy that I my boiler is working to heat the DHW as best as it is likely to do. The boiler now fires in two firings, the first lasts approximately 15mins, then cools for about 15mins before the second firing which lasts about 4mins, shortly after the second firing cycle the DHW cylinder thermostat is satisfied and the boiler turns off and no longer remains on for the full one hour that the schedule. Thanks for the help with this.

As I have a system boiler will the boiler control the pump speed based on the flow and return temperatures therefore if I restrict the cylinder flow via the gate valve won’t the boiler just increase the speed of the pump to maintain the required flow?

I agree, the gate valve might need some adjustment, the problem I have though is that when the boiler reaches set temp it stops the burner immediately. There is no stabilised period. I will experiment though as I know that having the gate valve open 2 turns from closed doesn’t seem to effect the boiler. As always with the internet advise is confusing. Some say open the gate valve just enough to allow flow and some say open it fully for unrestricted flow.

Having listened to the advice already given, thank you, I have now adjusted the boiler flow temp to 70c and the cylinder stat to 50c. The cycle has improved, the boiler now burns up to the set temp in about 22 mins and then turns the burner off, approx 2-3 mins later the cylinder stat gets to its set temp and ceases to request heat resulting in the boiler completely switching off (normal behaviour). I measured the temp out of the nearest tap and it was approx. 55c.

An s plan has two normally closed motorised valves, one for heating and the other for DHW, they can be operated independently of each other. I also have a gate valve on the HWC return which at present is set to 3 out of 5 full turns.

My thoughts are, during the heating off season, is to run the boiler flow temp at 75c, set the cylinder stat at 55c and only run the DHW heating cycle for 30mins at a time 2 times a day. During the heating season I would reduce the boiler flow temp to 65c which hopefully will satisfy the house heating and will mean the boiler will run longer hence heating the HWC at the same time.

Yesterday, when the HW was half way through a coasting session, I manually opened the hearting motorised valve, after 1 or 2 mins the boiler fired. I had a feeling the HWC was going to be the culprit and I think this proves it. Looks like something I will have to live with or spend some money on a new cylinder.

I might drain the cylinder and try to remove the immersion heater for a better look inside.

Should a 30 year old HWC need replacing? Other than limescale and leaks what can go wrong? The water in the system is very clean and runs clear, it has been a sealed system since 2015.

Hi, I have an s-plan setup. I have tried setting the flow temp higher and it makes no difference, the boiler behaves in the same way with just a slight variation in the temperature that the boiler restarts the burner. So boiler set to 70c, cylinder stat left at 55c, same sequence of events as before and cylinder never reaches the cylinder stat setting or even stays on long enough to reach the setting.

Unlikely to be scale as the house has always had softened water.

Recently I have been looking at ways to improve the heating of my DHW due to my boiler struggling to heat the HWC to the temperature set on the cylinder thermostat. To give some background………I have an 11 year old Worcester Bosch Greenstar 24i system boiler with the boiler thermostat set to about 65c and a 30 year old vented HWC with the cylinder thermostat set to about 55c. We have sufficient hot water for our needs even though it never heats to the cylinder thermostat setting, generally the temperature at the taps is about 50c. The issue I am having is that in a one hour DHW heating cycle I have noticed that the boiler fires for about 20mins in total in the one hour period, the rest of the time the burner is off but the pump is continuously running. BTW I have an ABV installed which I have checked to make sure it isn’t activating. A typical cycle is…..Schedule on, boiler fires, after about 15mins the boiler turns off the burner having reached the boiler set temperature, pump continues to run, about 35mins later the boiler turns the burner on again (flow temp at approx 48c), after about 5mins the boiler turns the burner off having again reached the boiler set temperature, about 5mins later the schedule turns off and the pump finally stops. Clearly the HWC is unable to absorb all the heat from the boiler resulting in a low deltaT of about 6c between boiler flow and return. Lowering or increasing the boiler set temperature by a few degrees either way makes no difference, adjusting the flow restriction valve on the HWC return makes no difference and Worcester Bosch tech support tell me the boiler can not be range rated so am I just stuck with a poor performing boiler? I know this issue will go away once the heating season is back but I am wasting electrical energy in the mean time due to the pump running continuously during the “ON” schedule. Any suggestions on what if any improvements I can make would be appreciated, other than to scrap the boiler and buy a lower rated one. Thanks

I would keep the boiler until it reaches end of life.

Mine is an older Greenstar 24i which doesn’t have a digital display just the old fashioned dials. FWIW mine is set by the dial to 65c but rarely gets above 60c at the flow pipe, not great accuracy but better then you are experiencing.

My boiler delta between flow and return is also around 10c after the boiler has been on for a few hours and it doesn’t cause any issues. I have a WB Greenstar system boiler though.