DHW heating and boiler efficiency.

[MrPotts]

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

[-rick-]

Sounds like scale on the HW coil in the cylinder. Descaling likely hard, so maybe time to think about a new cylinder?

[MrPotts]

29 minutes ago, -rick- said:

Sounds like scale on the HW coil in the cylinder. Descaling likely hard, so maybe time to think about a new cylinder?

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

[JohnMo]

So are you on an S or Y plan? Assume you are. If so the flow temp of the boiler will be the same for heating and DHW. So would suspect you have turned the flow temp down to be efficient, but you now have a flow temp too low to heat cylinder. Set to 70 degs flow temp and see what changes

[MrPotts]

32 minutes ago, JohnMo said:

So are you on an S or Y plan? Assume you are. If so the flow temp of the boiler will be the same for heating and DHW. So would suspect you have turned the flow temp down to be efficient, but you now have a flow temp too low to heat cylinder. Set to 70 degs flow temp and see what changes

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.

Edited Tuesday at 21:14 by MrPotts

[JohnMo]

1 hour ago, MrPotts said:

and a 30 year old vented HWC

 

1 minute ago, MrPotts said:

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 maybe the 30 year old cylinder is showing its age and it's time to buy a new one. If you do get one with a big coil for fast reheats and heating at a lower flow temp.

 

1 hour ago, MrPotts said:

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

Maybe scale on the inside of the cylinder protected, but could just as easily be bunged inside the coil. So you could clean with chemicals. But a 30 year old cylinder the risk is yours to take.

[-rick-]

Drain the cylinder and stick a borescope camera in and have a look? Cheap cameras are £10 on amazon.

 

EDIT: it's a vented tank so maybe there is something other than scale in there.

Edited Tuesday at 21:23 by -rick-

[MrPotts]

18 hours ago, JohnMo said:

 

So maybe the 30 year old cylinder is showing its age and it's time to buy a new one. If you do get one with a big coil for fast reheats and heating at a lower flow temp.

 

Maybe scale on the inside of the cylinder protected, but could just as easily be bunged inside the coil. So you could clean with chemicals. But a 30 year old cylinder the risk is yours to take.

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.

[MrPotts]

18 hours ago, -rick- said:

Drain the cylinder and stick a borescope camera in and have a look? Cheap cameras are £10 on amazon.

 

EDIT: it's a vented tank so maybe there is something other than scale in there.

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

[marshian]

17 hours ago, MrPotts said:

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

Sorry but that's the wrong order

 

Try to remove the immersion heater with the tank still full first (Obviously don't remove it but crack off the joint) then once it's cracked you can drain down knowing that the immersion has been loosened

 

If you try to remove an immersion heater that's been in a copper tank for a while with the tank drained down - you will need a new tank anyway as the soft copper will ruck up around the immersion boss and then fracture

[marshian]

On 04/08/2026 at 20:15, MrPotts said:

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

 

Your 30 yr old HW water tank will have a very small coil (0.3 m) designed for a non condensing boiler set up running much higher flow temps.

 

Your Boiler has a min output of 7 kWh - it can't go below that so instead if the Tank stat is still calling for heat the hysterisis of the boiler means it will have to see a temperature lower than the set point in order to fire again and with little of no heat transfer that could take some time.

 

Half way thro the HW heating cycle (when the boiler is coasting after a burn) run a hot tap for 5 mins - this should introduce enough cold water into the tank to drop the return temp and the boiler should fire again much sooner - this will tell you that the coil in the tank is way too small for the boiler min KW

 

If you want a quick fix turn the flow temp up to max for HW and cut the time down - do the water heating in one burn and rely on the tankstat to say satisfied to end the HW reheat cycle

 

PS if you are running low flow temps to improve efficiency you really are wasting your time - You aren't going to get much condensing efficiency doing HW recharge with a tiny coil in a 30 year old tank anyway so you might as well hit it hard and just get it done

[JohnMo]

1 hour ago, marshian said:

coil (0.3 m)

If that's the case you may need max flow temp from boiler to get anywhere.

 

On 04/08/2026 at 20:15, MrPotts said:

We have sufficient hot water for our needs even though it never heats to the cylinder thermostat setting,

Turn your thermostat down to what it achieves, then that is the simple no cost solution. Walk away and have a cup of tea. Other than that your options are spend money.

 

External plate heat exchanger and pump or new cylinder with a big coil (do 3m²) then you can flow at 60 degs and get 55 Deg water and heat in about 10 to 15 mins.

[MrPotts]

3 hours ago, marshian said:

 

Your 30 yr old HW water tank will have a very small coil (0.3 m) designed for a non condensing boiler set up running much higher flow temps.

 

Your Boiler has a min output of 7 kWh - it can't go below that so instead if the Tank stat is still calling for heat the hysterisis of the boiler means it will have to see a temperature lower than the set point in order to fire again and with little of no heat transfer that could take some time.

 

Half way thro the HW heating cycle (when the boiler is coasting after a burn) run a hot tap for 5 mins - this should introduce enough cold water into the tank to drop the return temp and the boiler should fire again much sooner - this will tell you that the coil in the tank is way too small for the boiler min KW

 

If you want a quick fix turn the flow temp up to max for HW and cut the time down - do the water heating in one burn and rely on the tankstat to say satisfied to end the HW reheat cycle

 

PS if you are running low flow temps to improve efficiency you really are wasting your time - You aren't going to get much condensing efficiency doing HW recharge with a tiny coil in a 30 year old tank anyway so you might as well hit it hard and just get it done

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.

Edited 9 hours ago by MrPotts

[MrPotts]

1 hour ago, JohnMo said:

If that's the case you may need max flow temp from boiler to get anywhere.

 

Turn your thermostat down to what it achieves, then that is the simple no cost solution. Walk away and have a cup of tea. Other than that your options are spend money.

 

External plate heat exchanger and pump or new cylinder with a big coil (do 3m²) then you can flow at 60 degs and get 55 Deg water and heat in about 10 to 15 mins.

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.

[marshian]

1 hour ago, MrPotts said:

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.

I thought S Plan was either HW or CH but I guess it can be configured to do both CH and HW at same time in which case yes running both together over a longer period would work (I used to have Y plan set up and did similar but I did have a gate valve on the HW coil return to stop the HW tank stealing all the flow

[MrPotts]

2 hours ago, marshian said:

I thought S Plan was either HW or CH but I guess it can be configured to do both CH and HW at same time in which case yes running both together over a longer period would work (I used to have Y plan set up and did similar but I did have a gate valve on the HW coil return to stop the HW tank stealing all the flow

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.

[MrPotts]

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.

[marshian]

1 hour ago, MrPotts said:

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.

it would probably be worth a little experiment to optimise the gate valve setting ready for winter

 

mine is set to 1 3/4 turns out from fully closed in all year round.
 

Easy to find the optimum - fire up on HW only let the boiler stabilise at target flow temp then slowly shut the gate valve 1/8 of a turn at a time until the boiler trips out due to temp rising above set point (too little flow) 

 

back out again 1/4 turn and thats your setting for summer and winter (open enough for HW heating in summer to keep boiler happy and closed enough that in winter when running both CH and HW together for a period of tome that the route thro the coil isn’t stealing all the flow and starving the CH circuit

[marshian]

PS mine is now set up on X plan (DHWP) so can’t do both at sane time but the gate valve still is in use just slowing the flow down to keep the boiler from throwing full power at the re-heat

 

(viessman 16kW boiler with 4kW min output - vented 115 litre HW tank with stupid 0.3 m coil - although I have new unvented 180 litre tank to fit with 2.7m2 coil that will initially be set up as vented)

[MrPotts]

38 minutes ago, marshian said:

it would probably be worth a little experiment to optimise the gate valve setting ready for winter

 

mine is set to 1 3/4 turns out from fully closed in all year round.
 

Easy to find the optimum - fire up on HW only let the boiler stabilise at target flow temp then slowly shut the gate valve 1/8 of a turn at a time until the boiler trips out due to temp rising above set point (too little flow) 

 

back out again 1/4 turn and thats your setting for summer and winter (open enough for HW heating in summer to keep boiler happy and closed enough that in winter when running both CH and HW together for a period of tome that the route thro the coil isn’t stealing all the flow and starving the CH circuit

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.

Edited 1 hour ago by MrPotts