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 2 hours ago 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 2 hours ago by -rick-