Improving boiler efficiency

[Andy Miles]

I recently paid British Gas to replace my old boiler with a new condensing one. My question concerns how to get the maximum efficiency from the new boiler when heating only the hot water.

My house is a four bedroom, two story detached. The new boiler is a Worcester Greenstar 24Ri. It is an open vented fully pumped system with 13 radiators that are mostly doubles and a 3-port diverter valve. The pump is a Grundfos UPS15-60 Super Selectric.

With my old boiler, the pump was set to speed 2. To achieve 20°c drop across the new boiler I have reduced the speed to setting 1. The central heating now takes longer to heat the house but the 20°c drop across the boiler is maintained for most of the heating cycle. I have measured this with radiator thermometers clipped to the flow and return above the boiler. However, when heating hot water only, the flow and return temperatures are very close, typically 70°c flow 65°c return, so the return temperature is above the condensing level. The boiler cycles on and off until the cylinder stat switches off. The boiler gas valve modulates correctly.

My question is, would there be any gain in efficiency by fitting an in-line balance valve in series with the hot water cylinder coil and adjusting it to reduce the rate of flow and therefore the return temperature during hot water heating. Would this achieve the aim of forcing the boiler into condensing mode during hot water heating, thus reducing my gas bill?

Andy Miles

 

[superdupergasman]

Might be suggested that you run your pump at 2 or 3 as you have quite a large system to pump the water round and the fact that it is taking longer to heat suggestes that the boiler will run for longer and therefor use more gas during this period.

I would always like to see a balacing valve on the return from the hot water cylinder to allow flow reduction, water will always find the easiest route and may be pinching flow from your rads.

you will never keep a 20 degree difference however as the water comes upto temp the return temp comes up and brings the boiler out of condensing mode but the balacing valve may prolong the temp difference.

 

[gas4you]

Leave the hw as it is, as this way it will heat the dhwc a lot faster then shut the boiler off sooner.

Is this a standard cylinder or a fast recovery cylinder? If the latter then definately do not put a valve in it.

Put the pump back to 2 and balance all rads with the lockshield.

Also make sure the boiler flow is set to no more than 65C

 

[bab]

If u can cope with the slow warm up of the rads leave it on a slow speed. The load to the rads is a lot greater than that to to cylinder. But if its a oldish cyl with standard recovery there should be a ballance valve on the pipe work to slow the flow to help stop cycling of the boiler, & will also aid the circulation around the rads when both are on if the low is cranked down to the cylider. Most fitters fit the cyl ballancing vave on the return to the boiler just after its come out of the cylinder which can help on replacement of the cyl without draining the heating

 

[Andy Miles]

Many thanks for the advice so far. I don't have a fast recovery cylinder, it is a standard one. I will try out some of your suggestions, any more ideas?

Andy Miles

 

[superdupergasman]

Also make sure the boiler flow is set to no more than 65C

Never heard this one?? would you not struggle with getting you DHW up to 60 degrees??

 

[Agile]

I recommend you leave the pump at "1" and add a balancing valve on the cylinder coil return.

That promotes efficiency over warm up. But you can compensate by starting the hot water earlier.

Tony

 

[DP]

I would like to see this 13 radiator manor house heated by a standard 15/50 set on 1 achieveing 20 degree differentail across each radiator.

 

[Andy Miles]

Just to clear up a few misunderstandings:
The pump is a 15/60 not a 15/50
I have not measured the temperature drop across each radiator, just the boiler. It maintains at least a 20 degree drop for most of the heating cycle.
The DHW cylinder stat is set to 55 degrees and the cylinder seems to heat up quite quickly. I agree that adding a balance valve may slow this down.

My sole aim is to reduce my gas bill. Therefore I am keen to achieve heating and hot water while running the boiler to its maximum efficiency. British Gas did a grand job fitting the boiler, but my entire system was designed for an old non condensing boiler. I suppose my question is, what adjustments would you make to pump/cylinder balancing after changing to a condesing boiler?

Many thanks for the advice so far. Andy Miles

 

[namsag]

And why do you think it is cheaper for your rads to be on far longer to heat the property to reasonable temp..
What uses less gas ,a boiler running at 90% efficency for 3 hours or a boiler at 85% efficiency for an hour and getting the house hotter and turning off..