Worcester Boiler switching off(cycling) when ONLY heating HW

[Agile]

The boiler modulates down its power.

But most boilers only reduce the power to about 33 % and in a seriously oversized situation like yours the boiler will then have to cycle on/off.

That is not very efficient but thats the result of an over powered boiler which is why its so important NOT to oversize.

Furthermore the anti cycle delay will result in the boiler only actually firing for about 25% or less of the time. That will cause a much longer reheat time. Again another result of an oversized boiler.

Some boilers enable the anti cycle to be adjusted to the minimum and many allow the max power to be reduced to a minimum. Doing these two will extend the firing time and significantly reduce the reheat time.

Tony

 

[St0rmer66]

there is one bit I really just do not understand:

let's say I have a house that requires 40kw and I have a boiler sized to meet that requirement. what is supposed to happen in the summer months when the rads don't require any heat and only hot water is being heated and, as you said in an earlier response, the cylinder can only consume, say, 10kw when cold?

i.e. if you need a large boiler because you have lots of rads required for the winter, what is supposed to happen in the summer months when you just want to heat only hot water with the same boiler?

You either have a powerful enough pump to flow the water through the boiler fast enough on the now smaller circuit or you have a low loss header to maintain a constant flow through the boiler. The boiler does not instinctively know that it only needs 10kw, it just knows that when it fires up at nearly full rate that the flow temperature is rising too fast. You could argue that smarter software could fix this and you'd be right, different manufacturers do it different ways. Some fire at minimum rate for the first minute then modulate upwards for example.

The problem you've probably had is that the new boiler, with a much more restrictive heat exchanger than the old one (to increase heat transfer, ironically), has been fitted onto an older system with the existing pump and now the minimum flow isn't being met. If the system was designed properly then it wouldn't have happened. Keep in mind that as the maximum output of the boiler rises so does the minimum output. Your 40kw boiler can't turn itself down enough as the cylinder gets hot so even when you fix this flow problem it will cycle quite a lot on HW only.

 

[theSouth]

UPDATE:

I just put a bit of tape on the copper to get a better reading from the infra-red thermometer and when the boiler is running normally I get this:

boiler digital display: 72
flow:70
return ~58 (it's not easy to get to the return with the infra-red tool)

so this seems to be ok - around a 12 degree difference

 

[theSouth]

ok, I think we are drawing to a close on this - thanks both for all the help; I can see you have both helped others with similar questions

Just one final point, it's the same question actually

> That is not very efficient but thats the result of an over powered boiler which is why its so important NOT to oversize.

surely it cannot be an 'oversized boiler' if the rads + water required 40k but the hot water only required 10k - the boiler has to be sized to meet the maximum demand, right? or the winter will be cold

BTW I calculated that I do need ~40k boiler - 22 rads, detached house, not all walls cavity (yes, I know they should be), yada yada

 

[St0rmer66]

A high recovery cylinder (which can soak up 20+kw) would be the best option to combat that, you're right that you have to cater for the highest demand.

 

[theSouth]

>> A high recovery cylinder

any recommendations?

if I understand this, this would consume some of the excess heat from the boiler to be used later? or is this a swap for my current cylinder?

goes to Google "high recovery cylinder"...

 

[fluxeyfingers]

Try googling "Albion Superduty"

 

[fluxeyfingers]

Dunno where my sig's gone detailing my previous username - maybe you don't see your own sig?

 

[St0rmer66]

It's a replacement cylinder, not additional! If you're staying open vented then the Gledhill CondenCyl is one of the cheapest and a popular brand.

 

[theSouth]

I was literally looking at this page when you replied

http://www.plumbnation.co.uk/site/gledhill-condencyl-he-high-performance-vented-hot-water-cylinder/

 

[fluxeyfingers]

Much cheaper than the Albion - Thanks Stormer

 

[Agile]

I really would be very surprised if you needed 40 kW.

Have you used the whole house calculator?

Have you added the heat output of each rad together?

I cannot see any advantage in replacing your cylinder just to prevent a little cycling!

It may well be possible to change the boiler parameters!

Tony