pipe sizing help please

[geezah]

Hi, Im here to see if I can get any help with my boiler cycling which I think may due to wrong bore pipework in my old house.

Ive installed a thermal store indirect coil tank in the loft, connected to my existing system with a condensing boiler, pump and 2 x 2way valves, one for CH and one for DHW.

Cycling only seems to occur on DHW and Ive noticed that within a total run of 24m in the primary circuit,(12m flow,12m return) which is mainly 22mm and 1" pipe, there is a run of 15mm pipework for a total of 8m (4m flow,4m return).

The thermal store ramps up to around 55c and then the boiler starts to cycle and it takes forever for the store to reach its operating temp of 75c. The boiler itself gets very hot so I suspect that the 15mm pipe is not wide enough to remove the heat from the boiler in sufficient quantity to heat the store and is shutting itself down to prevent it overheating.

Am I right in assuming this and should I change the 15mm pipe sections to a wider bore, if so 22mm or 28mm, I want as fast a recovery time for my thermal store as possible but will the 28mm pipe work (for 8m run) have pump sizing implications? The top of the coil in the store is nearly 7m vertical above the boiler/pump, I have a 6m head grundfoss pump which seems to hack the existing set up ok.

thanks

Geezah

 

[BLOD]

Not sure if I've read your post correctly, but have you got 15mm pipe between boiler & thermal store?

 

[geezah]

Yes that's right BLOD, total of 8m out of 24m run in the primary coil circuit from the boiler to the store.

 

[Cds]

Not sure that I fully understand your post. Is this a thermal store or DHW cylinder? A thermal store will have a flow and return from the boiler and then the CH and DHW will be taken off the thermal store. With a DHW cylinder a flow and return will go to the cylinder and also to the heating controlled by a 3 way valve or 2 two port valves.

Which ever you have the primary pipework should be sized to match the coil rating and the boiler output. 15mm is too small and that all of the primary pipework should be 22mm or 28mm depending on the coil rating. If the pipework is undersized you would get the symptoms you are experiencing.

 

[geezah]

Thanks Cds, yes its a thermal store indirect with 2 coils in it, one is the primary coil connected to the boiler and the other is the DHW draw off fed directly from the rising main to provide mains pressure hot water.

At the boiler end, there is a central heating pump feeding into 2 x 2 way motorised valves one for CH and one for DHW.

So, I understand that the 15mm pipe should be changed to 22mm or 28mm (that sounds perfectly logical to me) depending on boiler output (condensing 120,000btu) and coil rating. But can you tell me what the correlation is and how do I calculate it? I don't want to replace the 15mm with 22mm if it should be 28mm and vice versa.

 

[Cds]

The coil rating will be provided by the store manufacturer. However as you already have 22mm in the part of the circuit I would replace the 15mm with 22mm. This should allow the boiler to run without the short cycling.

If you go for 28mm you would have to replace the whole DHW circuit from the boiler to the store in 28mm.

 

[geezah]

Thanks very much for your common sense reply Cds, I've only managed to replace the 15mm flow (4m run) with 22mm copper so far but the difference is like night and day!

The temperature in my thermal store ramps right up to (70c) 5c below operating temperature (75c) very rapidly without cycling and the hot water delivery is much improved, the shower now blows your socks off and the hot water never runs out.

I'll let you know the final outcome when I replace the 4m return run of 15mm. Hopefully the store will hit 75c in one go.

There is one other issue which you may be able to help with, I am getting a substantial fall off in flow rate at all my hot water outlets after about 10 -20 seconds, most noticeeable in the shower outlet. The only thing I can think of is that perhaps I had not flushed the system properly after I installed it and that there is some debris in the blending valve in the DHW run. Any ideas?

 

[Renewable M]

again sounds like undersized cold main to the thermal store and probably too small pipework after it.

With no taps or showers open, the pressure equalises after a short time to the main, so you get a short burst of good pressure but then the resistance of the small pipes exceeds the supply..... so you get the fade.

a point, what you have done upgrading the run of 15mm pipe is you have halved the hydraulic resistance..... so flow rate increases..... so more energy gets to the thermal store.

 

[geezah]

I have now replaced the return 4m of 15mm with 22mm and I can't say I've noticed an improvement over just changing the flow. But the overall result is that the boiler short cycling has been cured and my thermal store reaches its operating temperature, so thanks to all you guys its mission accomplished.

Your theory is bang on Renewable M but in my case the pipework is fine but the following wasn't: I cleaned out my blender valve and made sure the filters were clear, reassembled but it made no difference. I noticed the 22mm isolating ball valve fitted to the incoming cold water main didn't look too clever so I bought a 22mm gate valve and on removing the ball valve isolator a bit of plastic dropped out........It was a flow reducer, got rid of it and fitted my new gate valve (larger bore than the isolator ball valve) and hey, I'm getting 30ltrs/minute at my hot outlets! Of course, who needs a shower at 30ltr/min? but the purpose as far as I'm concerned is that you can run 2 decent showers simultaneously...or even at the same time! There is only a slight fade on the hot outlets, hardly noticeable. Oh and its cured the 'groaning' sound whenever I turn on a hot outlet.

But, how come, there is no fade on the flow from the rising cold main (4.2 bar)?

Once again thanks for your very helpful contributions.

Geezah