DIY Doctor

Pipe sizing help please

Postby geezah » Wed Jan 20, 2010 10:28 pm

Hi, I have a thermal store indirect coil, connected to my condensing boiler with a pump and 2 x 2way valves, one for DHW, one for CH. The store is new, the system is inherited when I bought the house.

Ive noticed my boiler cycling when on DHW only and the thermal store in the loft does not ramp up to operating temperature (75c). It reaches about 55c then the boiler starts to cycle.

The primary circuit is around 24m lenght, 12m flow and 12m return. I have uncovered a run of 15mm pipe for a total of 8m, 4m on flow and 4m on return. The rest of the pipework in the primary curcuit is 22mm and 1".

The boiler gets really hot and is shutting down before the thermal store gets up to temperature.

I suspect (im not a plumber) that the 15mm bore pipe is not able to deliver the heat away from the boiler to the store fast enough or in sufficient quantity so that the inbuilt boiler stat shuts it down to prevent it overheating?

Am I right in the above, I conclude this because the CH doesnt cycle and Ive noticed that it uses 22mm pipe before it branches off to the rads.

Should I replace this 15mm pipe on the DHW on both flow and return to a wider bore, if so 22mm or 28mm, I need a quicker recovery time as poss for my thermal store.

The top of the primary coil in the store is almost 7m above the boiler (thermal store tank is in the loft) and I have a grundfoss 6m head pump which seems to be doing the job fine. Is it a case of the wider the bore the better? I only intend to replace the 15mm part of the circuit (its reasonably accessible) or will there be pump sizing implications if i go up to 28mm?

Thanks in advance,

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Postby plumbbob » Sun Jan 24, 2010 12:07 am

Although it is unusual to see an indirect cylinder coil supplied by a 15mm pipe, I would not automatically assume this to be the cause of your problem.

It is not unusual to find a boiler cycling if it is only being used to heat the secondary coil because the heat transfer ability of the coil is far less than the boiler capacity. As the water temperature rises in the store the coil becomes less able to transfer the heat and the last few degrees takes a long time.

One possibility is if the boiler temperature is set at near to or even below the store thermostat, the water will never reach temperature consequently the boiler will cycle indefinitely. Don't rely on any markings on thermostats as they are inaccurate.
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Postby Perry525 » Mon Jan 25, 2010 8:21 pm

I notice that you do not write that the hot water is too cold?
If the water coming out of the hot taps, is hot enough to use?
Then there is no point in doing anything.
If the hot water at the taps is too cool, then I suggest you buy and use an infrared temperture gauge, this will enable you to check the hot water at any point along these very long pipe runs.
Question: Are these pipe runs insulated?
There is the matter of heat loss on long runs and loss of pressure due to friction in the pipes.
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Postby geezah » Mon Jan 25, 2010 11:00 pm

Thanks plumbbob for your helpful reply, I have in fact changed the 8m run of 15mm pipe in the primary circuit as described above to 22mm pipe (Thank goodness for compression fittings!) this weekend and I'm pleased to say that the difference is like night and day!

The boiler cycling problem has all but been cured. The store now ramps right up to 70c in one hit, which is still 5c below the operating temp of 75c and it cycles as it struggles to raise the extra 5c, I assume for the reason that you state, which makes sense. The central heating boiler actually shuts off now into 'dormant' mode whereas before it never got the chance!

My condesing boiler is set to maximum, as im told it should be for a thermal store so I imagine that this setting would be above the 75c thermostat setting in the store.

Once again, thanks for your help.

PS, my original primary circuit was in 28mm but a builder remodelled my kitchen 10 years ago and it was his Heating Engineer (Corgi registered) who reduced this part of the run down to 15mm!! (I can only assume to make the pipe runs slightly easier - or thats what he had in his van at the time!) the same person who nearly gassed my family and I to death by leaving the main gas connector to the boiler hand tight! I guess you can see why my faith in the trade has been dented and why despite the regulations I prefer to do my own work on my home, because I know that it will be done properly and safely. Not to worry, I would never attempt anything that I didnt feel comfortable with. I know my limitations.
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Postby geezah » Tue Jan 26, 2010 8:37 pm

Thanks for taking the time to read my post Perry525, yes the water was always coming out hot, its just that there wasn't enough of it and it took forever for the store to come up to temperature because of the continuous cycling issue.

Ive insulated the pipes wherever possible, very difficult where pipe clips are used and the pipe is so close to the wall - any ideas?

Would you say that a total run of 24m on the primary circuit is particularly long?

How do I maintain the stated 20c temperature drop between the flow and return pipes (condensing boiler) if the boiler is always run at max pelt as recommended for a thermal store?
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