New boiler cuts out ? pipework

[Alangas]

Agile you are right in the advise you've given the op.
Any RGI would of worked out the heat loss and probably found the original boiler was too big to start with.

 

[alanb122]

Thanks for all your advice. With the extra pump in place we have now had a weekend without running problems. This morning the flow temperature was 75 C as required and the return was 59 C, so within range. With hindsight possibly the 28 kw boiler would have been a better choice but estimates of the heat requirements for the house have been assessed at 30 kw or above based on the rads in place which were originally sized based on room sizes. Presumably there is no point recalculating based on room sizes etc unless you are going to change the rads, which was not part of the plan.

The installers were rather slow to react to the problem but have made substantial efforts more recently to solve the problem. They were prepared to put larger pipework in as a next step, but hopefully that will not be needed.

For me one frustration was to be told that there was inadequate water flow but there being no means of measuring what flow actually was. ie how far off the required 1600 litres per hour we were. Also, there seems to be no scientific way of assessing the pipework and pumping requirements except by trying things out.

Alan

 

[D_Hailsham]

For me one frustration was to be told that there was inadequate water flow but there being no means of measuring what flow actually was. ie how far off the required 1600 litres per hour we were. Also, there seems to be no scientific way of assessing the pipework and pumping requirements except by trying things out.

Actually there is a method; it's called "calculating the index circuit".

This works out the pressure loss in the system due to pipe sizes, number of bends, size of rads, system layout etc. This info, together with the required flow rate,enables the engineer to find, from the pump manufacturer's catalogue, the correct sized pump.

The problem is that the procedure is time consuming, so installers don't bother. You also need to know exactly where the pipes run etc, which is OK for a new system where the pipes are installed from scratch, but on an existing system the pipes can be buried behind walls and under floors.

In addition, most installers can guess what size pump is required, based on past experience.

The problem, in your case, was caused by two things: an oversized boiler and too small a pump.

You mentioned earlier that you have a large 4 bed house with solid walls, loft insulation and some double glazing. My last house was a five bed Edwardian semi over three floors. Solid walls, loft insulation and the majority of windows were the original single glazed wood framed. The boiler was a 24kW floor standing cast iron Potterton Kingfisher. The house was always nice and warm.

 

[Agile]

The size of installed rads is only a guide towards the boiler size required.

Its pretty clear to me that no one ever calculated the house heat loss or spent much time calculating the rad heat output. Merely assuming 1.5 kW per rad is not scientific.

The flow rate is not really very important but can be calculated from measurements of the temperature differentials and the heat input.

Its all very well adding an extra pump but has anyone checked the setting of the bypass valve or properly balanced the rads?

Tony