CH Bolier Cycling

gazza007

I recently had my Central Heating pump replaced as it failed . I also improved the system by moving the wall stat to the hall area from the lounge/diner as it used to cut-off too soon due to the proximity of the kitchen etc & upstairs never had a chance to get warm.I have installed TRV's on all the rads apart from the hall/bathroom. The balance of heat in the house is much better and also must be saving some gas. One thing I do find is that the boiler cycles more than it used too but it isn't short cycling, I just think the boiler stat is doing a correct job. I have boiler stat set about 2/3 max & the pump on setting 1. Do I turn up the pump speed or the boiler stat or leave it alone? System seems to be balanced as the rads are all running nice & hot when required & hot water is ok too.
I just that I had the PCB replaced last year & I don't want to over-work the new one.
Advice appreciated. Thanks

moneysavingplumber

Balancing a heating system isn't as straightforward as it often sounds. Every system is different and has different dynamics and factors. The type of boiler will make a significant difference, modern low-water content boilers can be prone to kettling if the system isn't balanced properly.

If you had a PCB replaced I'll take a wild guess that you have a Potterton Suprima, which is a low-content boiler. I would personally have the pump and boiler stat set higher. Often the immediate pipe sizing from the boiler tails will be too small, which will also cause cycling and kettling over time.

New or updated systems need to have a programmer, room stat correctly sited, TRVs on all radiators with an automatic by-pass fitted to the system, and balancing requires that a constant temperature differential is achieved between flow and return at all points on the system (60/40 for condensing boilers), which is far easier said than done. A correctly piped and successfully balanced system should not have heat building up at the boiler flow stat, so cycling should be minimal.

But as any heating engineer will tell you, updating existing systems is rarely so straightforward. Sludge or undersized pipework in the system will all have an effect.

gazza007

If you had a PCB replaced I'll take a wild guess that you have a Potterton Suprima

Thanks for prompt reply

I'll give you 9/10!!! it is a Prima 40F. It cost me a few £'s too for the PCB as my regular plumber was on his hols.
There is a bypass just after the pump before the 3-way motorised valve (I also replaced the motor bit of the valve earlier this year after long delays when calling for heat only & I could hear it buzzing). To be honest I always ran the boiler at full stat before the new pump & TRV's. I guess as the TRV's close I will have more heat in the water returning to the boiler so more likely to cycle especially as I don't have it on full now.

:beer:

moneysavingplumber

gazza007 wrote: »

Thanks for prompt reply

I'll give you 9/10!!! it is a Prima 40F. It cost me a few £'s too for the PCB as my regular plumber was on his hols.
There is a bypass just after the pump before the 3-way motorised valve
:beer:

Well the PCBs on the old Primas and Profiles do go but they aren't anywhere near as much trouble as the early Suprima ones, which were eventually completely redesigned after much publicity.

Is your bypass an proper auto-bypass valve or just a cranked down isolating valve? The idea of the auto by-pass is it reacts to difference in pressure in the system when the TRVs close, where-as an isolating valve won't compensate.

gazza007

Just a bypass, T-junction with smaller pipe & valve before it dissapears under the floor, I guess it joins the return circuit.