Riva Advance HE m110B.32sm/c - Intermittent Central heating advice and thoughts

[Burkle]

Hi All, I have been experiencing issues with intermittent heating over the last few weeks. Sometimes the central heating works fine and all radiators are hot, but other times it is much more intermittent (e.g. for the first 10-15 minutes the radiators are hot, but then they go slightly warm or cold... every now and again they will go hot again but not always during the heating cycle).

Boiler is a Riva Advance HE m110B.32sm/c, and has been in at least around 9 years going from the installation book (we have been here just over 7 years).

I have been observing the boiler for the past couple of weeks to try and figure out a pattern to this - there isn't anything that stands out to me.

I always make sure there is a call for heat and the boiler is actually on, and the temperature of the stat (works off a single Hive stat) is not met, and have tried this by bypassing the stat and using the manual switch on the diverter valve which controls the radiator central heating (there is also a valve controlling underfloor heating separately). I generally don't have them both scheduled together (UFH is connected to its own wiring centre back to a panel at the boiler with everything else so the CH and UFH can be controlled independently) as the UFH will be on first to warm the floor, then radiators on separate, but have found that if i run them both together the heat output from both often doesn't seem to be as good. For information, the UFH circuit covers around 60m² on the ground floor, then for the radiator circuit there is 2 radiators on the ground floor, 4 radiators on the first floor, and 1 radiator in the converted loft.

All radiators have been balanced, and when the system is working they all get hot throughout the full radiator (including the flow, and the cooler return).

I have fitted a magnetic filter on the return near the boiler and the system has had x400 in it for just over a week now, which i am checking / emptying every other day at least. It is picking up slight fragments and the water is starting to look a bit brown, but nothing significant - i will be leaving it in the system at least 2 weeks (up to 4 weeks as suggested) then will flush out the system using the mains filling loop going around 1 radiator at a time). This is more on the basis that previously there had been several older steel radiators which have been removed (only 1 older rad remaining), and in case other rads are sludged up anywhere to try help.

I wanted to see if anybody might have any thoughts or advice on what the issue might be with the heating system being great some days and not good at all others even though the boiler is on and looks to be firing up still, with no errors.

A few things i have checked and other observations:
- i have taken the pump apart to check this and it is clean and spins freely - when the boiler is on it feels like it is working when touching it, but is there anything i need to be checking?
- the micro switches on the diverter valve all appear to be clicking, and the pins on the valve pushing freely in/out - i am thinking of taking the valve out to change all washers etc. and grease / change parts as part of a maintenance kit, just to see if this makes any different.
- at times the boiler will fire up and maintain longer than others, and you can hear the water being pushed out to the radiators (with the front case panel off), and feel it as soon as it hits the nearest radiator. The flow pipe is hot, then once water has moved it gets cooler initially. Sometimes i have noticed that the flow pipe is cooler from the boiler, but the last say half half a metre of the return pipe directly to the boiler is very hot. I dont know if this is normal at all with a combi at times, or suggesting a pump or diverter issue?
- pressure in the system is fine (typically set around 1.25)
- there is never any errors showing or lock out
- i have not investigated the primary heat exchanger, but will be looking to get a plumber to service the boiler sometime soon
- DHW heat exchanger has been cleaned out - no issues with this, and no issues with the hot water at all
- possibly a minor observation or something which makes no difference; when looking through the manuals i have noticed that the bottom half of the boiler doesnt match that of the m110B.32sm/c (the pump and diverter valve look different on pictures, and on the parts lists from suppliers / websites), but the fan, air manifold etc. in the top half all match. The bottom part appears to be the same as the parts and picture listed in the m110.32sm/c manual.

I appreciate there's lots of information above, but would anybody happen to have any initial thoughts on what might be the issue before i get the plumber booked in? Any particular known / common faults with this boiler to be mindful of?

Any advice would be appreciated, and if any other information might help then let me know

many thanks in advance!