marshian

Ahhh that makes perfect sense although two boilers mean two service bill/maintenance costs and I'd be fretting about which one was driving the bills but that's me all over

It really is amazing how much detail is in the install manuals if you dig into the detail - first manual I ever read was for my glow worm - I only read it because it was driving me nuts and I thought I could dial out it's problems - I got to understand the valiant/glow worm anti cycle settings which was a complete revelation when trying to manage a boiler with a minimum output 2.5 times the heat loss of the house. Despite the boiler being a cheap end of the market the manual at least laid out the critical basics like min flow rate required which is often absent in a lot of manuals I read

Fair point - but as I said I quite like reading boiler install manuals and it never hurts to check

That's the thing about boilers they don't always respond in the way you think they will - range rating my old glow worm boiler to 10 kW consistently reduced the warm up time by about 20 mins - yet it was a 24kW boiler Right now it's modulating up and down like a whores drawers - get it in a more steady state and it might actually improve the house warm up and reduce consumption

My favourite pastime is reading "Boiler Installation" manuals Item 3 - Flow temp sensor at the heating block.............. Seems the boiler can be range rated although I'm not sure how low it can be rated - looks like it can't be range rated to min of 5.1 kW but it can be throttled back to around 15 kWh as a max Settings under "heating" menu it seems Also seems like the pump curves can be adjusted too but I think I'd start with capping it's enthusiasm to ramp up to 90%

A few years ago when I was trying to make a glow worm boiler with 24kW rating and 10 kW min heat a house with a 4 kWh heat loss I had an issue with the boiler hanging or faulting out on warm up - the boiler did not like a DT at the boiler greater than 20 Deg C. If it faulted out it required a manual reset when I got up which also meant a cold house - I wasn't popular when this happened. The situation was always worse the colder the outside temps got - it wasn't unusual for the rad/room temps overnight to drop below 16 deg C when we were on scheduled heating slots (one of the reasons I wanted to go 24/7 with weather comp) Boiler flow temp was set to 55 deg C so on start up the flow would quickly get to 55 but with 130 litres of water in the rad circuit the initial return would be 16 Deg C (55 -16 = 39 Deg C delta so you can see why the boiler might not be happy) If it was just over 20 deg C it would modulate down to 10kW min until the DT at the boiler dropped below 20 Deg C at which point it would ramp up to max driving the DT to greater than 20 Deg C and at this point it would hang again dropping down to min 10 kW and then wait while the DT dropped below 20 again where it would rinse and repeat until all the rad were feeding back water lest than 20 deg below the boiler set point. By this point the boiler would then short cycle as the circuit couldn't cope with 10 kWh being fired into it to replace a loss of 4 kWh I did a number of things to limit the impact Range rated the boiler to 10 kW (unfortunately the range rating is ignored on start up so once the house was warm it frequently short cycled) Set a towel rail rad to be fully open on the lockshield - it was in a toilet (so small room and had a TRV so it provided a useful warming of the overall return until the TRV shut it down after 30 mins) by which point the overall circuit was at a temp level that meant the DT at the boiler was less than 20 Deg C Reason for the Glow worm story is I wonder if this differential is driving the modulation behaviour I'd be inclined to actually go the other way - drop the target flow temp and run the heating period for longer and if you can range rate the boiler down I'd do that for sure especially if you know your house heat loss?? PS how is the HW (Hot Water) heated if not by gas boiler?

Good call on that as a potential reason but the system and boiler are relatively new I wouldn't expect it to be clogged up like mine was once

@EinTopaz probably needs to explain this better - right now it reads that the boiler is running for 10 mins and reaching it's target before overshooting I fail to understand how a flow pipe leaving the boiler can be 30 deg lower than the water temp reached inside the boiler - it's likely to be 30-40 cm of pipe max so unless it's going via a blast freezer there is heat going somewhere

10 mins to overheat that's not exactly a short cycle scenario IMO.............. My old glow worm 24kW with a min of 10kW could heat up, overshoot and shut down in less than 4 mins if the house was up to temp and so not a lot of heat transfer going on from the rads. The water leaving the boiler would be same temp as the display said it was!!! Here it seems to be a big disconnect that doesn't make sense

If the boiler is producing 70 deg heat (according to the display) then unless the display is miles out or it's indicating a set point rather than an actual temp out - I would expect a similar temp at the flow pipe - so I'd wonder where the heat is going!!!

They aren't normally too bad for accuracy as long as they are tightly encased in some foam lagging - if just cable tied to pipes they are pretty poor temp indicators

I didn't read it that way either - so is the boiler short cycling? @EinTopaz How has the system been balanced - have you screwed the lock-shields down to get a DT at the rads of 20 Deg C? It's very easy to concentrate on the DT at the rads and forget that the boiler will have a min flow rate required to be happy!!

If this is CH Only then HW is covered by some other system? I'm guessing you have a fairly large house with a high heat loss if you are running 70 deg flow temp with T22 rads and triple columns I've never worked out my T50 rating for my rads as I sized them for a 40 deg flow temp (I'm actually running lower than that) but having done a quick check it seems my 13 rads work out to be 22 kW Do you have a Automatic bypass anywhere on the system - if you do what pressure is it set too?

It's very much house dependent and you have to find the fit that works for your needs, in harmony with the house and of course at a cost that you can afford. I used to heat scheduled - there was a small benefit in terms of energy consumption but at the expense of hot period, cold period, hot period and a house that had internal temps that dropped rapidly when the hot period was stopped Heating 24/7 the fabric of the house stabilises (let's say takes in a chunk of heat and that heat has a initial cost - I was seeing that during weekend heating when CH was on all day) and if the heating is stopped it gives it back and the house loses temp much slower than it previously did (without any insulation changes) I've just started to play with set backs again and I'm finding that I'm getting better results.

Then as @SimonD said the important question is "is the house warm enough" with the current rad temps? Opentherm is talking to the boiler - as outside temps get colder it will increase the flow temps accordingly to maintain the house temp (based on the room stat feedback and set point) Leave ECO and comfort modes alone for the moment. Are you running heating according to a schedule or 24/7?