"Main" (Baxi) 24HE A Heat only boiler temperature setting

[Ric2013]

Hi. Bored and wondering about a friend's boiler that was fitted a few years ago and that, having been involved with the heating system, I take an interest in. One of these jewels:

Document

baxi-public.partsarena.com

As it was a friend's house, I went there after and cleared some post-installation airlocks and checked the approximate balancing. It did seem to be around DT 20, but I couldn't get it accurate as the boiler kept cycling. In fact, the boiler cycles a lot and with a lot of hysterisis. I've now been online and read the instructions yet again and worked out where the temperature adjuster is (well-hidden in the Commissioning section of the manual).

As it is in a friend's house and I have access, I'd like to look at this again really when I finally get back to the UK. With a 9.14-10.1kW minimum output, with all radiators on (estimated outputs total = 16kW) and TRVs to max, the boiler should be well within its range so I'm not sure why it's suffering 10°C or more hysterisis (IIRC) on its flow temperature and cycling instead of modulating to keep a steady flow temperature. In my experience, even zombie boilers can keep within a degree or two so long as they aren't massively oversized, and they can't modulate at all.

Is it just a case of not being a very good boiler? I'd have thought the boiler should manage 16kW of radiators without cycling, at least with all TRVs fully open, but it just seems to be running through the Section 3 Operation Cycle as if it is unable to keep the flow temperature down via modulation. Or should I be suggesting my friends get an RGI in to check the boiler does not have a fault with its controls?

The manual's a bit weird as it states that the boiler responds to reaching set temperature by switching off for 3 minutes, yet elsewhere it states 'the boiler Flow temperature is controlled by varying the fan speed (and thereby the gas rate) to achieve optimum operation', which suggests the boiler should stay just below the set temperature.

Any thoughts appreciated.

 

[John.g]

Based on this statement, wonder if the 24HE modulates, if not it will cycle with a anti cycle time of 3 minutes or if it does modulate and its min output is greater than the heating load then it will do the same but not as often.
in modulating mode all boilers will maintain the SP temperature and will cut out at the SP+5C if min output still too great, and cut back in at SP-5C but in not less than the anti cycle time.

NOTE: The 12,15,18 & 24 are self-regulating dependent upon the system load. The 30 will modulate between inputs of 33.76kW and 10.3kW. The 30 input is factory set at 24.5kW and can be altered to 33.76kW - see section 10.8. No adjustment of the gas valve is permissible.

 

[Ric2013]

Hi John. Thanks, but I'd seen that bit.

My point is this:

'the boiler Flow temperature is controlled by varying the fan speed (and thereby the gas rate) to achieve optimum operation'

The boiler is a 24 model (not a 30) with a 9.14-10.1kW minimum output and is running 16kW of radiators. In theory, therefore, it should modulate, so I've half answered my own question lol.

Anyone have any personal experience of these boilers? I'm just wondering if there could be a system-side issue that is confusing the boiler controls. Perhaps the external bypass is letting by?

 

[John.g]

Some boilers make a awful meal of modulating down before the flow temp exceeds SP+5C except that they are range rated so reducing from 24kw to a max of 16kw may help, also on ignition they fire up at 70/80% of full output (irrespective of any range rating) and some hold this setting for 60 secs or so which doesn't help either.
The bypass should definitely be looked at first though.

 

[Ric2013]

Yeah, I think that's the way forward as I have a feeling the bypass has issues.

Sounds like this boiler cannot be range rated though (?). By an awful meal, I take it you mean they take a long time to make the decision and sometimes they have to cycle before they decide to reduce the output?

 

[John.g]

A RGI may be able (on advice) to range rate it.
The "awful meal" is IMO to do with the controllers, I have watched a relations Vokera 20S firing from cold with a heating load ~ 6kw (or less) and the flow temperature rises very rapidly initially but you can hear the fan slowing down well before it reaches the SP temperature and it will only overshoot its SP by 2C normally before settling back down to the SP, even when it re fires (3 min anti cycle time) after cutting out due to heating load < min output, it still doesn't overshoot by more than 3C but will eventually cut out at SP+5C if minimum output too high.