Sabre (Vokera) HE 29 fluctuating DHW temp CH early shutdown

Joined
2 Feb 2013
Messages
3
Reaction score
0
Location
Essex
Country
United Kingdom
Hi guys
Have had the boiler installed for 6 years, and been problem free until now.
When demand for DHW is called for, the boiler fires up as per normal, and delivers some hot water, but after a short period the boiler shutsdown with unheated water being delivered. This cycles on and off every 30 seconds or so. A bath is therefore a non event as the fill can never be hot enough. i have wound back the DHW temp control and this seems to help, but only a little.
Also, when a heat demand is placed on the central heating, the boiler fires up as required, but cuts out before the desired temperature is reached. It then cycles for a time until the set temp is reached with 3 minutes dwell time between firing up. I have read in the manual that when the primary thermistor see's 6 degrees above the set temperature that it will shut down the boil and instigate a 3 minute anti cycle. After three mins the boiler will fire again if temp has dropped. So the three minute delay makes sense, but why is it overheating, limescale?
I am led to believe by others that the issue could be either heat exchanger / diverter valve / thermistor related.
Any ideas on what to tackle first?
I am an electrical engineer so can check operation of the thermistor (I guess)!!
Currently running some F3 through the system before draining down and refilling with water + F1. Now would be the time i guess to pull out the secondary heat exchanger (DHW) for a flush.
 
Sponsored Links
I cannot give you a definitive cause as you have not given any temperatures.

There could be a gas setting/fault which we dont advise on here.

If the CH reaches the set temp too quickly then the CH power output is set too high. There should be a pot on the pcb which the nupty installer probably left at max. You should turn that down to just above min and see how that manages. I always think the makers should sent them out at min so the nupty installer has to turn it up!

The hot water fault seeems to indicate that the boiler is not modulating. That could be a faulty or scaled up NTC. Or a faulty PCB or a faulty/unadjusted gas setting.

Of course it could be just one fault preventing modulation affecting both hot water and CH.

F3 seems pretty weak! I use it on clean systems!

Tony Glazier
 
Thanks Tony,

Agree with your gas comments, the boiler is coming up for a service in the spring, but wanted to get an idea on the fault and whether an easy remedy could be applied before the service.

I put two bottles of F3 in, would you suggest anything stronger for a 6 year old system clean out?

Referring to your comments, when you say the CH is reaching set temp too fast, you are referring to the boiler CH water temp setting? That would make sense if the boiler shuts down while the room stat is still calling for heat. I must admit, I haven't adjusted it since install, but probably haven't noticed the boiler shutting down early as the DHW fault wasn't occurring then!

Will investigate the NTC as a first step to see if that is affecting the DHW issue, and get a replacement to hand before dismantling. OK to leave the F3 in for a week if required?
 
F3 is pretty weak. F5 is better. Sentinel X800 is best as a non acid cleanser. They are chellating agents and dont dissolve anything.

But you seem to be panicking and throwing chemicals at it without know what if anything needs cleaning. Different chemicals are required according to what dirt may need clearing.

The plate HE might well be blocked and that needs removing if it needs to be dealt with.

But the flow and return temps need to be measured to diagnose that problem.

Tony
 
Sponsored Links
thanks Tony,

no panic, just hadn't cleaned the system for a few years so running the cleaner through before refilling with F1.

Will try to get the flow / return temps later for completeness. Not as concerned with the CH issue, more concerned with the DHW cycling. As you say the two issues may be related and could simply be a scaled up NTC.

More later once I have the temps.

Paul
 
A scaled up NTC does not affect CH as the time constants are so much longer.

In any case there are probably seperate NTCs for CH and DHW.

Sealed systems done need regular cleaning.

Tony
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top