Hot flow, tepid return (again)

[COYS]

Typically heating misbehaves just as it starts to get cold!
Boiler: Vaillant Eco-Tec 24kw. 8 Rads, 2 towel rails.
Boiler set on max, flow piping hot out of boiler, return cold. With all rads open both sides, next to no heat. Balance rads right down, still only tepid. Operate one at a time, heats up ok.
Flushed & refilled, no apparent blockage as individually each rad gets hot. More than one, barely warm.
Boiler fires for a minute or so but goes back to standby quickly & before return is warm.
Status screen says pump operational & it seems to be running. Blocked or inefficient maybe?
No problem with DHW just the heating side. Should add that system just a couple of years old, all was ok when heating last used around June & no modifications since. Pipe sizes seem ok with 22 copper up to 4.5m from boiler thereafter in 15 hep, very few elbows etc.
At a loss & freezing! Any advice much appreciated. I have had a look through the similar threads without success thus far.

 

[kozak1968]

Has it been commissioned to MI

 

[Centralheatking]

Coys you must be a well versed in Ch to post such a technical explianation of your problem

- maybe the mods can determine if it ok for us to quote help CHK

Typically heating misbehaves just as it starts to get cold!
Boiler: Vaillant Eco-Tec 24kw. 8 Rads, 2 towel rails.
Boiler set on max, flow piping hot out of boiler, return cold. With all rads open both sides, next to no heat. Balance rads right down, still only tepid. Operate one at a time, heats up ok.
Flushed & refilled, no apparent blockage as individually each rad gets hot. More than one, barely warm.
Boiler fires for a minute or so but goes back to standby quickly & before return is warm.
Status screen says pump operational & it seems to be running. Blocked or inefficient maybe?
No problem with DHW just the heating side. Should add that system just a couple of years old, all was ok when heating last used around June & no modifications since. Pipe sizes seem ok with 22 copper up to 4.5m from boiler thereafter in 15 hep, very few elbows etc.
At a loss & freezing! Any advice much appreciated. I have had a look through the similar threads without success thus far.

 

[COYS]

Has it been commissioned to MI

Well versed maybe, but always willing to listen & learn even after 30 years in the building industry.
Yes, of course it has been commissioned. It is originally a BG installation, now out of the guarantee period as system was fitted for the previous owner of my current house. The only modifications being towel rails for rads to the bathrooms up & down, single to double in hall & moving a bedroom rad locally to make room for furniture.
My post was based upon my own knowledge/experience, & the varied advice of colleagues within the trade. 4 heating engineers = 4 differing diagnoses (balancing, blockage, sludge/scale, pump) = problem unsolved. Hence, turn to the fountain of all knowledge (the web) & maybe someone has an alternate opinion.
It's not a massive drama, but I didn't realise it was so closed shop.

 

[chris watkins]

I'd go with your last mates idea, has anyone whipped it out & had a look at it's condition ??

 

[COYS]

I'd go with your last mates idea, has anyone whipped it out & had a look at it's condition ??

Tomorrow hopefully, but it looks favourite so far, as all other touted solutions have been a dead end. We are a major hard water area, aided but not totally cured by Sentinel CombiGuard so it may be a factor on pump output, though to be fair little evidence of build up when flushing or in the scale reducer. Cheers for the input.

 

[Jonwells]

Ecotech it will be a lazy pump, it's hard to picture the system design but if the installation has once worked and nothing has changed I would change the pump 1st.

 

[Gas]

New pump, then balance system and by pass if fitted

 

[kay-jay]

Check system pressure....
Had a couple of jobs presenting similar issues. Changwd pump on first one to no avail. Turned out pressure guage was stuck on 1.5 bar but was not working. Actual pressure way less and giving impression of lazy pump

 

[Gas]

If it is pressure prob on a UF screed job KayJay, suggests a fundamental problem.

 

[kay-jay]

If it is pressure prob on a UF screed job KayJay, suggests a fundamental problem.

Im drunk but cant see where je mentions ufh?

 

[dancinplumba]

I'm bemused

 

[dancinplumba]

I'm bemused

No I'm not.....I'm Dancin ))))

 

[COYS]

Check system pressure....
Had a couple of jobs presenting similar issues. Changwd pump on first one to no avail. Turned out pressure guage was stuck on 1.5 bar but was not working. Actual pressure way less and giving impression of lazy pump

Good shout. Not pump - renewed with little improvement

 

[COYS]

System pressure a good shout.
Not pump - renewed with little improvement.

 

[COYS]

Apologies for duplicate post.
Not pressure either.
Powerflush as well as new pump yesterday. Double checked all TRV pins & lockshields.
Unit still stuck in a sequence thus.
From cold; fires & runs hard, 22 flow heats quickly well beyond first tees @4m+ (suggests carcass ok) Rads start to warm in sequence.
5-10 mins; boiler falls back to standby & starts cooling. Rads not hot, return barely lukewarm. Screen temp falls from 78c to 50c, & so it continues without the rads ever getting more than warm & return tepid at best.
No weather compensator etc, but could it be a rogue sensor causing micro cycle

 

[kay-jay]

Have you got a return valve with a mesh screen in it? Possible partial blockage.

 

[kay-jay]

Faulty thermostat?

 

[Taylor]

Could be as simple as putting some airpips on the drops seen this before plenty of times