DHW HE or DHW Thermister

[GH blackpool]

Hi guys, quick recap of my issue.

Ravenheat RSF82ET, currently 8/9yrs old. Heat Exchanger replaced 2 years ago due to kettling - not sure actual underlying cause, but also had DHW symptom of hot water, cold water, hot water, cold water etc.

Current problem:

Hot water for say 30 secs, then turns off the burner for 10 secs, fires up and repeats. You can tell that it's going to do it as the water goes scalding hot before it cuts off, despite the HW contol being set to low.

HOWEVER - this is intermittent. For example yesterday it did this first thing - scalding hot then a cut out. But later in the day, it worked as normal - ie not the hottest water in the world but reflecitve of the setting and hot enough for washing etc - and more importantly - constant.

In my limited knowledge, I'm thinking either HE knackered again (just by association with my last major prob), or could it be the HW thermister. For info, I've unplugged the thermister, but same thing has happened - scalding hot then a cut out.

Flow etc is unaffected. Problem defintely increases in regularity if I try turning the HW control higher.

Thanks!

 

[gas4you]

Try replacing the ch thermistor. These control the overheat function as well and are notorious for going on these boilers.

 

[GH blackpool]

Just to clarify, ch is fine and dandy. Would removing both thermisters be an adequate test ie neither is connected so in theory the overheat control isn't there?

 

[gas4you]

Just to clarify, ch is fine and dandy.

So it could be.

Try swapping the thermistors around. This may help to prove something

 

[GH blackpool]

Right then, I'm not keen (nor qualified) on messing about with these things, but a bit more advice would be appreciated in respect of next action.

Had a friend out, and he said it was the HE. I've no reason at all to doubt him and he's a good guy, only thing is, I'm trying to get my head around this and I'm not (yet) quite convinced - but then I'm not a heating engineer!!!

When heating is set above the minimal level (and higher than the hot water setting which is always set to lowish - this is plenty hot enough), the hot cold hot cold cycle doesn't happen. When I turn the heating off, or right down to below the normal minimal setting, it does. The HW goes scalding then the burner cuts out.

I'm trying to logically connect this to the CH thermister, in hope more than anything else What you think?

 

[GH blackpool]

To explain my limited logic, I'm thinking that for some reason the CH thermister, when the heating is off or especially low, is butting in and cutting out the burner for the HW.

Then again though, why doesn't it do this when the heating is at it's normal setting, and why does it let the water get scalding before cutting out if this is the case....?

I think my theory is massively flawed

 

[Agile]

If it was the HE that would be a constant problem.

There is some other problem!

For a start is the hot water being brought on by turning on a tap when the heating is off?

Get a new thermistor and plug it onto the hot water cable and leave it dangling and then run the hot water and tell us what the boiler does.

If its still the same then replace the system thermister in the same way.

DO NOT leave the boiler operating with either thermister connected this way. This is for test purposes only!

Tony

 

[GH blackpool]

If it was the HE that would be a constant problem.

There is some other problem!

For a start is the hot water being brought on by turning on a tap when the heating is off?

Indeed it is Agile, but as I say, on the few times I've tested it this way (heating is on more or less constant it being winter and all ), I get scalding hot water then the burner switches off for a short time.

Right then, go on, I'm feeling brave.

Can you give me a quick idea on how to change the thermisters?

Is it just a case of twisting them off (unscrewing) with spanner/wrench/pliers?

Does the boiler need draining first, or switched off at mains? Also, are there special types for a ravenheat and do they differ depending on whether they're for the HW / CH side?

Also, one last question - how much can I knacker the boilder up good and proper if following your clear instructions?

 

[weareleeds]

Phone ravenheat & ask for a dry pocket thermister (they are available) just connect wires & clip onto pipe between u/s thermister & o/h stat also change dhw thermister in usual way

 

[GH blackpool]

Thanks

Any chance of a quick recap on changing the thermisters? Presume I power off at mains, and empty the boiler via the PRV?

They just unscrew off do they once I've disconnected the wires going into each one?

 

[GH blackpool]

Forgot to say also, that apparently my friend checked the ch thermistor with one of those things you techy guys have that check for, well...resistance and ohms and the like. Apparently it was ok.

Agile - knowing this, should I still do what you said, and can you just tell me how to do the replacements if so.

Many thanks to all. So far this is the difference between, what, £20 and an awful lot more.

 

[clarkerothwell]

are you in a hard water area?

Why was last heat exchanger changed? was it your DHW heat exchanger?

Best bet is get a new thermister (both heating and DHW are the same) and dont fit but connect it up dry to the heating one first and try your water. If same probs then try the water one, if same probs then its prob Heat exchanger and over heat stat cutting boiler off.

 

[gasmick2]

hi some technical informaton aquired a while ago on a course i attended
at Ravenheat " if the C/HEAT NTC is faulty then only the pump will run, but if the DHW NTC is faulty it will NOT cause the boiler to fail

*** quote If the DHW NTC fails then HW is still provided but NO modulation of the gas valve takes place and the water temp is higher with more noise***
Which will account for the very hot water as you say even with the temp sat turned down low
so it does look like the DH thermister has gone faulty

hope this helps mick

by the way this information applies to all ravenheat boilers as RSF84E/84ET .....100E/100ET.... 82E/82ET....CSI 85E/CSI85ET

 

[mario500]

Gasmick - very useful information there - will put it in the grey matter - or maybe write it down lol



Al

 

[GH blackpool]

Good stuff thanks fellas

Also tends to do it if the HW is set anything from midway between middle and high, regardless of whether heating on. Suggestive of HE being the prob?

What is meant by fitting the thermister dry? Just plugging a new one onto the lead and leaving the old one insitu?

Area isn't particularly hard acording to United Utilities. Unsure exact prob with last HE, but similar symptoms. Albeit that HE would have been approx 6yrs old.