Ravenheat RSF84 - DHW goes hot and cold

[richw]

Hi all,

I've got a Ravenheat RSF84ET combination boiler. It was installed by the previous owner of the house (I've been in 2.5 years). For the first year I used it, the boiler worked great, but since then the DHW has been on the blink. The CH works perfectly.

If I open a hot tap, the boiler fires up. The gas increases to the full rate (big flame) and within 30-40 seconds, I have hot water at the tap. Then it starts to get very hot, followed by a slight hissing sound from the boiler (perhaps because the water is getting too hot?), then the flame dies completely, so I get cold water. After 10 seconds or so, the flame fires up again and I get hot, then very hot water. Shortly there will be hissing, and the cycle continues.

A Corgi engineer checked the min/max gas pressure for me, but then tried to sell me a new boiler, so he's out of the picture.

I've checked that the gas valve is able to modulate correctly by connecting a variable power supply across the modulator terminals. By changing the voltage, the flame will vary in size.

I've checked the air pressure switch. Blowing/sucking on the tube causes the switch to click on/off.

I've checked the diverter valve. Running the hot tap always makes the boiler respond, and the CH pipes do not get hot. When the heating is turned on, they do get hot as you would expect.

I've replaced the HW and CH thermistors. Ravenheat now supply 'dry' (clip-on) sensors instead of the 'wet' (inserted into the flow) ones so this was easy enough to do.

I'm completely stuck with what to try next. The boiler looks so simple, so what else is there to fail?!

I'm in an area with nice soft water, so I can't imagine any of those nasty 'scaling up' problems would apply. Does that make sense?


Many thanks,
Richard.

 

[wizbongre]

Hi,

Not sure if either of these help but here you go:-

//www.diynot.com/forums/viewtopic.php?t=65081&highlight=

//www.diynot.com/forums/viewtopic.php?t=73798&highlight=

Good luck!

Gareth.

 

[richw]

Hi Gareth,

Yeah, I forgot to add that I've read pretty much every Ravenheat thread going. I'm still searching and re-reading them though!

It sounds like I have the same HW problem you had (my flue is OK, though!). I've measured the voltage across the CH and DHW thermistors and it does decrease as the water temperature increases. I really can't believe the HW potentiometer is faulty, as that is such a simple thing!

When the boiler first broke, but before I replaced the thermistors, I did try messing with the HW potentiometer (it was on max). It seemed to enable the boiler to run hot for longer before cutting out, but not completely fix the problem.

I'm now re-testing this theory. So far, I tried the dial at the 12 o'clock position, and found that the boiler didn't cut out but the water wasn't very hot! I think the temperature still varied though - it wasn't constant. I'll keep moving the dial and trying it.

Thanks for your comments.

Does anyone know a sure-fire way to test the potentiometers? Are there particular voltages I can look out for as I turn it from min to max? (just wondering if it's bust)

 

[Agile]

It sounds like the symptome of a blocked plate heat exchanger.

However I think your boiler has a single double purpose HE. Perhaps its a similar effect in yours due to dirt?

Its less common on your type because the water passages are larger and dont block so easily.

It was a common problem on older Radiant boilers though.

It needs a thermal survey to confirm!

Tony

 

[richw]

Hi Agile,

Thanks for your reply. Yes, I believe the boiler does have such a heat exchanger. The manual calls it a 'double parallel heat exchanger'.

How would I go about proving that the HE is blocked? Are there any simple fixes such as putting some chemicals into the CH side?

Or do I need an engineer to actually drain-down and remove the HE from the boiler? (which they presumably then clean out or replace)

If it's an engineer job, can anyone recommend someone in the Teesside area? One engineer has already looked at it, but he was useless.

 

[Slugbabydotcom]

Phone Ravenheat and they will give you my number

 

[corgiman]

Phone Ravenheat and they will give you my number

awww you are recommended by ravenheat, what evil did you do in a past life to deserve that mate

 

[donametchi]

try turning down the flow of water entering boiler(black knob on right hand side found underneath the boiler) .

 

[Rob]

I've checked the diverter valve.


Many thanks,
Richard.

If your boiler has a diverter valve then it must have a plate heat exchanger

 

[Agile]

Not so Rob!

Also think about the Potty Puma! That has a diverter valve and a four pipe main HE!

Slug must have said something nice about Ravenheats tio John Wilson or even Louis!!!

Tony

 

[Slugbabydotcom]

Phone Ravenheat and they will give you my number

awww you are recommended by ravenheat, what evil did you do in a past life to deserve that mate

I was going to tell you what my hypnotherapist said to me but its too sick. Lets just say I was in charge of diesel engines until I was shot by the Russians in 1945

Mock if you must but I find that wandering about under the smog munching parmo's and throwing thermistors by the bucketfull into these things is an enjoyable pastime.

I've checked the diverter valve.


Many thanks,
Richard.

If your boiler has a diverter valve then it must have a plate heat exchanger

It does have a diverter valve and doesn't have a plate heat exchanger

try turning down the flow of water entering boiler(black knob on right hand side found underneath the boiler) icon_lol.gif

You can adjust the flow rate at the DV.

Whats your flow rate at the hot tap?

Slug must have said something nice about Ravenheats to John Wilson or even Louis!!!

It took me ages to get the brown stain off my tongue but with the jobs they pass me I can now afford plenty of mouthwash
Did you ever manage to get on their list Tony?

 

[Rob]

Not so Rob!

Also think about the Potty Puma! That has a diverter valve and a four pipe main HE!

Because these use the Gianonni type with diaphragm flow switch.. my mistake

It does have a diverter valve and doesn't have a plate heat exchanger [quote/]

 

[richw]

Phone Ravenheat and they will give you my number

Funnily enough, I did approach Ravenheat, and after telling them I'm in TS20, they gave me a name/number. Unfortunately, I'd just had a bad experience with another engineer, so I was too scared to try this one!

Are you 'FCA', and your last two phone number digits '61'? If so, I think I'll give you a bell on Monday! Failing that, I'm not sure if Ravenheat will help me when I ask for 'slugbabydotcom'

 

[richw]

Mock if you must but I find that wandering about under the smog munching parmo's and throwing thermistors by the bucketfull into these things is an enjoyable pastime.

I suppose I'm in that part of the world, but honest, I've never had a parmo!

You can adjust the flow rate at the DV. Whats your flow rate at the hot tap?

I was wondering about the flow rate adjuster, but the thing that put me off is that it was all working OK (once upon a time!). I have not yet measured the flow rate with a bucket, but the cold and hot come out pretty fast.

Before the HW went iffy, it could really chuck out hot water. I thought that this cheapo boiler was amazing! Not it comes out of the tap at the same rate, but the temperature varies (and it cuts out!).

 

[Slugbabydotcom]

Are you 'FCA', and your last two phone number digits '61'? If so, I think I'll give you a bell on Monday! Failing that, I'm not sure if Ravenheat will help me when I ask for 'slugbabydotcom'

Yep that's me. You can catch me in on that number between 11 and 12.30 ish

I'd just had a bad experience with another engineer, so I was too scared to try this one!

Don't worry. It's fixing boilers that makes me tick. I don't charge a penny until you are happy that it is fixed.