Erratic DHW on Savio 424s

[steviebobs]

HI again all.

I have had some great support from this site in the past so here's hoping all the good guys are still around (Tony Agile etc !!)

MY thermo shower (Grohe) has recently been delivering varying temperature HW, going from hot to cold and back within seconds. At first suspected the cartridge in the shower, but before buying an expensive new one decided to do some boiler 'tests'.

When the shower was running I felt the DHW flow pipe from the boiler, it indeed went hot to cold and back, switching boiler off and on again each time. I still suspected the shower thermo was closing down the hot flow and this was telling the boiler there was no demand.

BUT, I then tried running my kitchen hot tap, and the same happened, boiler off and on and off etc, so now suspect something like the DHW thermo in the boiler, or sadly, a blocked heat exchanger.

Worked like a dream for a long time (since 1999), with only one having to get the diverter cleaned. Any suggestions? Am I on the right track or miles away?

SB

 

[Agile]

Unfortunately I am still here!

Yes, it does sound as if the plate heat exchanger is blocked. You could confirm that because the flow and return temperatures will be more than about 15-20° different. You will probably be able to touch the return but the flow will be too hot!

There are lots of posts about which tell you how to clean or replace it yourself.

If you are going to replace it yourself you can buy the part for a Biasi Prisma 24S as its the same one. No one will know about a Savio!

Tony

 

[steviebobs]

Hi Tony. Pleased you are still about.

Ill give it a go - sounds good. I have no issues with a boiler replacement, this owes me nothing after nearly 10 years, but I actually like it. It has been a good and faithful servant !!

Steve

 

[steviebobs]

Tony

Problem getting to flow return pipes on DHW HE, need to try again from below. But I do have another symptom. My overflow is dripping - Pressure had dropped so low and I just had to 'pressure up' in order to get boiler to fire up again.

Does this fit with the blocked DHW HE diagnosis or have I unearthed another monster? I noticed earlier that if I reduced the DHW temp (via stat) then it seemed to work ok and not go off and on.

S

 

[steviebobs]

Tony - UPDATE

Two items appear un-related, I have had a go at the PRV (on/off/on/off etc) and the dripping has stopped.

Also it is a 428s not 424s, but dont think that unduly affects diagnosis, just a different DHW HE if I need to replace.

Steve

 

[steviebobs]

To confirm. 24 hours no drips from PRV, so that sorted.

Stilll getting fluctuations in DHW temp, but less so when DHW temp control reduced. Going away for a few days so will remove DHW HE on return and clean/replace.

Incidentally I am based in the NW with quite soft water, do the HE's still scale up, or just corrode internally after 9 years...

Original boiler blurb suggested that it wouldnt scale due to 'controlled internal temp of less than 75 degs....'

S

 

[steviebobs]

HI all

Sorry for delay in replying but wanted to get into the throes of Summer before taking the boiler down.

My last reply was in response to Tony Agile who agreed that it did indeed look like it was the plate heat exchanger that was blocked causing the on/off cycle of burner during hot water draw-off.

I have now taken said exchanger off and have to say it seems as clean as a whistle ! A power hose had no problem with flow and there appeared to be no restriction there.

On refit, the same problem re-appeared.

There appears to be no change if I open the hot tap less (reduced flow) or alter the DHW temp knob from low to medium to high. Burner is 6 seconds on, 6 seconds off, regular as clockwork. Have to admit have lived with this with no discomfort for quite some time but he pedant in me seeks a fix.

More thoughts?

Thanks for reading

Steve

 

[Agile]

Firstly you cannot always see anything inside the plate to lessen the heat transfer. Its best to clean it anyway with a chemical. HCl is best if used carefully on both sides at about 8% and clod.

The 6 sec on/off would not usually transfer into the hot water because the thermal time constant would be too long.

It sounds as if its a flame detection or fan/air problem.

But in that case it would usually do that on CH as well. Does it?

I may have to be a little careful as some of these aspects are related to gas work and I cannot give you that information on an open forum.

Tony

 

[steviebobs]

Tony - wow what a fast response (as usual)

It doesn't affect the CH at all - thats great, just DHW. Re the cycle, yes I agree as it is only six seconds theres no real detection of water temp change at tap. I do have to say that I am sure when this started, the cycle was longer than 6 seconds as there was a noticable change in temp detected in the shower, making me think originally that it was the shower stat (Grohe) but found it did same on kitchen DHW etc.

Understand and appreciate your comment re gas work.

May leave well alone as it doesnt cause any discomfort, as I said earlier, it is probably the pedant in me wanting to solve lifes latest problem !!

As an aside, in the past we discussed the risc pot. Is this at the minimum when at clock or anticlock end of its travel?

Regards again and appreciate your assistance.

Steve

Steve

 

[Agile]

There are a couple of things you can do.

First see what the temperatures on flow and return pipes when on DHW. You can just feel with your hands. The return should be a bit lower than the flow but usually too hot to touch. About 60/45 C for example.

When the plate is blocked or flow restricted the temperature differential rises.

Poor DHW can also be caused by a restricted flow. Look in FAQ on this site for the finger test on the pump if you want to try that. It also gives a too high differential on flow and return but in that case the cause is different.

You can also substitute a 10k resistor on test for the DHW sensor in case its failing. That would keep the burner on max.

If all those are OK then it could be the other aspects!

Tony

 

[steviebobs]

Thanks Tony

Bit of homework - will do over next few days and report back.

Best Wishes

Steve

 

[steviebobs]

Tony ( and spectators !)

Have had a go at a few things you mention.

Simple one first - subbed 10K for sensor and symptoms the same - suggesting the DHW probe is actually ok. Reconnected.

Unsure which are the flow/return pipes in order to identify differential. Boiler is in tight location and cant get to rear.

Looking from the front of the boiler, the DHW HE has 4 ins/outs. Two left (top and bottom) and two right (top and bottom). Im not sure which is which, with regard to cold feed in/hot feed out and main HE flow and return?

Unable to find FAQ with details of pump 'finger test'. Can you point me to it?

Hoping it isnt one of the 'other things' (which I guess refers to 'gas issues' and out of my remit) I am wondering whether to take a leap of faith and just replace the DHW HE. £50 + delivery seems fine. But do have to emphasise that when off the boiler, the DHW HE seemed to offer little restriction to a hose attached and flow seemed fine to my untrained eyes. A little black crap but no more, my area is very soft water so dont expect too much scaling.

Regards and thanks for your time as always.

Steve

 

[Agile]

On the plate HE the top connections are the boiler primary circuit. left for flow and right for return. There should be a noticeable difference but the return should not be cool! About 60/45°C as typical.

The lower are the DHW and inlet on right and outlet on left. About 45/12 ° at this time of year.

I am not at all sure the problem is the plate until I have further details of the temperatures involved round the plate.

An increased differential is a sure sign of a reduced heat transfer.

Tony

 

[steviebobs]

Well Tony

After expriencing a 'senior moment' (which lasted a few days...) I can report back on the differential between flow and return. I was previously puzzling how to get at flow and return on the plate until finally realizing that the flow and return from bolier and above pump are effectively the same)

Anyway, with the hot tap fully open (delivers 10L per min) I can report that the temp of the flow pipe from the main heat exchanger is high and the pipe 'unholdable'. The pipe/union nut above the pump returning to the main heat exchanger is warm, and easily holdable.

This, I think, highlights reduced flow and hence the Plate HE. So even though there seemed to be adequate flow thru Plate when off the boiler using a hose, it is ovbiously restricting the flow to an extent enough to trigger the DHW temp probe.

Sounds like I bite the bullet and buy a new Plate?

Regards

Steve

 

[Agile]

That does sound as if the plate is restricting thermal transfer.

That could be the primary or secondary side.

They can be cleaned by chemicals. Ideally HCl for the secondary and citric acid for the primary although HCl will also work.

Use about 8-10% solution cold and take the appropriate precautions and only if competent. It needs considerable rocking to ensure the acid gets all round the plates. It may need several applications of acid and if badly blocked a lower strenth will be needed to revent excessive fizzing.

Bubbling will indicate that lime or oxides are being dissolved.

On refitting feel the flow and return pipes again and tell us what you find.

Tony