Suprima 120 / Megaflo 250: boils & trips out

[jakey_200]

Potterton Suprima 120 / Megaflo 250.. annual dickathon... worse than previous years.. can't keep it alive now... trips out and shuts down.

Trip is real.. preceded by gurgling and sounds of boiling and displaced water out of overflow. PCB is new... boiler stat works.. motorised valves work... pump works...

Any thoughts... interval between episodes between 30 mins and 4 hrs now...

The geyser from the overflow will boil someone alive if it doesn't get fixed!!

 

[ChrisR]

The 120 is a lot of power in a small box. They do tend to overheat. I wouldn't fit one for that reason. Potterton recommend bigger pipes than they used to - must check the M.I. - online now. What size are the pipes just after they come out of the boiler - they need to INCREASE from the connector size.Do you have an automatic bypass valve - brass lump with a knob to set the bypass pressure - will adjust between 1 and 6 .
If you have insulation on the boiler pipes, try taking it off for now.

Is it a sealed system or is there a header tank? Wondering what you mean exactly by "overflow".

Sounds like there's slightly more resistance to flow than there was, maybe the pump's impeller is a little worn.

The boiler stat (adjustable one, which uses a thermostor as sensor) should eb cutting the boiler off before the overheat stat. You could try a new one - only about £15, plus you will need some thermally conductive paste to put on it.

 

[jakey_200]

Pipes from boiler are 28mm - no lagging.

There is no automatic by-pass valve on the pump. There are 3 rads with no TRV Have had room stat and all TRVs set high, to try to keep flowrate up in heating circuit.

There is a 4 gallon expansion (header) tank in the loft. When the boiling occurs, steam displaces water and shoves (a lot) of hot water to the tank and out of the overflow which exits the building at the front.

Boiler stat seems to work.. boiler starts and stops as temp knob is turned up and down... but response is relatively slow.. a few seconds. I have not changed the temp sensor, but I have smeared some more heat transfer paste in the fitting, to try to ensure good thermal contact. Trying to "survive" hoiliday period with poorly heating, have had boiler stat set near minimum - which does extend interval between eruptions - and room stat high.

Whole installation was new 3 years ago. Big house needs a boiler of this output. On the whole system has been good - megaflo hot water is very impressive. Have had the usual catalogue of Potterton PCB misery (now on number 3), but there is something different this time.

Had a visit from "chosen at random from Yellow Pages when I was away" heating engineer, who proclaimed the plumbing and wiring in the airing cupboard is an abortion.

Sounded quite plausible when he was here... he did not think the arrangement with a mid-position 3 port valve plus a 2 port valve on the HW circuit is correct. He proposes to cut and shut the pipes to convert to a 2 port valve for the CH circuit (keeping existing 2 port valve for HW) plus automatic by pass.

After he left, I found plumbing and wiring diagrams in the Megaflo installation guide (diag 7b, pg 10) that correspond exactly to what I have (except that pump is, correctly, controlled by the boiler, not direct from stats and valves. So now I am concerned that we may embark on an expensive re-plumb to get to a layout that the guy recognises.. but which does not fix the problem of the boiling and overflowing..

It seems to me there is insufficient flow at the crucial instant. Like Chris says, Suprima 120 is a lot of power in a small box, so overheating is rapid.

Pump is working, but is only a common or garden size Grundfos (can't remember model number at the mo). Doubt it's worn out, but if spec was always marginal, who knows?

Any tricks for thoroughly bleeding air out.. rads and hot water coil seem ok.. but after all these episodes of boiling I'd not be surprised if there is air somewhere..

Any other possibilities?

Thanks.

 

[breezer]

what if (this is only a suggestion) you put in a bigger pump or even two in series.

in my job i have been in basements (etc) of big houses and i was amazed the first time i saw one, that grundfos make a pump THAT big

i have also seen 3 pumps on 3 pipes, and even a small house with "twin boilers"

but as i said its only a suggestion (i am only saying what i have seen)

 

[DP]

Two shortfalls in the system are as follows.

i) No bypass. If boiler in running, HW and CH demand met, residual heat in the combustion chamber will boil water in the heat exchanger resulting in water and steam from vent. This will be a random thing.

ii) Sluggish control on the boiler appears to be the culprit. One would need to test the boiler to locate the component that is keeping the demand on when in fact the boiler has reached the set thermostatic temperature. Several things spring to mind. Gas valve, control potentiometer, sensor etc. Best to diagnose.

It is possible there is no primary control. Boiler might be controlled by high limit thermostat which is a little too late resulting in 100 degree C plus temperature. High limit is usually 95.

 

[ChrisR]

It shouldn't be overflowing! The level in the F/E tank should be LOW, so that the water can expand and still not overflow. If it overflows when it gets hot, it will take in more water when it cools down. Thios causes constant intake of fresh oxygenated water. This often causes sludge/scale in the feed pipe whee it meets the circuit. That means flow problems.....

I downloaded the MI form potty only to find it only covers up to the 100 Btu. Then found 120's on my shelf. You need to get latest version but I'll quote a couple of bits:
"It is recommended that 35mm pipeworkis used for the flow and return to within approximately 300mm of the boiler" They show 35mm up to pump, with Vent pipe going off in 28 mm and the feed from the f/e recommended 22mm.

Using a 3 port valve on one of these is "unusual"! The have a higher resistance than the same sized 2 port valve. You must keep the valve provided with the HW cyl as it's part of the approved package. . Yes you would be better off replacing the 3 port with a single 28mm 2 port for the heating.

Sounds like your bypass arrangement should be OK.
Pumps bigger than a 15/60 do make a noise. Yes Grundfoss et al make some seriously big pumps!

Potty's requirement of 300mm of 35mm sounds a bit odd - especially when the boiler conns are 28mm, does it not. Possibly it's an afterthought to get rid of a slug of very hot water quickly. But if that's the case I don't see why they bother to put it on the return pipe for 300mm. Bright ideas anyone?

just off to check pumps..

"This is a high putput boiler requiring a pump producing a high water flow rate ...."
"Most commonly used domestic circulator pumps do not produce the performance required, therefore a higher duty/light commercial pump should be used"

"The boiler resuiresa 22mm bypass for correct operation of the pump overrun, The total length of the by-pass circuit taken from the boiler connections should be greater than 4 metres. It is recommended that the bypass circuit should eb fitted with an Automoatic Bypass valve"
Alternative bypass arangement shown uses bathroom radiator connected across boiler, after the pump , fitted with 2 lockshield valves so it can't be turned off

 

[jakey_200]

Thanks DP.

Re no by pass, the boiler keeps the pump running for several minutes after the gas is switched off, to protect against boiling from residual heat. There are 3 rads with no TRV, so heating circuit is never closed. There does appear to be the risk of this happening in the "CH off, HW on, HW temp demand satisfied" state, when the 3 port valve is "HW only" and the 2 port valve in the HW loop is "closed". When in this state, the pump is dead headed when the boiler runs the pump on after the heat turns off.


During the "trouble", I have run the system CH only, both with valves operating and with motor head removed and 3 port valve set manually to CH only.. in both cases the boiling still occurs.

So I am inclined to favour the suggestions about boiler temp control.

What does anybody think about the proposal to change from 3 port valve to 2 port valve for CH loop?

Thanks all.

 

[ChrisR]

Just checked pump graph -

Head available is only about 2.5 metres with a 15/60 which is probably what you have, at the flow rate you need (45 l/min). Checking boiler instrs - loss across boiler is 1m at that rate. So you only have 1.5 metres head available to pump the whole of the heating system. All motorised valves are prety restrictive as they have a ball or similar inside which just moves off the end of a pipe... The three port ones leave a very small hole so I'd change it.

Simple and cheap is to comply with the instructions ( ! ) and change some pipe for 35mm.

You need to repipe or connect an ABV so that there is circulation for the pump overrun.

You need to check the level of water in the f/e tank is right. This is critical. Overheating boilers scale up! Constant supply of fresh water means they do it much quicker.

You need to check for scale at the feed/circuit junction, if the level in the f/e has been too high, as explained above. Simple matter to cut the pipe out if you can get at it.

Boilers always overshoot the temp set on the boiler stat - whether it gets to the cut-out stat temp, or boiling, is a matter for the flow rate. Yours is overshooting so radically that even the overheat stat isn't stopping it boiling. If the boiler spends more than a few seconds boiling then you need a new overheat stat!

It's quite possible that not havng a proper bypass has caused sufficient scaling and flow restriction that it now boils in steady ch operation.

This is just another case of boiler installation instructions being ignored - the boiler gets the blame instead of the prat who did it![/img]

 

[jakey_200]

Thanks for all the legwork ChrisR.

I agree with your sentiments about prats that ignore installation instructions.. makes life hard for the rest of us. In this case, wish I had bought a big cold house with no heating, then could have specified something sensible and would have done the homework pre-installation rather than post-failure! Anyway, c'est la vie - the house came with radiators and water!

The F/E tank level is ok.. it can cope with normal expansion and contraction of water in system with no overflow. Overflow only happens when boiling occurs. With system at working temp, level in F/E is at its highest (no overflow though) so when water arrives pushed along by the steam, level rises and then overflow happens.

Boiling does not happen with system steady state.. it does only last a few seconds.. so seems boiler over temp stat is OK.

Can't realistically rework to comply with latest Potterton instructions without ripping entire CH pipework and starting again. So objective must be to get as close as I can.

Based on ChrisR's comments and manufacturer instructions for all the ingredients (which I now have - ain't the internet a wonderful thing..) I propose the following plan:

1. Ditch the 3-port valve.
2. Fit a 2-port valve for the CH circuit plus an auto by-pass valve.
3. Fit a bigger pump.
4. Check for scale at feed/circuit junction - renew pipes locally.
5. Renew thermistor and setting pot for boiler temp control (rest of controller lives on pcb that is new anyway).
6. Re-check level in F/E tank (reset ball-cock if needed so level is low when system is cold). Ensure no overflow or system refill in normal heat-cool cycle.

Any more for any more?

Thanks again,

 

[ChrisR]

Fair enough - pity you can't get at the flo pipe at least - but I know what you mean about access, you're lucky you can get to the others. If it was OK when new, or more OK , then it's worth a try.

The resistance of the pot which adjusts the temp should be stamped on it. Have seen cracks on pcb mounted ones so it's worth checking. The sensors do "go" and don't cost a lot.

If those don't fix the problem (!) then I'd do the rest - but leave the pump til last, because the bigger ones do make a racket which might be a nuisance.

It would be nice to know what the outcome is.

ANd where did you get the right instructions from? Make sure they're for the 120!

 

[jakey_200]

Yippee.. no more eruptions.

Posting this is final stage of exhaustive test procedure; designed to provoke more trouble by claiming to have fixed it!

£6.88 for new thermistor (plus a tenner for the new pot I bought but don't need) lovingly fitted with a blob of thermal paste and all has been quiet for the whole afternoon. Water is hot, radiators are toasty, pump is quiet - no sign of any boiling anywhere. RESULT!

Not 100% convinced yet, but boiler is behaving normally and nothing bad has happened. Have just increased set point for boiler stat and started to put TRVs back to their normal settings. Yesterday this would have caused system to boil within a few mins. Fingers crossed!

So I think I found the explanation for the total carnage of the last few weeks; there was indeed a failed part! But all the stuff about the poor installation is still true, so I will continue with the rest of the plan... I know whats needed for 2 port and bypass valves, so next step is to identify the biggest pump that is interchangeable with the Grundfos 15-50 that I have... best I have found so far is Grundfos 18-65. Is there a bigger one still without changing from 130 mm length and 1.5" bsp fittings? Can anybody give me a leg-up through the pump catalogues?

Thanks...

PS Thanks again ChrisR. Boiler instructions came in the post from Potterton so are kosher for 120 - but internet was great for valves, pump, this forum...

 

[ChrisR]

Glad it turned out to be simple. Whenever I've needed a bigger pump thyan a 15-60 it has come in the boiler (system boiler) or I've used two boilers & 2 pumps! I'm surprised yours was a 15-50! The 15-60 gives you about 1/3rd more available head at the flow you need. I think that might give you enough, without the pipe alterations!

Have you hot a resistance meter - if so could you measure the R of the old sensor (room temp) ? Wouldn't blame you for having drop kicked it though!