Potterton Suprima 100, new cylinder and lots of issues!

[watertight]

Hello all,

Sorry in advance for the long post, but I hope a lot of detail will help you help me! I'd appreciate some assistance from you experienced guys with a persistent problem relating to a heating system I've been working on for a customer.

I recently fitted a indirect cylinder for a customer after their original one split, a relatively easy job however there seem to have been a number of issues with the heating system since I installed it.

I'll give you a little background information. Their system comprises of:

- Potterton Suprima 100
- Vented direct cylinder, controlled by a two port valve.
- 2 Heating zones, upstairs and downstairs, each controlled by a two port valve.
- 2 Room stats, one for each heating zone.
- 1 FP715 programmer controlling upstairs heating and DHW.
- 1 TS715-si controlling downstairs heating.
- 1 Frost Stat
- 1 Grundfos pump.
- The central heating system is sealed.

Issues started soon after the installation, upon refilling the system after replacing the cylinder and firing the system I noticed that the boiler kept locking out, not on a regular basis but maybe 2-3 times a day, with the red light on the boiler flashing 5 times in 10 seconds, this could be reset after about 10 minutes so obviously it's an overheat fault.

During the initial few days of it failing I took air out of the system on a number of occasions and topped up accordingly, however the fault continued. At the same time the heat exchanger of the boiler developed a leak and had to be exchanged. I'm not Gas Safe so this was carried out by another engineer.

The boiler is 18 years old and the engineer explained to the owner that it was probably due to wear and tear. The original overheat fault continued. At this point the homeowner called the Gas Safe engineer in and he checked the two thermostats and swapped the PCB with a spare he had that worked, however the problem of it locking out persists.

On top of all this the frost stat shorted and kept tripping the heating's electrical circuit, easy to diagnose and replace so this is now working.

As the system was working without issue before I replaced the cylinder I feel obliged (quite rightly) to sort the issue, the problem is I'm stuck now. I've electrically tested the pump, valves, cylinder stat and room stats but the boiler still locks out on a irregular basis, still 2-3 times a day.

The homeowner has gone on holiday for a couple of weeks so it's given me a chance to work on it and get it sorted.

The only other thing to mention is that I had the system working on 'AUTO' for a couple of days with the room stats turned to about 12 degrees, as soon as this was turned to 'ALL DAY' with the stat turned up the boiler locked out again, not sure how long it fired for as I went away from the property.

I have also noticed that the boiler only ever fires for about 2 minutes at a time and there seems to be a kettling or boiling sound when it is fired. Could it be a blockage?

Many thanks to you all in advance, if you need further information please let me know.

Mark

 

[muggles]

So you've replaced a direct cylinder with an indirect one, and assumed that this would be OK? What size tappings were on the old cylinder? What size coil is in the new one?

You've said that the old cylinder was direct, but also that it's a sealed system. Sorry, but that's impossible. Was it sealed before, or have you sealed it yourself?

Have you tried bleeding the pump and checked for rotation? Did you definitely re-open the pump valves, if you closed them?

 

[watertight]

Sorry,

That should have been both indirect. Obviously didn't proof read as well as I'd thought.

Tapping size is 1", not sure about the coil but can get info from the new cylinder and cut open the old one, I doubt it's related though as the fault occurs when there is no demand for hot water, I'm willing to be told different though.

Pump checked, system fully bled including pump. Two AAVs fitted at point where water and heating branch in airing cupboard at highest point.

Popped in earlier, boiler was still in after 24 hours with room stat at 25 degrees and programmer set to 'ALL DAY'.

M

 

[watertight]

Pump valves are definitely fully open too.

Mark

 

[MrTherm]

>> I have also noticed that the boiler only ever fires for about 2 minutes at a time and there seems to be a kettling or boiling sound when it is fired. Could it be a blockage?
...............

Watertight, Do you in fact have *three* two port valves?

1 zone valve for upstairs switched by room stat
1 zone valve for downstairs switched by room stat
1 zone valve for cylinder coil switched by cylinder stat

The boiler burner must never light up unless one of the three valve's 'end-of-travel' micro switches make electrical contact proving that at least one valve is open prior to pump run - otherwise over heat lock-out will occur due to no circulation via the pump through the heat exchanger - nowhere to dump the heat.

Hope this helps.

 

[kbdiy]

Further to the last post, this boiler should have a bypass circuit as there is a pump overrun feature, rather than rely on one of the motorised valves being open, as inevitably there will be times when all 3 are satisfied but the pump will still need to circulate through a bypass.

I know this because I have a Suprima myself, not because I am a plumber or heating engineer.

 

[MrTherm]

Spot on kbdiy

From the Suprima 100 install manual...

Systems fitted with controls which allow the boiler to operate
when both the hot water and central heating circuits are
closed i.e. mechanically operated thermostatic control
valves, must be fitted with a by-pass circuit of:-

1. A minimum length of bypass of 2m of 22mm dia. pipe.
2. Maintaining a minimum water flow rate through the
boiler of 9 litres/min (2 gal/min).
...........................
It does indeed sound as if there is no by-pass or it may be turned off.

 

[kbdiy]

In addition I think the kittling could indicate a problem such as a dirty system. I had similar noises from mine but a good clean and flush has sorted that.

The OP also comments that the boiler fires for about 2 mins, stops for a short time then restarts. As far as I can tell this is normal operation - it fires to heat the circulating water, reaches the set temperature, stops firing while the water circulates and cools, then restarts to run the cycle again until the various stats stop calling for heat. Then the overrun kicks in until the boiler has cooled sufficiently.

 

[Agile]

There is obviously a lack of circulation through the boiler.

Best identified by a temp differential greater than about 11 C at the pipes to boiler.

Pump needs finger test and checking impeller is clear.

Seems replacing a cylinder is easy but I would always recommend a good boiler engineer is available for when something goes wrong.

Tony

 

[watertight]

Guys,

Many thanks for all your advice, the bypass had crossed my mind however I thought I'd tested this theory by putting one of the motorised valves into it's manual position and letting the system run, the lockout occurred again so didn't think that this was an issue, I also reinstalled the pipework 'as was' as it'd be working for the past 18 years in that way.

The lack of flow through the boiler makes perfect sense so I'm happy to stand corrected! There was quite a bit of air in the system when refilling and for some time there was some pipe banging and clattering. This has stopped (I think) upon repeated bleeding, I'm thinking that this could be a factor too?

The system has been running now for 48 hours without fail, not a sign it's fixed I've learned from experience with this system, I'm wondering if to flush it if it fails again, could a blockage really cause the intermittent lockout?

The heating zones both have their own 2 port valve, sorry this should have been much clearer!

Once again many thanks for your help to date.

Mark

 

[kbdiy]

Some Suprimas do have a flow switch which will cause a lockout if the system isn't fully bled. As you have repeated the bleed process it may be that you have now removed all the air and hence solved the problem.

I experienced something similar with mine after the BG engineer had changed a motorised valve. He didn't bleed it properly before he left and it locked out literally minutes after he had gone. Rather that go through all the hassle of calling them out again I re-bled the system fully and it was fine after that.