Bypass valve question...

[Mottie]

Hi.
We have an old boiler - Potterton Profile 60E - 26 years old that has been working fine. Don't shoot me down but I've never had it serviced or had any inhibitor in it! Only problem over the years has been the PCB. It's on its third replacement now. The system was a conventional system with a header tank for the heating and a cold water storage for the gravity hot water. It had a three way diverter valve for HW & CH.

Recently had the system upgraded as we wanted more pressure in the shower so we had an unvented hot water cylinder fitted. At the same time we had the system header tank removed and the heating system was converted to a sealed system. We also had a system power flush, the three way valve changed for two separate ones, a new digital room stat and programmer and a boiler service. I believed what we had was a 'Y' plan setup and now we have a 'S' plan one as a bypass valve was fitted next to the pump.

Initially it was fine apart from the last three remaining 50+ year old rads springing a leak (probably due to internal corrosion and the now pressurised heating system) so I changed them myself.

Now to my problem! Within a week or so and completely at random, the safety button on the bottom of my boiler would 'pop' and the system would not heat up. Never seemed to do it while running but normally happened during the night. I had noticed that the boiler was not 'running on' when the hot water or heating reached temperature so I got the heating company back. They replace a part in the boiler and it was back to how I remember the pump would 'run on' with the boiler flame extinguished to cool down before shutting off.

I still get the random shutdown and the heating company are telling me it's probably because the old boiler is scaled up inside and that I need a new one. I don't mind that if I have to but I'd be pretty ticked off if I had a new boiler installed and I still had the same shutdown problem due to some other reason. Is it because the old boiler cant cope with the new setup? Could it be to do with the pump and/or relief valve settings? Any advice from a heating engineer would be welcome. I've tried a few different combinations of the above and I'm now on what I think is my last combination option!

This morning I shut off all rads with TRV's (7) and completely closed the return relief valve. I then set the heating high leaving just the three rads that don't have TRV's (Bathroom, landing and Living room). I had the pump on speed 3 (max). When the pipes got too hot to touch, I slowly opened up the relief valve until the relief pipe started to get warmer indicating that the water was now circulating back to the boiler. I then closed it off a little. When I turned the temperature down, the heating zone valve closed, the boiler flame went out and the pump kept running proving the overrun worked and I could hear and feel the water returning to the boiler via the relief valve circuit. It shutb off after about 7-8 minutes. At a guess, I'd say I opened the valve to around 15% of its total adjustment.

I still won't know for a while if that was it - the boiler can go a couple of weeks before 'popping' or just a day. I had to reset it on Saturday and Sunday mornings this weekend.

Sorry for rambling on and I apologise if I've used the wrong terminology. As my user name suggests, I'm an MOT tester so if any advice is needed on that, feel free to ask!

 

[Mottie]

80 views and no suggestions. I knew it was going to be bovver! FWIW, it's still working. Fingers tightly crossed.

 

[fezster]

There is a "proper" way to set the ABV using your pump curves and the minimum flow of water required through your boiler (in the MIs). The instructions for the Honeywell ABV are here:

http://www.honeywelluk.com/documents/Installation-Guide/pdf/951.pdf

But the way you have done it is essentially how the ABV works. You can refer to the post here to confirm:

How to set a honeywell auto bypass valve?

 

[Mottie]

Thanks. Still okay this morning - but that could just be luck. Starting to wake up before 5.50 am each morning now when the heating is due to come on to see if it fires up. Was up and waiting for it this morning with my hand on the pump piping praying for it to get warm! Wife says I'm becoming obsessed with the damn thing and says I should just get a new boiler fitted. I really don't mind if it needs replacing but I don't want to get a new boiler fitted and still find out it was the pump/valve settings wrong. Wouldn't even mind so much if boiler was in the kitchen and easily accessible - it's up in the loft which means getting the ladders out each time to go up and press the overheat reset button. If it turns out to just be a matter of a balancing act with pump and valve I'll be a bit annoyed that I've had all that work done and the plumbing company didn't bother to set it up properly. They should have done that as part of the works (shouldn't they?) but they just said leave all the settings in the middle - valve half open, pump on speed 2 of 3. Trouble is, it's getting warmer now so less likely to get up to temperature. Might even run fine on the hot water only all summer and then play up next winter when all plumbers are busy. See, I mustn't keep thinking like that. Half of me is hoping it will explode or something!

Correct me if I'm wrong but for that overheat button to keep popping (and, believe me, it has popped no more than four times in 26 years before this work to the system was done) there could only be two things causing it to overheat: Not enough water being pumped through the boiler on overrun, due to pump speed and bypass valve settings incorrect OR bypass valve open too much resulting in the same water circulating when boiler is heating, resulting in the circulating water getting hotter and hotter until it exceeds the permissible temperature of the boiler. Is that about it or am I taking too simplistic a view of things?

 

[Gasguru]

If the bypass was open too much the boiler thermostat should knock off the burner.

What you have done sounds about right...you'll normally find the 22mm auto bypass valves only need setting to around 1 to 2 ie. they have a springrate that's far too strong for most domestic systems.

When the heating and/or hot water zone valves close, the boiler needs a circuit for the pump overun to shift water round...in that scenario you need to ensure the bypass is open sufficient. Most boiler manuals specify a minimum pipe length loop/size in order to dissipate some heat.

There are many other possible causes...weak pump, pump valves not fully open, blockage, an out of spec boiler thermostat or pump overun stat. Sometimes it's worth taking a punt and getting the stats replaced...it's certainly been the issue a few times for me. This is strictly an RGI job as the boiler requires opening up and specific safety tests carried out on the casing, the Profile being considered a higher safety risk due to its positive pressure casing.

It is also thought that continual overheat stat tripping can affect the trip temperature point...

 

[Mottie]

........... Most boiler manuals specify a minimum pipe length loop/size in order to dissipate some heat.

There are many other possible causes...weak pump.........pump overrun stat. Sometimes it's worth taking a punt and getting the stats replaced...it's certainly been the issue a few times for me. This is strictly an RGI job as the boiler requires opening up and specific safety tests carried out on the casing, the Profile being considered a higher safety risk due to its positive pressure casing.......

Hi thanks. I did consider the heat loss issue as basically, the 22mm run from the boiler (in the loft) to the bypass (in the airing cupboard downstairs) and back is 90% insulated so I did wonder where the heat loss took place on overrun. Mind you, that is the same run that was there with the old system and I didn't have a bypass circuit before but would I need one with a 'Y' plan system? I was thinking of adding some extra piping into the bypass circuit to help with heat loss when the bypass is in use. Do you think that would be worth it? It would certainly give me more heat in the airing cupboard - there's virtually none since the well-insulated hot water tank was fitted there in place of the old one. I changed the pump not long ago and it was a good quality named brand pump (cant remember off top of head but I think Grunfoss). A couple of weeks after the installation I got the heating company back and they replaced two valves in the boiler - overheat and overrun I think? - They only charged me for parts and not labour as they said that in light of the work they had recently done, they would class it as 'still being on site'.[/QUOTE]

This is a picture taken when they were installing the hot water cylinder showing the pipework. Not much of a bypass, or is there?

 

[Gasguru]

Previously the "bypass" would have included the hot water or heating pipe loops etc....since a circuit would have always been left open.

Set the pump to a fixed speed setting...don't use any fancy operating modes as they are not compatible with many boilers...the eco claims are miniscule.

See how it goes for now....