Banging Baxi Solo 3 PFL

I had my 25 year old Boiler replaced,in June 2003, with a Baxi Solo 3 PFL.

Just recently I have been having Boiler bangs with steam/hot water being pumped up into the header tank.

Looks, initially, as if I might be running the boiler too hot.
When it was banging I measured the water temp at the pump and it was 80 C.

Turning the boiler control down close to minimum gets this temp down to 60 C and the banging goes away.

Now this raises a couple of questions (or three!).

1) What would be the optimum water temperature for a CH system like this (15 rads, 3 speed pump set to 3, TRVs on all rads, Electronic room stat in main room, etc)?

2) Should they have installed a bypass circuit during this boiler change? They didn't - but would it have helped if they had?

3) I read about sludge & slime causing blockages but this system has always had Fernox in and has also had the benefit of being fed from a water softener.
Should I bite the bullet & have it flushed? or, maybe I can do this by adapting my Karcher pressure washer to the pipes?

Note that I believe there is no air in the system.

Advice appreciated.

1) What would be the optimum water temperature for a CH system like this (15 rads, 3 speed pump set to 3, TRVs on all rads, Electronic room stat in main room, etc)?

Depends on the weather. WOuldn't normally want it over say 70 though hotter will heat your HW faster, but tends to scale boilers.

2) Should they have installed a bypass circuit during this boiler change? They didn't - but would it have helped if they had?

Baxi's instruction are craap on this point. But YES if you have ALL trv's.

3) I read about sludge & slime causing blockages but this system has always had Fernox in and has also had the benefit of being fed from a water softener.

WS is a definite DISADVANTAGE! Lots of nasty chlorides. LImescale is not relevant in normal ch systems.

Should I bite the bullet & have it flushed? or, maybe I can do this by adapting my Karcher pressure washer to the pipes?

Forget the karcher - hopeless!


There could be all sort wrong - check the water in the header tank, check it fills system by draining some out from pipes. Pump could be spinning but doing nothing. How are you measuring 80 and where? 80 is normal, banging is not. Banging could be a knackered pump, boiling other than where you measured, etc.

Thanks for that reply.

>Pump could be spinning but doing nothing. How are you measuring 80 >and where? 80 is normal, banging is not. Banging could be a knackered >pump, boiling other than where you measured, etc.

The pump is upstairs in the airing cupboard and I am measuring on the out pipe from the pump to the CH system and also (with a second clip on thermometer) on the pump to Cylinder pipe.

The pump is failry new ( under 6 months) and the 3 way valve was new last week. The pump is running at high speed (3) and I'm not happy to turn this down as the speed of the water flow through the boiler must have a bearing on all this.

I've now adjusted the water temp up to 70 C and it all seems to be OK for now. 80 would probably bring the problem back. Maximum is 85 according to the manual.

I wonder if this problem has existed since the boiler was changed (June 2003) and has been "hidden" because of the system's ability to lose heat when the ambient temperature is low.

If TRV,s are fitted to every radiator a by-pass MUST be fitted with this boiler.Otherwise when the themostats shut down when the system is providing heating only there will be no circulation thru the boiler.In any case current regulations call for the use of an automatic by-pass to be fitted where trv,s are used on every rad.Boiler manufacturers instructions assume that installers are conversant with all relevant legislation and if every pertinent piece of information was included you would end up with something the size of the bible.Many installation problems are caused by the installer failing to read the instructions.The information regarding the fitting of a by-pass is included in the Solo instructions.

drbagiva said:

If TRV,s are fitted to every radiator a by-pass MUST be fitted with this boiler.Otherwise when the themostats shut down when the system is providing heating only there will be no circulation thru the boiler..

Click to expand...

Thanks for that.

I'm studying the Solo HBK for a way of sorting this out. It appears that a single radiator will be acceptable as long as it has lock shield valves at both ends and is a minimum of 4 square feet.
My bathroom radiator is a double, with a total area of 10 square feet, so I guess it would be suitable if I took off the TRV and replaced it with a lock shield valve.
Alternatively I might install the bypass circuit if I figure out how it should be fitted.

Could also be a fault boiler stat the solo 3 's are a bit glitchy so i would get it looked at as it sounds like you have a serious problem