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Our fairly recently installed central heating system (about three years old) produces banging noises at this time of the year every time the boiler ends its firing after achieving temperature. This happens when most of the TRVs are not calling for heat. We keep the hall rad fully on to try to compensate for this as it seems that the boiler is trying to heat too much water for the demand. This does not happen in the summer when it is only used for hot water and not so often in the middle of winter when the TRVs are calling for heat. The boiler is a Potteron Suprima 60L and we do not have a room thermostat only TRVs. Boiler temperature is set at two points below max. There is no air in the rads. Is it something to do with the pipework installation, three way valve? The system has an inhibitor in the system and I would have thought it was too new to be firred up.

I have a similar problem since replacing my motorised 3 way valve in the summer. The heating system makes a terrible noise and hot water is being vented into my expansion tank which is boiling hot and has melted the plastic lid. Like you this seems to happen when the TVR's stop calling for heat and the boiler switches off. I thought it was scale in the boiler until I noticed water dripping from my loft. I am today going to do a full drain down and force mains pressure water into the pipe from my expansion tank to see if blocked. I am not a plumber but have had this suggested. I will let you know if it helps.

Perhaps a heating engineer can put us out of our misery.

Sounds like what both of you need is a automatic by-pass valve this is a valve that fits between the flow & return pipework close to the boiler ie it makes a bridge between the flow & return. The idea of this (which is now mandatory under Part L of the building regulations since April 2006) is to perevent water hammer and excessive pump wear in the event of the TRV's in the heating system closing down. How it works is that your pump runs at a constant speed, when all the TRV's are calling for heat i.e. open you need all of the pumps speed to get the water around all the radiators, however as the TRV's start to close the pump is still pumping full speed until it is pumping against a closed circuit, the boiler shuts down and the pump goes through it's overrun again against a closed circuit, the pressure increases as will the noise and in a open vent system the water goes over the open vent, in a sealed system the pump goes through a real tough time and will reduce the pump life. When a automatic by-pass valve is fitted this senses the increase in pressure and opens up, thus providing an escape route for the pump. The water leaves the boiler through the pump, travels through the by-pass in to the return back to the boiler in a loop. As the pressure decreases because TRV's start to open again the by-pass valve closes and the water goes back along the flow to the radiators missing the by-pass. You will need a 22mm automatic by-pass valve costing no more than £30 + vat.

Thank you very much for that reply. I am arranging to get one fitted. I am, however, told that an automatic by-pass valve is not required in a system with a 3 port valve even though the system has TRVs because the valve is always open for either heating or hot water It would be needed if it had two way valves. Is this correct? My system is an open vent type.[/url]

Bobbee the short answer to your question would be yes. However, the three way valve may well be open but some if most TRV's could be closed or worse partially closed this could lead to an increase in velocity and could lead to what is known as pump cavitation this = noise and increased wear & tear on the pump and will lead to the pump runing less efficiently. This is why automatic by-pass valves are required to be fitted by building regulations on all new systems and boiler replacements. Some combi/system boiler models claim they have an inbuilt by-pass, but this is the boiler by-pass for pump over run and NOT the system by-pass that is required to deal with closing TRV's that is why building regs spell it out that the boiler by-pass is not sufficent to meet current regs.

DIY Doctor thanks for the post, I today changed the boiler thermostat but the problem still persists. When the boiler shuts down after the heat has been on for a while it bangs and whooshes and hot water squirts out in the expansion tank. I have a 23 year old glow-worm Fuelsaver F55, microbore Y plan with 3 way valve and individual TVRs. Will the automatic bypass be okay with this system. Bobbee sorry for piggybacking on your question!

Hi DavidM, there is no compatability problems when using a auto by-pass valve.

Thanks Heating Doctor for your replies and DavidM you are welcome to join in.
Just one other factor in this situation. I have a bathroom towel rail with two lockshield valves that is plumbed on to the boiler flow and return. The installation instructions for my boiler says that this is an alternative by-pass arrangement to an automatic by-pass. I understand, however, that the Building Regs call for a by-pass but I wonder if you are surprised that I have the banging when I have this alternative.
Another factor on the system which may or may not be related is that the first floor rads heat up when the heating is off and hot water is on on the programmer. What would the cause of this be?

mmmmmmmmm more info, first let me tackle your second question first the cause of up stairs radiators heating when hot water is on is gravity. Hot water rises, hence it is only upstairs that heats, your 3 way valve must be passing or the water is finding another route, which ever it's all down to gravity and of course is wasteful as your paying to heat radiators you dont want to. It can be overcome by finding out what route the water is taking and fitting a non return valve in line on the return loop, this will prevent the gravity circulation as the non return valve will offer to much resistance to gravity but will open on pump pressure.

As to the towel rail & three way valve being open yes I am kind of surprised that this does not provide enough of a by-pass.....unless..... the towel rail is part of the primary loop, the side that feeds the hot water cylinder which is not uncommon. Then if the 3 port valve is closed to the cylinder it would close the towel rail off. Then if the TRV's started to close no by-pass and your problem is still there.

As you can see there is no absolute in central heating only possibilities!