Boiler banging and TRV problems

[Andrew316]

Hi,

I'm currently having problems with the TRVs on my radiators and wondered if anybody here could offer some insight. The first problem is that when I turn it down to say 3 the radiator will turn off. So in other words i either have a radiator on or off there is no middle ground where I can turn 1 room down a few degrees. Is this normal. I have a BAxi duo tec 28 he boiler which is linked to a thermostat for the house. Does this thermostat render the TRVs useless?
My second problem is that the boiler will occasionally make a banging noise usually but not exclusively when it is starting up. This does not happen all the time but when it does it can be quite loud. I have previously spoken to the plumber who installed the boiler and was told the banging could be due to air in the system and to give it a couple of months. However I have just noticed that the first radiator after the boiler has a TRV on either side of this. Once again is this normal? Or could this be the cause of the banging noise. For the record one of my radiators, in the bathroom, does not have any TRVs on it
Final question sorry do TRVs always go on the right hand side of the radiator as you look at it?

Thanks very much for any help that can be provided

 

[Agile]

You dont understand how TRVs work. Once the room is up to temperature they turn the rad off until the room cools.

In practice they often do stay partly on but not if the circulating water temperature iss too high. It should be no more than 70°.

The TRVs can go where the installer thinks its best. Its actually better they are placed on one end in each case.

There is never any need to have TWO TRVS on one rad!

The banging might be because the installer did not range rate the boiler to the property.

Tony

 

[Andrew316]

Cheers Tony, you were right I did not know how TRVs worked so thanks for the advice.
Could the banging be due to 2 TRVs being on one rad or is that no possible?

 

[Agile]

I would not expect it, but you can easily check that by turning both TRVs off and see if that stops the problem.

If it does, then you can remove one of the TRV heads. Ideally replace it with a decorating cap and adjust this so that its not fully open otherwise it will upset the balancing.

Tony

 

[TicklyT]

I would expect problems balancing the rest of the system if a radiator had two TRVs fitted, as both could be wide open and robbing the flow from the other radiators.
It may be a conntributing factor to the banging if the return water temperature is getting higher than expected at the boiler.

Adjusting the flow to balance the system with a decorating cap may be a little finnicky, and rendering the setting 'tamper resistant' may be problematic, but certainly worth a try before changing one TRV for a lockshield.

 

[Goldspoon]

TrV's will be one way flow or two way flow. If two way flow all is well. If one way flow then pipes will bang if flow is going the wrong direction through the TRV. Little arrows on TRV body show if one or two way and direction of flow if one way.

 

[D_Hailsham]

You dont understand how TRVs work. Once the room is up to temperature they turn the rad off until the room cools.

Sorry to say so, but you are wrong. A TRV is not like a switch, open or closed. It is a proportional control. The TRV will stay fully open until the temperature is within about 2°C of the set temperature. It then start to close down until the required temp is reached. At that point the TRV should be allow just enough flow to maintain the required temperature within ±2°C. If the TRV is turning the rad on and off, the system is not properly balanced. It is possible to set up TRVs so the differential is less.

Read TRV4 Commissioning Guide

In practice they often do stay partly on but not if the circulating water temperature iss too high. It should be no more than 70°.

Where did you get that idea from? The influence of the water temperature on a TRV is negligible.

There is never any need to have TWO TRVS on one rad!

Did you know that the Drayton TRV4 body can be used as a balancing/lockshield valve? Drayton sell a special cap for this (not the decorators cap).

 

[Agile]

Whilst I dont disagree with what you have said, the actual operation in many cases is as I have described.

It will only operate as you say when its set up properly with an appropriate flow temperature, rad size and has been correctly balanced. The practical heating world is not the same as your textbook world.

The OP's system does not seem to be set up or operating correctly as he had come to the conclusion that it was intended to turn on/off to control the temperature.

It may surprise you but I did know about Drayton TRV4 bodies and lockshield caps but I concluded the OP would not have them but ought to have the decorating caps.

Tony

 

[D_Hailsham]

Whilst I don't disagree with what you have said, the actual operation in many cases is as I have described.

It will only operate as you say when its set up properly with an appropriate flow temperature, rad size and has been correctly balanced.

But why continue perpetuating the myth that a TRV is nothing more than a fancy switch? Surely it's more helpful if you tell the OP what should be happening and how to put it right. I agree that rads need to be sized according to the flow and return temperatures. Obviously an undersized rad will never get the room up to temperature, so the TRV will stay fully open all the time. Correct balancing is essential.

To the OP

Read How to Balance a CH System

 

[Agile]

I was responding to the OPs problem rather than explaining how they should be working in an ideally set system.

The reality is that most systems are not set correctly and even when they are they are often operated by the occupier in a less that ideal way.

The correct operation of the TRV is to partially close and adjust the flow so that a constant temperature is maintained.

To be able to do that there are several factors which need to be correctly set.

# Radiator sized to give the required heat output.

# Flow through the rad set ( hence differential temperature on flow/return )

# Flow temperature ( this should not be excessive in relation to the heat output and will be different during colder/warmer weather )

# The radiator and TRV should be unobstructed so there is a correct airflow over the radiator and the TRV.

If all these requirements are met then the TRV will control the temperature proportionally without overshooting and turning the rad off which is what the OPs system seemed to be doing.

In any system there can be one or more of the above which cause the temperature to oscillate around the set temperature. The time constant of a room/rad is long and typically 30-50 minutes. The liquid filled TRVs like the Drayton TRV4 respond far quicker than the cheaper wax element types.

Tony

 

[D_Hailsham]

I can't disagree with most of your last post.

Balancing of Radiator Systems is a helpful and interesting read, even though it is primarily concerned with commercial systems.

 

[Agile]

Which bits do you disagree with then?

Of course to keep my posting short, I had chosen not to mention the effects of hysteresis within the TRV.

Whilst new TRV4s seem very good, some of the older rad valves do exhibit considerable hysteresis and that just by itself can create and on/off action even when everything else is correctly set.

I was reminded of just how much resistance can be provided on an operating pin when a friends diverter valve was sticking after only 10 months to such an extent that it would sometimes not bring on the boiler and Susan was complaining about cold showers!

Not wanting to have to replace the whole valve again on an outdated Sime boiler I extended the pin and cleaned, wiped and lubricated it with silicone grease and to my great pleasure it then worked so well that Susan gave me a big kiss.

I will now be recommending that as part of the annual maintenance all TRV pins should be greased !

I think that he has at last realised that the boiler must be changed this year. I am not so sure about his Lexus though. He has been muttering about replacing it for the last two years.

Tony