Boiler by-pass with TRVs?

[SteveHo]

We've had a Ravenheat Combi for a few years and we've had quite a lot of problems with it. Is it all because there is no room stat or automatic by-pass valve?

Its only a small system with 4 radiators, each with a thermostatic valve, and no room thermostat. We've had two new pumps, a new expansion tank, new three-way shunt valve, all the TRVs sticking, banging noises quite often, various leaks inside the boiler, and the system has always lost pressure and needed topping up every week or two (when heating on). Various plumbers have repaired the boiler, but the sticking valves, noise and loss of pressure have remained a constant problem.

I'v just found an article at www.heatinghelp.com/newsletter.cfm?Id=152 which sounds like it might explain our problem. Although that system was a much bigger installation, it seems to me that the cause of the problem could be the similar - but possibly worse in our case. When the timer puts our heating on but all the TRVs are closed because the weather is mild, there is no automatic by-pass valve to allow circulation. The Ravenheat manual gives the following "general guidance": "The flow through the heat exchanger must exceed 650 l/hr when the burner is firing. It is important to ensure that this rate is achieved when sections of the system are shut off either manually or automatically. THEREFORE A BY-PASS MUST BE FITTED (shows a valve between the heating supply and return) If the circulation is too low, the boiler water temperature will rise too rapidly (which) could cause noise ... or the safety thermostat to trip". I assume that the boiler doesn't have any sort of internal by-pass a) because its not shown in the functional diagrams and b) because they stress the need to fit one externally.

What the installers did fit is a manual by-pass valve, which might be of some use during maintenance, but is no good at all when the TRVs operate in normal use.

Seems to me that when the TRVs are closed the boiler temp is rising too fast causing the noise and air venting. The pressure is rising too high and blowing the relief valve which in turn causes the system to lose pressure so that I have to keep topping it up to above 1 bar. The frequent excessive pressure is causing the leaks in the boiler and caused the pumps, expansion tank, and shunt valve to fail. And on top of all this, the frequent feed of fresh water (in our hard water area) will have caused everything to badly scale-up, and caused the TRVs to stick shut creating a vicious circle!

But I'm no expert
Am I likely right here or have I overlooked something?
- and is it dangerous?

 

[suffolklad]

im just a diyer but my boiler has built in by pass pipe , but also guy that installed mine said one radiator must not have trv just two lock shields and thats the bathroom rad . mrs likes to be hot in there anyway lol

 

[Softus]

We've had a Ravenheat Combi for a few years and we've had quite a lot of problems with it. Is it all because there is no room stat or automatic by-pass valve?

That depends on its problems, and whether or not you have a manual bypass valve.

Various plumbers have repaired the boiler, but the sticking valves, noise and loss of pressure have remained a constant problem.

In that case you one or more unresolved leaks. The constant topping up with fresh water will lead, and will have led, to yet more problems.

What the installers did fit is a manual by-pass valve, which might be of some use during maintenance, but is no good at all when the TRVs operate in normal use.

I don't see why not. Why do you say that it's of no use?

Seems to me that when the TRVs are closed the boiler temp is rising too fast causing the noise and air venting. The pressure is rising too high and blowing the relief valve which in turn causes the system to lose pressure so that I have to keep topping it up to above 1 bar.[/quote]
Unclear - are you saying that there is evidence that the PRV is opening?

Am I likely right here or have I overlooked something?
- and is it dangerous?

Largely you're on the right track, so your research and attention to detail have paid off. It's essential to stop all leaks, so if it's the PRV then I would replace that first, then concentrate on finding out why it's actuating (i.e. why the pressure is rising).

Have you checked the condition of, and the pressure in, the expansion vessel?

Also, what's the temperature setting on the boiler thermostat?

 

[Gasguru]

You clearly know more about heating systems than the original installers (and more than many installers I come across)

The article refers to a commercial setup with high head pumps but there are similarities with a domestic setup.

As you only have 4 radiators each fitted with a TRV I assume they are fitted in the living room, bedroom, bathroom, hall or kitchen. I would suggest the you remove the living room radiator TRV head or replace the valve with a lockshield. This radiator should not have any user adjustment.

Install a room thermostat in the living room (note the positioning in the room must be correct for it to function properly). You should set the lockshield sufficient to ensure this room heats up last. This way when the whole property is up to temperature the stat will turn the boiler off thus saving energy.

Next remove the manual bypass valve. Instead fit an automatic bypass. I suggest the Honeywell - it has good adjustment and does not appear to leak. This can be set to give a minimum flow through the boiler when the remaining TRV's shutdown. Most combi's are fitted with 6m pumps that must be left on their maximum speed setting. On small systems this will create high flow velocities resulting in excessive noise and the maximum differential pressure across the TRV's will be exceeded. Again the automatic bypass will solve this problem.

Some manufacturers advise fitting the bypass at a certain pipe distance from the boiler - this is rarely done since it is not often practical (it doesn't seem to cause a problem anyway) - check the manual.

Cheaper TRV's are not bi-directional, can have reduced bore and the pins can stick - if so replace with Honeywell, Danfoss ot Drayton.

With a cold pressure of 1 Bar I would expect a your system to hit no more than 1.5 Bar when hot. Any higher than this indicates the expansion vessel is low on air pressure. Should be about 0.8 Bar with zero pressure on the boiler gauge. Topping up the boiler on a regular basis will cause corrosion - sort out the pressure and any leaks and use a quality inhibitor.

Many (but not all) combis can be range rated. This means the boiler heating output can be set to match the heat load of the property. Unfortunately most installers are lazy and fail to do this. As a result the boiler will kick out its full output and the radiators are unable to disipate the heat. The boiler sensors are too slow to react and excessive boiler noise results. It is possible your boiler is set to maximum resulting in noise. With your property you will only need the boiler on the minimum setting. You need to check the manual. It is a simple procedure to calculate the heating output the boiler is currently set to from the gas meter test dial. If the output requires adjustment you will require the services of a registered engineer for the gas pressure setup.

This work doesn't guarantee all the problems will be resolved as the heat exchanger may be excessively scaled - is the boiler quiet when delivering hot water?

Tell us the boiler model.

 

[Agile]

Or if you dont want to bother then just open the manual bypass one turn.

It is clearly turned off ( closed ) from what you say!

Close it and then open it one turn.

That should make the system stable!

I think that the Ravenheat probably has a CH power setting on the PCB for the installer to set. They are usually turned anticlock to min. Thats usually ignored and left on full power probably the cause of much of the problems.

Tony

 

[SteveHo]

Thanks for all the advice - I've been busy doing it, and it has helped a lot. I've opened the bypass valve one full turn, installed a room stat, and now leave the TRVs in that room fully open. The CH setting on the PCB is only set at one quarter so that should not be a problem (we get enough heat).

The boiler is much quieter now (no banging) and the temperature control is much better, but it still loses pressure and needs topping up every week. I disconnected the outflow from the PRV and there is nothing coming out there. There are no leaks inside the boiler or from any of the radiators or pipework.

The only escape seems to be from the air vent on top of the boiler. This is set into a small recess in the top panel and is always full of water - when I dry it up it just refills over a day or two - is this normal or is this causing the pressure loss?

Is the problem likely to be made better or worse by having the system de-scaled?

Any other suggestions welcome (within reason).

Cheers
Steve

 

[Agile]

No of course not.

Clearly the auto air vent is leaking and needs to be replaced.

Tony