Low Pressure Switch in a Sealed CH System - Mandatory ?

[self builder 123]

Hi

Is it manadatory to have a low pressure switch incorporated in a central heating system ?

From what I understand if there is a leak and the sealed pressure drops off the switch will activate and shut off the boiler.

Does it have to be fitted when plastic pipework has been used as part of the CH system ?

I take it that if it was not incorporated and the system sprung a leak the boiler would shut down on either the boiler stat or the overheat stat if the boiler continued to run ?

Cheers

 

[Jezclayton]

No.

In a domestic situation, a low pressure switch is something often incorporated in a boiler and is there to protect the boiler against dry firing as opposed to protecting the system. There is no requirement to fit one, though it can be beneficial on large systems where there are numerous expensive pumps that can be damaged should the system run dry.

 

[self builder 123]

Thanks Jezclayton

I see. If there was a leak the circulating pump could run dry and potentially burn out the bearings due to the lack of cooling.

Without such a low pressure switch is it only the pump which would be at risk ?

I take it that if the boiler was to dry fire it would shut down as soon as either the operating stat or the overheating stat temperature was reached ?

 

[Jezclayton]

Where the boiler to dry fire, it would eventually shut off, but bearing in mind most stats are on the internal pipework and not the heat exchanger, it is possible that damage would be done to the boiler.

Pumps running dry are normally the first indicator of a problem since they tend to screech painfully, before seizing.

Most boilers designed to be fitted in a sealed system will incorporate a low pressure switch and this should not be removed.

 

[self builder 123]

Thanks again.

I have a Grant condensing oil fired boiler where the thermostat phials are fitted to the side of the boiler as shown in the attached diagram.

Thermostat Phial

• self builder 123
• 8 Mar 2009
• 1

I believe that this is the side of the secondary heat exchanger ? Is this correct ?

In the event of dry firing would this reduce the risk of damage to the boiler ?

 

[Agile]

Many older boilers had a low pressure switch.

Most later boilers have a system flow switch.

The LP switch does not protect a boiler against pump failure.

A system flow switch can protect the boiler and pump because the PCB only allows the pump to run for a few seconds if the flow switch is not activated.

Slowly, boiler design has improved as computer control techniques can be cheaply integrated into boiler design.

Tony

 

[Jezclayton]

It would be better if you just said what boiler model you had. Is it a system/combination boiler or heat only?

Seeing as the flow/return tappings on this boiler are taken from the top of the heat exchanger, unless the boiler itself was drained, it is unlikely (failure otherwise) that the heat exchanger would ever be empty of water and the stats would trip.

Fire the boiler with no water in the heat exchanger and you are likely to damage it.

 

[Jezclayton]

I recognised your boiler from the picture posted of the thermostats and the manufacturer makes their position very clear. The following is the relevant extract from the installation manual:-

Sealed systems - If plastic pipe is to be used, the installer must check with the plastic pipe manufacturer that the pipe to be used is suitable for the temperature and pressures concerned. Pipe must be Class S to BS
7291: Part 1:1990.

The system should incorporate a low-pressure switch to shut off power to the boiler if the system pressure drops below 0.2 bar. A suitable low pressure switch kit is available to purchase from Grant Engineering (UK)
Limited, Part No. MPCBS 62.

If the above is relevant to your system , I suggest you incorporate the recommended switch.

 

[self builder 123]

Many thanks !

 

[mrgas]

For a sealed system I think you will find you need an overheat stat on the boiler.