Where is Thermomatic?

Thermomatic, manufacturer of the (in its time) fantastic RSM range of (non-condensing) combination boilers is unfortunately one of the latest victims of the current recession.

Thermo who?

Thermomatic were an Italian manufacturer of a number of products but with regards to the UK, mainly the Thermomatic RSM 20 and RSM 25 combination boilers.

And what is this RSM model?

This was before condesning boilers were anything more than a niche specialist product and was one of the few designs (in fact some might argue the most successfull) that stored an amount of water within the boiler for imediate use rather than waiting for the boiler to heat up before supplying hot water.

It managed this by acting like a small cylinder with the DHW in the coil and the “jacket” of water surrounding the coil supplying the radiators and so not wasting any heat. The “jacket” of water was kept at the desired temperature all the time the boiler was on so that this would transfer the heat into the coil and whenever a tap was opened the already warm water in the coil would leave the boiler and the mains water replacing this would heat as soon as it started its journey through the coil.

This design meant that for small amounts of DHW the boiler would not neddlessly fire and the store of water could be kept at temperature with a few short burst of heat throughout the day.

The design was in fact so efficient that it had to have a thermostat (called the anti-condensing stat) specificaly to stop it condensing.

If it is so good why is it still not around today?

The RSM model was a very simple piece of equipment with just a large heat exchanger/water jacket, gas valve, ignition PCB and a few thermostats. The only problem at that time was the weight and size. Although not large for its day, it was not small either and because of the size of the copper heat exchanger it was a two man lift (even when Health and Safety wasn’t so forceabley imposed).

Its real achilies heel was what set it apart from its competition. Most boilers had a basic heat exchanger (mostly low water content) which could be replaced with a condensing alternative (with a few other changes) whereas the large copper heat exchanger could not. So as soon as the laws in the UK changed the end of the RSM was in sight.

Within a few years Thermomatic stopped production as Europe followed with similar Condensing boiler laws and incentives. Although Thermomatic made a range of products and boilers, they have been facing many separate threats over the last five years and ultimately the family run business was let down by their banks and had to close the business in November 2010.

Why are you telling me this?

Whilst most of the industry have never heard of let alone seen or worked on a Thermomatic, there are a number of the thousands sold still out “in the wild” and because their design was so simple (not to be confused with basic) many of them are still going strong.

It is for this reason that as the sole importer of Thermomatic spares we bought any and all spares stock left for the models when Thermomatic went out of business.

We do not have everything for every model /version of the boilers as even the newest RSM 20 Mark 1 would be over 30 years old however we do have 90% of the parts and the technical knowledge to help if you are in trouble.

So if you see a Thermomatic in trouble, give it a bit of TLC, give us a call and before you know it you could have a working unit that should still go on for a few more years.


Paul and The Gas Boiler Parts Team.

Edit November 2016

Since this post was written we have had a number of requests for help from around the world. Since we can only help with the UK versions (which may or may not be the same as international versions) I have scanned and linked (below) copies of the manuals we still have for the mk3, mk4 and mk5 RSM 20FB (the 25FB was similar) so from these manuals you should be able to find some information to help.

Thermomatic RSM 20 FB mk3 Manual (706 downloads )

Thermomatic RSM 20 FB mk4 Manual (1201 downloads )

Thermomatic RSM 20 FB mk5 Manual (1155 downloads )

Thermomatic RSM 20 FB User Instructions (1173 downloads )

16 Responses to Where is Thermomatic?

  1. Paul Arnell June 28, 2011 at 8:26 pm #

    I have a friend who is having trouble with his Thermomatic RSM 20. It will not fire but is trying to ignite. I have done a resistance check on the gas valve and the coils appear to be fine. The only thing I can put it down to is the Honeywell PCB attached to it. Any advice would be great as not many people know about this unit.



    • Paul June 29, 2011 at 10:02 am #

      Hi Paul, to help we would need to confirm the model version (Mark 1, 2, 3, 4 or 5). I am assuming from your description that it is a Mark 5.
      If this is the case, have you checked the common issues –
      1) Gas at the appliance, with correct inlet pressures (21mb min)?
      2) The process for ignition is: Fan starts, Air pressure switch activates, gas valve opens, gas flows to burner AND spark at electrode.
      If the above is happening gas should ignite so problem would be flame sensing however if there is no flame then work backwards.
      Most common ignition faults are Control box, electrode and lead then Gas Valve.

      We have control boxes – http://www.gasboilerparts.co.uk/products/1048-control-box-%2C-honeywell.html but we will need some info to get the right lead and electrode set (length of lead and type of connector, bullet or flat) but we have both in stock. The gas valve will need to be checked as well but they rarely fail.

      For reference here are some tips to find the right version of a Thermomatic RSM 20 –
      Version 1, 2 and 3 have the control box at the bottom of the boiler with the red reset button.
      Version 4 has the control box at the top left of the boiler.
      Version 5’s control box is on top of the gas valve (red plastic box).



      • Paul Arnell June 29, 2011 at 6:38 pm #

        Hi Paul,

        Thanks for your quick reply. I have checked the components as you advised and they are all working as normal and am getting the spark but no gas.

        I believe it is a mark 5 which has the red Honeywell cvi control board attached directly to top of gas valve. The control box has a little adjuster wheel which I believe controls the gas on ignition at a lower rate.

        I will ask my friend who owns the boiler if he wants to take the chance and purchase one. I will let you know in due course.

        Once again, many thanks for your help.



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    • Paul February 4, 2012 at 11:24 pm #

      Thanks for the comment, we have a couple of articles to release soon so hopefully they will also be of interest.

  3. Richard June 17, 2013 at 4:04 pm #

    My Thermomatic Combi RSM 20, version 4 fails to light intermittently. It runs for some weeks or days no problem; it then has increasing periods of lighting cycle failures, say one in twenty. The devil is that it works when watched making diagnosis difficult!
    Fan always starts on demand.
    The air pressure/flow switch seems to function ok – all the 3 connections all show positive even when no gas lit.
    Plumber confirms gas valve and pressures in order. Exhaust gas measurements all good!
    I have not caught it in the act, but the control box may not click on demand.
    The electrodes appear to function. The leads sometime show a very low grade spark when resting against metal parts (viewed in the dark!).
    Can you indicate which part(s) may be malfunctioning and the order in which they might be substituted?
    If not the solution, can parts be returned?
    I am reluctant to change the boiler after 15 years – by all reports it might last another 10, spares permitting!

  4. Paul June 17, 2013 at 5:00 pm #

    Hi Richard,

    I know this is difficult as it behaves when you watch it but –

    When the boiler is NOT alight when it should be. Does the fan continue to run?

    This will confirm if the Air Pressure switch is at fault as there should never be power at all three connections!



  5. Richard November 6, 2013 at 9:05 pm #

    The air pressure switch was replaced as have both electrodes and cables with some improvement but the boiler still on occasions fails to light. The gas control valve operates erratically. Sometimes it operates with little noise and lights the gas progressively but more often the solenoids operate with an aggressive “clack” and the gas lights with an oomph! Before getting the plumber in again and deciding to repair or replace, is this unit likely defective (and in stock) or might the problem be with the control unit?

    • Paul November 11, 2013 at 4:32 pm #

      Hi Richard,
      Sounds like gas valve although depending on the model if it is a gas valve/controller combo then it could be the controller.
      In all honesty as Thermomatic are gone the remaining items are getting “rarer than hens teeth”.
      Unless you are sure which part you need I would consider thinking about replacing the boiler if/when it fails.
      I can check the availability of the controller and gas valve if you can supply the serial number of the boiler so we know exactly which model/revision it is.
      Either reply here or email directly (info@gasboilerparts.co.uk).

  6. Keir November 14, 2013 at 4:59 pm #

    I have a Thermomatic boiler (RSM 25 I believe but could be RSM20 – it was the middle of the range at the time) & it now occasionally stops with the red light at the bottom coming on. Originally I know this was a gas pressure fault as some idiot (me) painted over the top of a gas meter blocking a hole which was something to do with pressure balance. Since then the meter has been replaced by the supplier & moved to an outside cabinet so I am at a loss as to why this is happening again now. It can go for a while with no problems then stop many times in 1 evening. Any ideas ?

    • Paul November 28, 2013 at 6:41 am #

      Hi, the red lockout could be a number of things related to ignition and flame sensing or control.
      When was it last serviced?

  7. JOHN WADE August 28, 2016 at 9:45 am #


    • Paul August 30, 2016 at 9:05 am #

      Hi, we have a number of manuals we can scan and send but we will need some information from the boiler please –
      The serial number, as we have records of some serial numbers and their respective models but not all so also the make and description of the gas vale and pcb (a photo emailed would also help).
      From this info we should be able to get a close match manual.
      Note – the RSM 25 models did not have their own manual but used the RSM 20 manual depending on gas valve/pcb makes and configuration.
      email any info to info@gasboilerparts.co.uk

  8. Brad Morgan November 1, 2016 at 10:53 pm #

    Paul, have a RSM 25 PZ dated 30/01/1997, model #70036 that fails to light.
    Have had twp lots of plumbers but dont understand the controls on the face.
    There is the grey turn and push to light the pilot and on the right a black knob that rotates but its function is unclear and on the left an 8mm dia head that can accept a screwdriver for what purpose I dont know. Can you email a description of the functions of the control panel please so I can show a new plumber? thanks, Brad
    Edithvale, Victoria, Australia!!

    • Paul November 2, 2016 at 10:57 am #

      Hi Brad, since we only imported UK specific boilers I can’t be certain that the Australian version is the same (although the serial number is similar to ours).
      However the black know could be the output (boiler control) which controls the temperature of the output to the heating. The 8mm dia head could be the fuse cover but I am not sure what the grey turn and push is (possibly part of the gas valve).
      I have added the last 3 manuals (poor quality scans of scans I am afraid) and the user instructions which may help you and your plumber work out the details. They are at the bottom of the article.
      Sorry I can’t be of more help.

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