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  1. #1
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    Default Expansion vessels and Evohome controls

    I seek the advice and opinions of the experts in the forum!

    Three weeks ago I had my boiler replaced with an Intergas Xclusive 36. I use the Evohome control system and OpenTherm using the dedicated bridge. My house has 12 radiators if I include towel rails. One radiator is never on. All the rest have HR92s apart from one, a tiny one in the downstairs loo, a throwback to when I left one radiator “open”.

    I am running the central heating at a flow rate of 65c as opposed to 80c. 65c is fine. DHW is currently set to 55c. All works well. All rooms reach their set point and several rooms having the same schedule, all do so more or less together.

    However, reading the Intergas installation manual it says that the inbuilt 6 litre expansion vessel is good for up to 8 radiators or more specifically, 100 litres in the system. The manual states that Intergas’s advice must be sought if the system exceeds these parameters. On that point I raised it with Intergas and they have so far not responded at all! Seek their advice and you don’t get it! Richard from the Intergas Shop has been much more helpful.

    From enquiries made it seems many Intergas appointed installers are not aware of this,

    Now for the questions!

    The 100 litre maximum is I understand based on a flow temperature of 80C. It is implied that above that volume the 6 litre expansion vessel needs an additional external one. Does anybody know what the formula is if the system is run at a lower temperature? The volume level must go up but to what? In my case I am running at 65c. So far perfectly adequate.

    Next is the effect of Evohome. The benefit of Evohome is that only zones one wants heating are heated. Thus some, possibly most radiators, are closed off. Therefore a 12 radiator system does not mean 12 radiators of volume. Is there any way of calculating the “Evohome effect” on the capacity limit?

    So far I have had no indication at all that there is any issue with the volume of water in my system.

  2. #2
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    Quote Originally Posted by G4RHL View Post
    I seek the advice and opinions of the experts in the forum!

    Three weeks ago I had my boiler replaced with an Intergas Xclusive 36. I use the Evohome control system and OpenTherm using the dedicated bridge. My house has 12 radiators if I include towel rails. One radiator is never on. All the rest have HR92s apart from one, a tiny one in the downstairs loo, a throwback to when I left one radiator “open”.

    I am running the central heating at a flow rate of 65c as opposed to 80c. 65c is fine. DHW is currently set to 55c. All works well. All rooms reach their set point and several rooms having the same schedule, all do so more or less together.

    However, reading the Intergas installation manual it says that the inbuilt 6 litre expansion vessel is good for up to 8 radiators or more specifically, 100 litres in the system. The manual states that Intergas’s advice must be sought if the system exceeds these parameters. On that point I raised it with Intergas and they have so far not responded at all! Seek their advice and you don’t get it! Richard from the Intergas Shop has been much more helpful.

    From enquiries made it seems many Intergas appointed installers are not aware of this,

    Now for the questions!

    The 100 litre maximum is I understand based on a flow temperature of 80C. It is implied that above that volume the 6 litre expansion vessel needs an additional external one. Does anybody know what the formula is if the system is run at a lower temperature? The volume level must go up but to what? In my case I am running at 65c. So far perfectly adequate.

    Next is the effect of Evohome. The benefit of Evohome is that only zones one wants heating are heated. Thus some, possibly most radiators, are closed off. Therefore a 12 radiator system does not mean 12 radiators of volume. Is there any way of calculating the “Evohome effect” on the capacity limit?

    So far I have had no indication at all that there is any issue with the volume of water in my system.
    Depends what your precharge pressure in your expansion vessel is. I have a calculator that works it all out.

    At 65oc flow temp, with an expansion vessel pre-charge of 1.5 and a system fill of 1.5bar your hot working pressure should be 2.6bar on a 100l system.

    6L expansion is really not enough though, I don't think the 8L ones are enough on some of the combis tbh! You need a min of 10l really, otherwise you'll find it needs recharging more often or fails earlier.

    In regards to Evohome and expansion - yes it helps. So much to the point of my own expansion vessel had actually ran out of air, yet my system was never expanding - hence why I never noticed. I was only when I drained down for some other work and checked the expansion cause I'd drained off that I noticed it was flat.

  3. #3
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    Quote Originally Posted by mtmcgavock View Post
    Depends what your precharge pressure in your expansion vessel is. I have a calculator that works it all out.

    At 65oc flow temp, with an expansion vessel pre-charge of 1.5 and a system fill of 1.5bar your hot working pressure should be 2.6bar on a 100l system.

    6L expansion is really not enough though, I don't think the 8L ones are enough on some of the combis tbh! You need a min of 10l really, otherwise you'll find it needs recharging more often or fails earlier.

    In regards to Evohome and expansion - yes it helps. So much to the point of my own expansion vessel had actually ran out of air, yet my system was never expanding - hence why I never noticed. I was only when I drained down for some other work and checked the expansion cause I'd drained off that I noticed it was flat.
    Thanks. From what you say whilst 6 litre may not be enough it is in my current set up. I don’t achieve 2.6 bar.

  4. #4
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    Quote Originally Posted by G4RHL View Post
    Thanks. From what you say whilst 6 litre may not be enough it is in my current set up. I don’t achieve 2.6 bar.
    Yes it's probably fine, if you were to run at 75oc and had the system on a full demand for a good hour or so then it might be a different story.

  5. #5
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    I am adding to my original posting on this subject in case the info is of help to others.

    As mentioned, some weeks ago, I had installed a new Intergas Xclusive 36 boiler. This has an internal 6 litre expansion vessel. After that my attention was drawn to advice in the installation manual that if there is more than 100 litres in the heating system then an external vessel may be needed and Intergas’s advice sought.

    In my case I am over that 100 litres, but I suspect that because of how I run it the 100 litre maximum is not an issue. I have my flow temperature set to 65c, it could go lower, I use Evohome and the boiler peaks at around 1.8 bar. It seemed to me that my setup meant the advice in the manual did not apply. I have 12 radiators. That includes 2 towel rails. One radiator is never ever on. I am lucky if at any one time 5 radiators are calling for heat. I have a 4 bedroom detached house. I am lead to understand the calculation of 100 litres is based on a flow temperature of 80C.

    My approved installer knew nothing about the advice in the manual. Convinced the 6 litre expansion vessel was adequate. He is not alone. Through a friend I know of another approved Intergas installer not aware of this limitation or advice. It is in the installation manual!

    I am so well aware that if a problem developed and I had to make an insurance claim my insurers may well wriggle and try to avoid responsibility on the basis the boiler was not installed in accordance with the manufacturer’s recommendations. I therefore sought advice from Intergas. As required by the manual.

    They have kindly advised that the manual is being updated with a little more useful information. But also confirmed that in my case, because of the flow temperature and the fact I use Evohome which limits the number of radiators wanting heat at any one time, and because my pressure is usually around 1.6 to 1.8 bar I do not need a larger external expansion vessel. I quote their response:

    “The addition (this is the addition to the manual) expansion vessel will be based on system water volume and not the number of radiators you have, we only use this as a typical scenario, however you have mentioned operating your boiler with a target temperature set at 65C and an operating pressure of approximately 1.6bar so I would suggest that the expansion vessel is perfectly adequate otherwise you would be getting over expansion and pressure readings above 2.5bar.”

    I suspect that probably more than 90% of installers know nothing about the fine tuning and needs of a combi boiler to enable it to run at above 90% efficiency. Never mind not knowing about the need for larger external expansion vessels. Which is rather important to say the least.

    The benefits of Evohome here are quite clear. Properly set up and with a sensible flow temperature to enable the boiler to condense then the boiler should run at or above its stated efficiency of above 90%. A legal requirement I believe that in most cases, because of ignorance, is not achieved.

    Long live Evohome for it has other cost savings!

    In the 5 weeks I have had the new boiler running OpenTherm and Evohome with it I have noticed my rooms come up to the set temperatures and stay there. Nearly all at similar times. There is little movement. It remains stable. This did not happen before with my old boiler. I have reduced the maximum power of the heating side to 12.3kW and will reduce it further. I have had Evohome for 6.5 years and always appreciated its advantages, despite my criticism of Honeywell’s support of its end user, but the boiler with Evohome takes my appreciation to a higher level!

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