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Thread: OpenTherm with two boilers & other related questions

  1. #1
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    Default OpenTherm with two boilers & other related questions

    Hi:

    My house has two boilers wired in parallel. One BDR91 turns them both on for heating, and another turns them both on for hot water and operates the appropriate valves.

    I'm not sure if it really needs two boilers, they were there when we moved in.

    We've always had some noise from the heat expansion of the pipes when the boiler comes on. This is less if the boilers are at a lower temperature, but then the hot water takes a long time to get to 60 degrees. The boilers are getting oldish (20 years) and according to british gas some parts not replaceable although not yet at the stage of causing a servicing problem.

    Since installing evohome the heat expansion noises from the pipes are worse because of the TPI. The house is much more evenly heated though.
    So - first thought is whether opentherm would help - but can it control two boilers at once? (Assuming I swapped for opentherm capable boilers). If it would help, I also wonder whether I could actually have one boiler and the system was originally over-specified.

    The water heater is a megalow - we've never had the radiators flushed through or anything, so I do wonder if we did that whether it would heat more efficiently needing a lower temperature and reducing heat expansion noise from the pipes that way too.

    Any ideas gratefully received - just trying to avoid replacing all the pipework!

  2. #2
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    My desktop research suggests that I might need an opentherm compatible boiler sequencer? (Assuming that I do actually need two boilers to heat the house).
    And given at the moment both boilers run in parallel then there is probably quite a lot to justify this from a savings perspective.

    Does anyone have experience of multi-boiler opentherm products they could point me towards?

    Best wishes
    James

  3. #3
    Automated Home Legend paulockenden's Avatar
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    Is the house huge? Or particularly energy inefficient?

    Do you know what the rated output of the two old boilers is?

    A single modern boiler is surely the way to go. Especially if you've done everything you can to minimise the heat losses from the building.

  4. #4
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    Quote Originally Posted by paulockenden View Post
    Is the house huge? Or particularly energy inefficient?

    Do you know what the rated output of the two old boilers is?

    A single modern boiler is surely the way to go. Especially if you've done everything you can to minimise the heat losses from the building.
    It is a big house (perhaps some would say huge?). We have 29 radiators and four bathrooms. The current boilers are Valliant ecoTherm which I can see was available in a number of different sizes but I can't work out what capacity they are. Half the house is pretty old (stone walls) but the other size is a c.year 2000 extension.

    We haven't yet done everything we can to reduce heat loss - windows are still single glazed for instance - but I plan to replace them next year (we are in a conservation area so will need to apply for planning as well as making selection etc)
    Last edited by Jabes; 11th November 2018 at 05:32 PM.

  5. #5
    Automated Home Legend paulockenden's Avatar
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    I’ve stayed in smaller hotels!

    ;-)

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    I have 24 rads (which isn't enough - only one in some quite large rooms, for example), 6 beds, 4 baths, 4000sqft. The house is 1950s and a mix of OK-ish insulation and modern-ish extensions. I get by on a single 25kW boiler! To be honest, it's not really enough to heat the house from cold very well - we schedule the heating to come on the day before we come home after holiday, for example. But it's enough to keep the house running on a normal schedule. I chose it because of the lower end of the modulation range. I would choose a slightly bigger boiler (but not much) next time *IF* it had a good modulation range - many don't.

  7. #7
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    Quote Originally Posted by Jabes View Post
    Hi:

    My house has two boilers wired in parallel. One BDR91 turns them both on for heating, and another turns them both on for hot water and operates the appropriate valves.

    I'm not sure if it really needs two boilers, they were there when we moved in.

    We've always had some noise from the heat expansion of the pipes when the boiler comes on. This is less if the boilers are at a lower temperature, but then the hot water takes a long time to get to 60 degrees. The boilers are getting oldish (20 years) and according to british gas some parts not replaceable although not yet at the stage of causing a servicing problem.

    Since installing evohome the heat expansion noises from the pipes are worse because of the TPI. The house is much more evenly heated though.
    So - first thought is whether opentherm would help - but can it control two boilers at once? (Assuming I swapped for opentherm capable boilers). If it would help, I also wonder whether I could actually have one boiler and the system was originally over-specified.

    The water heater is a megalow - we've never had the radiators flushed through or anything, so I do wonder if we did that whether it would heat more efficiently needing a lower temperature and reducing heat expansion noise from the pipes that way too.

    Any ideas gratefully received - just trying to avoid replacing all the pipework!
    Ideally you need it set up for Hot Water Priority, where you can have two different flow temperatures on some systems. Vaillant's own controls system allows you to do this, but as you're already invested in Evohome it won't be much help.

    I'm not sure Opentherm is going to work well for you? If you've got creaking pipes it sounds like you need to investigate further and get loose pipes clipped etc. Seeming EvoHome requests 100% demand when any zone is more than 1.5 degree below set point, over 29 rads (Assuming you've got rad valves) and poor insulation it's more than likely going to be requesting 100% demand most of the time.

    Replacing boilers, depending on the outputs of your existing boilers you could go down the route of a light commercial one. Vaillant's are pretty good, but if you've got the room I'd always have two boilers. You've always got a back up then, along with in summer you can just run one for the HW. On a system so big you'll be needing a LLH too and a decent pump.

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    Quote Originally Posted by mtmcgavock View Post
    I'm not sure Opentherm is going to work well for you? If you've got creaking pipes it sounds like you need to investigate further and get loose pipes clipped etc. Seeming EvoHome requests 100% demand when any zone is more than 1.5 degree below set point, over 29 rads (Assuming you've got rad valves) and poor insulation it's more than likely going to be requesting 100% demand most of the time.
    I guess I'm possibly conflating two things. I'd like to get better efficiency out of my heating. If that helps with the creaking pipes then great, if not I'll have to deal with them. They're under the floorboards and I need to replace the carpet anyway but was trying to avoid a big floors up project!

  9. #9
    Automated Home Legend paulockenden's Avatar
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    I suspect insulation / double glazing / etc. would be a much more efficient use of compared to heating systems or boilers.

    Not saying don’t do it, just that there might be better priorities.

  10. #10
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    Quote Originally Posted by mtmcgavock View Post
    On a system so big you'll be needing a LLH too and a decent pump.
    Ah yes. I have a LLH (actually closely-coupled tees) because the boiler's built-in pump didn't have enough head left after the losses in the HEX.

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