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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorjms452
    • CommentTimeFeb 3rd 2019
    Hi all,

    What is the current consensus on high efficient high reliability regular gas boiler?

    Heat demand is lower following renovation so the 15kW boiler we've got could possibly dropped to 12kW.

    Valliant - ecoTEC plus 412 (4.7 - 12.8kW)
    Atag - i15S (14.7kw)

    The lower & variable heat output and wider range of repair expertise is probably pushing me towards the Valiant - but interested in opinions here. Also is there anything I should be looking for or requesting to make the system work better while I'm at it?

    Old system boiler - embarrassing efficiency about 75% for heating
    9 largish mostly double radiators on 15mm pipes
    well insulated hot water tank and two cold tanks in loft.
    basic controls.

    Considered the FGHR combi option but decided against this as:
    We may add solar hot water so want to keep the hot water tank
    We live in a supper hard water area and I'd rather have scale in the hot water tank than in the boiler (and don't want to add a water softener).
    • CommentAuthortony
    • CommentTimeFeb 3rd 2019
    No doubt about in my mind —-Veissmann every time for domestic
    • CommentAuthorjms452
    • CommentTimeFeb 3rd 2019
    <blockquote><cite>Posted By: tony</cite>No doubt about in my mind —-Veissmann every time for domestic</blockquote>

    Thanks Tony - Something like this then?

    I notice they have weather compensation options -is this an additional easy win?
    • CommentAuthordereke
    • CommentTimeFeb 4th 2019
    I had a Viessman fitted to our last house (on Tonys advice) and I was very happy with it. The weather compensation was brilliant. This was the one I had https://www.viessmann-us.com/en/residential/gas-boilers/condensing-boilers/vitodens-222-f.html

    However.. if you've lowered your heat demand would it not make sense to look at an ASHP?
    Much less CO2 will be emitted.
    • CommentAuthorbhommels
    • CommentTimeFeb 4th 2019 edited
    After reading about Viessmann on this forum, we now have our ancient back boiler replaced by the lowest power vitodens 200, with weather comp. Although it is early days, it looks like a brilliant piece of kit.
    I wanted the 200 as it can do 2 circuits with independent temperature settings without requiring additional mixing valves etc: one cicruit for the UFH and low temp radiators, the other for the hot water tank.
    And now for a bit of shameless self promotion: only after committing to the boiler did I think through the various options, and I my thoughts (for what they are worth) are here:
    • CommentAuthorjms452
    • CommentTimeFeb 4th 2019
    We could do the ashp option in terms of heat loss - I'd estimate around 3-5kW heat requirements based on the boiler duty cycle on a really cold day so 6kW or so should work nicely.

    That said it looks like an ashp would:
    -cost more to run than a mains gas boiler.
    -cost more to install than a mains gas boiler.
    -We'd probably need to top it up with an immersion erroding (any) CO2 benefits.
    -Most of the heat load would also be on winter evenings which is horrible for the grid.

    +On the plus side it would feel complementary with 4kW of PV.

    Am I missing something? ASHP doesn't seem that great with the current electrical grid when mains gas is available. (Sounds a bit like you cam to this conclusion bhommels?)
    • CommentAuthordereke
    • CommentTimeFeb 4th 2019
    My situation is a bit different as I don't have mains gas so for me it works out better to go with an ASHP.
    However where it is more marginal I'd like to think I would still go for the heatpump because the grid is getting cleaner and you can buy from a renewables only supplier that invests in more renewables.

    Personally I think going with gas now is a bit like buying a new petrol/diesel car - you run the risk of it becoming a stranded asset.

    Obviously you've got to go with what you think works best for your situation, finances etc. so if you do go gas get Veissmann with weather comp :-)
    • CommentAuthorjms452
    • CommentTimeFeb 5th 2019
    Hi Dereke,

    With no mains gas, ashp would make sense. I wish these has been more mature 20 years ago in our old house as an oil boiler and was expensive, high CO2 and the tank took up lots of garden.

    Now with mains gas it looks like Veissmann with weather comp wins for us but thanks for the suggestion. I'd much rather consider the wider options when I put something in place for the next 15 years.

    • CommentAuthorbhommels
    • CommentTimeFeb 5th 2019
    Posted By: jms452
    Am I missing something? ASHP doesn't seem that great with the current electrical grid when mains gas is available. (Sounds a bit like you cam to this conclusion bhommels?)

    That is what the numbers told me. Taking into account all factors, ASHP is nor cheaper nor lower CO2 than a modern gas boiler, unless the efficiency is 250% or higher. And then still it is massively more expensive. If you are off-grid the picture changes somewhat, although running a gas boiler off LPG should not be that much more expensive than the standing charge for mains gas.

    The CH system is now designed to work with very low temperature flow: the UFH has no mixing valve as the Viessmann can drive it directly (by modulating), and the upstairs rads have been replaced with low T ones, calculated to be OK when running off the same flow temperture as the UFH.
    So by the time the Viessmann reaches end-of-life, it will hopefully be a relatively straight swap for a heat pump of some kind.
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