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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.

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    • CommentAuthordelprado
    • CommentTimeMar 6th 2018
    I'm thinking about Viessmann, feedback wanted!
    • CommentAuthortony
    • CommentTimeMar 6th 2018
    • CommentAuthorDarylP
    • CommentTimeMar 6th 2018
    WB are OK, but their after-sales and repair services seem to be falling below standard recently.... :-(
    • CommentAuthorKenny_M
    • CommentTimeMar 6th 2018
    I'm in the market for a new boiler too. All of the installers who have come out have quoted for the same WB Greenstar CDi classic range (30kW heat output). I had one of these in the previous house and didn't have any bother with it, although I think it was maybe only around 6 years old when I moved.

    They have sized 30kW for the house which is fair enough based on the normal way of doing things, but i'm holding off at the moment because I am not convinced that I will ever need as much as 30kW based on my own usage, and the intention improve insulation etc. In fact I am in the process of working out the maximum heat output of my rads because I am not convinced that that even in high temp operation that they would total more than 24kW.

    I am more concerned with ensuring that the new boiler is sized so that it will run in condensing mode as much as possible and I was struggling to get what I thought would be basic information about efficiency over a different range of heat outputs from WB website. I posed the question to their support team, and got a call back from a guy, who was very helpful but seemed surprised that a member of the public was asking these kinds of questions! :)

    My understanding is that in order for the boiler to be running in condensing mode my rads could only run at around 60% of their nominal heat output. The other thing that came up was that according to the guy on the phone the CDi boiler lowest modulated heat output is around 8.5kW, and for large proportion of the year, when only a few rads are on, that means it will be switching on and off all the time, lowering the efficiency.
    • CommentAuthortony
    • CommentTimeMar 6th 2018
    The one suggested by the op has a ss heat exchanger with no microfins that can block us, I regard them as the RR of boilers. Well engineered, high quality few problems

    Probably engineers are better working for companies that have lots of problems and hence plenty of work.
    • CommentAuthorArtiglio
    • CommentTimeMar 6th 2018 edited
    There are many good boilers out there , baxi, main, wb are ones I currently deal with. Plus a number of 12-15 year old aristons and a truly ancient Balmoral and an even older Ideal. None give any real problems.
    When i need one replaced, i ask the merchant and heating engineer and go with their recomendation.

    In what must be approaching an equivalent of 150 years of combined use. Never replaced a pump, main heat exchanger, gas valve, expansion vessel, pcb.

    Most common issue was plate heat exchangers in the ariston combis, after this its air pressure switches, then maybe 3 fans, diverter vlave plungers ( o rings eventually harden and jam) , couple of flow sensor diaphragms.

    More important than the boiler is keeping the system clean, use inhibitor and have a magnetic “filter” ( not replaced a plate exchanger since adding these to systems) Flux that is allowed to stay in a system will ruin expansion vessels . Use a system cleaner every 3 years and then flush and refill with inhibitor and you give the boiler its best chance of working as it should.

    Beyond that sizing the boiler will probably be down to you, installers generally opt for “the next size up” from an already generous calculation. Plus if its a combi thats installed the extra kw’s make for better dhw supply.

    Some of the manufacturers offer long warranties, but anecdotally they’re pretty much worthless beyond the first year, with just about any issue in the system installation used as an excuse not to pay out.
    Safety outlet not considered to have been bent back towards wall far enough, external rubber flange on flue not tight enough to wall, just two recent examples. In each case had nothing to do with boiler fault.
    • CommentAuthorKenny_M
    • CommentTimeMar 6th 2018
    Posted By: tonymicrofins that can block us

    ? Is this a typo, or what is "us"

    Posted By: tonyProbably engineers are better working for companies that have lots of problems and hence plenty of work.

    :) I don't doubt it, but don't WB have a good reputation for reliability too? I only had mine for 6 years from new in the last house, but I didn't have any issues during that period at least, and I have heard other reports of the WB being reliable.

    I hear that in general modern boilers only have a 10-15 year lifespan, don't know if the Viessmann bucks that trend or is likely to be the same. This seems pretty disappointing considering my current boiler is still ticking away presumably nearly 30 years since it was installed - much lower efficiency than a modern boiler granted.

    I had a quick look at a Viessmann Vitodens 30kW combi online, its got a better modulation ratio (6:1) than the WB (4:1) which was one of the things I was looking for. Doesn't seem any more expensive than the WB either.
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018
    Posted By: Kenny_Mmicrofins that can block us
    microfins that can block up
    • CommentAuthorKenny_M
    • CommentTimeMar 7th 2018
    Posted By: goodevansmicrofins that can block up

    I should have got that, maybe earlier in the evening! :)

    Posted By: ArtiglioMore important than the boiler is keeping the system clean, use inhibitor and have a magnetic “filter”

    Couldn't agree more. First thing I did when I moved here was drain the system down, flush then fill with inhibitor. Magnetic filter on the list when boiler gets replaced.

    Posted By: ArtiglioBeyond that sizing the boiler will probably be down to you, installers generally opt for “the next size up” from an already generous calculation.

    I am getting exactly that impression, and I don't blame the guy. They have to size for the house, and the worst case scenario, as some customers will still expect their 1850's uninsulated property to be at 22 deg in winter. I'm happy to take the responsibility for sizing, once I have done the calculations, but as I have been deliberately running the boiler at 60deg all winter, I should be able to roughly size it based on the combined radiator outputs, and my gas usage history.
    • CommentAuthorsnyggapa
    • CommentTimeMar 7th 2018 edited
    as well as modulating, the ability to control the modulation is important. We recently went for a WB greenstar 30cdi (seems a common device) with a wave controller and magnetic filter. That collection of bits meant that WB guarantee the lot for 10 years which is pretty good in my book

    The wave controller is really necessary here - other controllers can only do a basic on/off control to the boiler which would then rely on the boiler stat to limit the temps. The wave is clever enough to learn the heat response of the house and modulate right down and run intermittently which at least in our place keeps the temperature at a very even level, which was good for comfort (and no desire to turn it up a degree to compensate for fluctuations) as well as boiler efficiency.

    It also knows the outside temperature (gets that from the Internet somehow..) and can use that in it's algorithms, but i haven't needed to turn that on.
    • CommentAuthordelprado
    • CommentTimeMar 7th 2018
    As a side note does anyone know anything about storage combis like the 170l Veissman do?
    Mine is a storage Combi - the 222F. I'm pretty sure we've got the smallest 19kw with a 100l tank.

    What do you want to know? It works. You get mains pressure water at multiple points simultaneously and you never seem to run out.
    • CommentAuthordelprado
    • CommentTimeMar 7th 2018
    So basically it’s a pressurised system, like a conventional boiler and, say, a megaflow attached in terms of performance but with the advantage of a combi boiler (assuming system boilers are somehow different from combis in anyway I don’t inderstand)

    What is interesting about your comment is that in thought for a pressurised sustemnthey normslly recommended 250l for a 4 bed house
    • CommentAuthordelprado
    • CommentTimeMar 7th 2018
    Blimey just looked at price why are they so expensive?
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018 edited
    It looks like there is about 100L of storage in the 222F but If you size the burner right the store will be enough to get over any short term peak usage. e.g. when three showers are running there may not be enough power in the burner to keep up and so the store is slowly used until one shower finishes. Its the combination of burner and store that allows a better result for the user 99% of the time. The other 1% of the time is when 2/3 baths are being drawn at the same time.
    • CommentAuthorgoodevans
    • CommentTimeMar 7th 2018
    Price wise I suspect it may be not much different to a boiler and store and the pipes/pump between.
    • CommentAuthorSimon Still
    • CommentTimeMar 8th 2018 edited
    Goodevans - yes, that's about it.

    - you have small tank, running at mains pressure like a 'megaflow'.
    - rather than heating the tank with a coil the boiler pumps cold water from the bottom of the tank, past a heat exchanger, and puts it back at the top .
    - The 19kW unit will deliver 18L a minute at 35C temp rise you're only digging into the cylinder capacity when you're using more than that. eg you could run 3 x 10L showers simultaneously for 8 minutes until you exhaust the tank.
    - in theory when the tank is exhausted it behaves like a combi so at that point water pressure would drop but you'd still have hot water.

    Actually, performance is better than that - your showers aren't using 10L of pure hot water. Not everyone leaves the shower running the whole time which gives the tank a chance to catch up, as does one person getting out of the shower and the next getting in (theoretically it can bring a depleted tank back up to temperature in 6 minutes).
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