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Thread: Boiler can't modulate down low enough

  1. #1
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    Default Boiler can't modulate down low enough

    Hi, just an observation, I've noticed that when I'm sat in my home office with just the office radiator on (evohome shuts down the rest of the house during the day) my boiler can't seem to modulate down low enough. I have a bypass rad in the bathroom, so only have 2 "active" radiators when I'm working in the office. However when the boiler is indicating it's lowest flame height, I can see the flow temperature slowly rising up on the boiler display until eventually it hits 78 degrees and cuts out - if I run up to the airing cupboard I can see that the pump is still running so looks like the boiler is cutting out due to excessive flow temperature. I can watch the displayed temperature then fall quite quickly as water continues to circulate, and a few minutes later the boiler fires and the same thing happens again, after a few minutes it cuts out again and so on.

    I've got a Viessman Vitodens 100 and I think I read the minimum output is around 6kw, I also read that they've produced a newer version with an even lower output, but my boiler is 3 years old which is before this new one came out. So I'm wondering if there's anything I can do - should I open the TRV on another radiator and effectively have a second bypass rad, or just not worry about this - I'm not sure if it's more efficient to keep the boiler fired on minimum flame height or allow it to cycle like this, I don't know what uses more gas because when it fires it does not fire at minimum flame height, it starts off a bit higher as you can hear the fan spin up and then slowly it slows down as the boiler settles into "minimum" mode, so there is a period during the ignition phase that it might be using more gas then if it were continuously running at it's lowest flame height.

    Anyone got any thoughts?

  2. #2
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    I have the same boiler and it behaves the same way. 26kw with a 7kw min. Its been in 6 years and has been very economical, my usage is way below that of the EPC, and until last week (when the flow switch died) had never had a problem with it.

    there is the "traditional" school of thought that says that boilers should be run for longer periods to avoid short cycling wear and tear, but there is other information on the internet specific to Viessman that says "don't worry about it, they're specced to cope with this".

  3. #3
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    Yes mine is also 26kw it's the WB1B variant, open vent heat only boiler, if you have the same, what position is your temperature dial? I have to turn mine to half way between position 6 and "reset" to get 70 degrees flow temp. So far I've been very pleased with it, I did take out the 10 year warranty so hoping I can rely on that if I have any problems. This will be my first winter with evohome so will be interesting to see what my gas usage is like.

  4. #4
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    I have exactly the same setup during the day.

    I have a 38Kw boiler (please don't ask why!) but then as I started making changes, my heating requirements kept going down not up. I now have my boiler de-rated to 18Kw. But that is not the point of the discussion here.
    What I found out was that by de-rating the 38Kw boiler to an 18Kw, I actually could hit a new lower threshold, that I couldn't get to before. This also makes some sense because Vaillant sell the same boiler in an 18Kw, 28Kw and 38Kw versions. Most of the specs are identical except the 18Kw has a lower min than the 28 and 38Kw versions. Just by de-rating the boiler, I was able to get to this new low, which is not on the spec sheet of the 438 boiler that I have.

    So might be worth looking at.

    With a zoned system, you are almost never going to need the full strength of the boiler, if you schedule your zone smartly.

  5. #5
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    its in a sealed system but exact same boiler. I run mine at "4" as much as poss, flow temps typically 50-55. thats enough to heat my house (210m2, 1973 brick detached). Of course every house is different, but worth you giving it a try to run it cooler?

    derating - I have read about this but various commentary saying not needed with a modulating condensor. How did you get it done? Is it a specialist job or any gas safe engineer?

    interestingly - after my flow switch died this week - I had to "cold boot" the house on friday morning after the service engineer fixed it. about 4C outside, 14-15 inside (no heating two days). Put every zone on to 20C. Based on the gas usage (smart meter, tracking it with hildebrand DCC via homeassistant) the highest it ran was 14kw, for about an hour. then down to 5kw and then 3kw.

    there is a such a lot of oversizing. still seems the case with heat pumps as well - I've spoken to a couple of companies and they've been muttering about 16-18kw pumps, needing 3 phase supply etc.
    Last edited by sandyman; 8th November 2021 at 10:11 PM.

  6. #6
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    Quote Originally Posted by sandyman View Post
    I run mine at "4" as much as poss, flow temps typically 50-55. thats enough to heat my house (210m2, 1973 brick detached). Of course every house is different, but worth you giving it a try to run it cooler?
    Ah I should have said, I'm on y-plan here so need minimum 65 degrees really to heat the hot water cylinder. Even to get that temp I have to have the dial on about 5.75. If I run it on 4, or wherever the "dot" is (supposed to be for maximum efficiency) I don't get anywhere near enough heat into the water for the HW cylinder or CH. I was quite surprised tbh having to have the dial up so high, didn't know if that was "normal" or whether something's not right. I guess slowing the pump down might help to get more heat into the water but then some of the radiators don't get very hot due to lack of flow.

  7. #7
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    hi
    I was wondering why you were setting it so high. fortunately, I have the exact solution for you as I have been there and done that! There is a demand terminal box for these boilers. https://viessmanndirect.co.uk/Catalo...al-Box-7574319
    connect the output side of that box to your boiler via the low voltage block terminals (diagrams in the manual for the box). the input side needs to be switched on (@230v) when you have DHW cylinder demand so this comes from your HW bdr or your HW zone valve end switch.

    when the demand box is activated, the symbol on the boiler display will change from a "radiator" to a "tap" and the boiler will target a flow temp of 79C, regardless of the setting on the 1-6 dial.
    very few installers know about this (mine didn't , I had to figure it out for myself) and its not very well documented either. you can read my saga of posts from 5 years ago if you want more background / the full war story.

    that should allow you to set the dial to a more realistic level for radiator-only flow temp.

    couple of things to bear in mind:
    if the rads are on, then there will be cold return water from the rads mixing with the return from the cylinder. that makes it harder for the boiler to get up to 79C flow - it really doesn't like running at a wide delta-T.
    My system is plumbed as X-plan (like S-plan, but the rad zone valve is N.O, and the HW zone valve N.C. , when DHW demand is active they reverse) meaning that rads are all shutdown when cylinder demands.
    After the cylinder has finished you'll have a primary circuit full of 79C water. if some rads are on it will dump there. If you have small children be aware - a 79C rad is not safe to touch. If rads are not on , you have to be careful about boiler overheat . I found the pump overrun of the boiler didn't last long enough to get all the heat out of the boiler's heat exchanger and so would get overheat (F5 I think) errors. I manually wired a 5 minute delayed pump off relay in and that sorted it.
    I ran like this for 4 years and it works fine.

    I have since however bought a much higher spec cylinder with a heat pump spec coil, and I heat it with offpeak cheap electricty (octopus go) - more or less that same price as gas per kwh (or better, given the current situation) and/or surplus PV, so I rarely reheat the cylinder on gas now. the heat pump grade coil is future proofing, I don't have a heat pump (yet).

    ta
    Ian
    Last edited by sandyman; 9th November 2021 at 09:54 AM.

  8. #8
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    Quote Originally Posted by sandyman View Post
    I have since however bought a much higher spec cylinder with a heat pump spec coil, and I heat it with offpeak cheap electricty (octopus go) - more or less that same price as gas per kwh (or better, given the current situation) and/or surplus PV, so I rarely reheat the cylinder on gas now. the heat pump grade coil is future proofing, I don't have a heat pump (yet).
    I am currently in the process of switching to Octopus Go (in spite of recent tariff increases, owing to mistakes made by British Gas which blocked my switch, Octopus are honouring the previously agreed rates). So your mention of using the cheap overnight rate caught my eye. What are you using to control the electricity supply to the immersion heater?

    I use Evohome (with Intergas Xclusiv / Joule HG Unvented / X-Plan) to control DHW demand so something to the side of that to use cheap electricity might be a good fit (although I haven't actually done the sums yet).

    TIA

  9. #9
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    Quote Originally Posted by Jon_Boy View Post
    I am currently in the process of switching to Octopus Go (in spite of recent tariff increases, owing to mistakes made by British Gas which blocked my switch, Octopus are honouring the previously agreed rates). So your mention of using the cheap overnight rate caught my eye. What are you using to control the electricity supply to the immersion heater?

    I use Evohome (with Intergas Xclusiv / Joule HG Unvented / X-Plan) to control DHW demand so something to the side of that to use cheap electricity might be a good fit (although I haven't actually done the sums yet).

    TIA
    I have a MyEnergi Eddi to dump PV into the cylinder. Since october I have set a manual boost on that between 0030 and 0430. That means it runs at full power between those times (until the tank is up to temp, obviously). I'ts pulling down around 9 or 10kwH each night. 50p's worth of electricity, doing the family's hot water for the whole day. If we get a sunny winters day, it dumps to the powerwall first (which also charges overnight on Go) and second to the cylinder - so almost never exports.

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