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Viessmann 100 short cycling while warming up, overall low heating?

Discussion in 'Plumbing and Central Heating' started by m_brom, 29 Dec 2020.

  1. m_brom


    29 Dec 2020
    Thanks Received:
    United Kingdom

    Recently I moved into a property built in 2004, which had a boiler replacement in 2014 with a Viesmann Vitodens 100 WB1B, 26kW. I believe the system is open-vent as there is no pressure gauge, tanks in the loft, a HW cylinder, and what looks to be only a feed and return pipe out of the top of the boiler, and a gas pipe plus another plastic pipe (condensate?) out of the bottom, and no others. I know that this boiler can be fitted with a weather compensation sensor but I'm not sure if it has been, or how I can tell.

    Since moving in it has been particularly cold inside despite having the CH turned on often - today for example, the temperature started at ~14°C and rose to about 18°C after having the CH on for around 3-4 hours, and stayed around this temperature despite being on for nearly 9 hours total. I have never seen temperatures in excess of 18.5°C since moving in around 2 weeks ago, even when outdoor temperatures were in the high single-digits. I think the insulation of the house is adequate - snow still remains on the roof from over 24 hours ago, and all windows are double-glazed.

    I have noticed that when the call for CH first begins, the boiler will begin by slowly climbing in temperature (on the display, which I'm assuming is the outlet/flow temp?) but then appears to modulate down as it starts to climb quite quickly, followed by the temperature then beginning to drop and it modulating back up again. This would be fine except once it surpasses something like 40°C it seems to have trouble with modulating down too much, followed by a rapid temperature drop and it modulating all the way up to full bore (5 bars on the gas-consumption gauge), making a slight bubbling noise which could be boiling water? followed by a click as the burner then turns off completely, reaching maybe only 45°C by then. It then does not fire for a good 30-60 seconds, as the temperature drops down to 30°C or lower, before firing up again and repeating the same procedure. This happens for at least 10 minutes after calling for heat however I've not hung around the boiler to see when it stops!

    Once it is up to temperature (set past the dot on the dial at 5/6 which gives a reading of 60°C, I am not sure whether I should try it on 6/6), it runs continuously and stable with minimal gas usage (very quiet, 1 bar on the gas-gauge). The temperature stays rock solid down to the 0.1 figure. I have also checked and the pipe from the 3-port valve to the HW cylinder is just warm once it is up to temperature, the pipes around the pump are too hot to touch.

    There is one single radiator which gets properly hot (but not painful-to-touch), a small 600x450 single-panel radiator with fins in the upstairs landing - medium distance from the boiler, however closest to the pump in the airing cupboard, which is set to 2 out of 3 possible levels (and makes a fairly noticeable but not overly loud noise while on). All other radiators get hot at the top, and just "pretty warm" at the bottom, with the exception of one bedroom radiator which is always only lukewarm (however I suspect this could be a stuck TRV) and the large 1600x700 radiator in the living room downstairs which is hot at the top but barely even lukewarm at the bottom. The lockshield valve on this radiator is fully open (no TRV), but I've yet to check any of the others, however all 6 TRVs fitted on other radiators are fully open.

    Since the boiler has had an annual service every year before we arrived, and the new boiler was fitted only 6 years ago, I've been putting off a balance of all eighteen (!) radiators in the hope that they should be properly balanced already - probably naive of me, but judging by documentation left by the previous owners there has been no change in pipework at all since the house was built aside from the fitting of the new boiler due to uneconomical repairs needed on the original. On this topic, should I be aiming for a 20°C drop with this boiler rather than 11°C if I do balance them?

    Other things to note are that the system is fitted with a magnaclean (though I am not sure of its location), and is due a service as it was last done around May 2019. The previous owners were an elderly couple so the house was unoccupied for a few months before sale, hence the lack of 2020 service - this will be arranged after the holidays! - but I am not sure whether the boiler was turned on at all during the summer/autumn period or left off. Additionally, the radiators are all fed with plastic piping which has an outer diameter of ~10mm. I don't know whether any inhibitor is present in the system or has ever been used.

    Office [upstairs] - 1x 900x450 single/fin/TRV
    M. Bed. Ensuite - 1x 400x600 single/fin
    Master Bedroom - 1x 1000x450 single/fin/TRV (not warm), 1x 700x450 single/fin/TRV
    Bathroom - 1x 500x600 single/fin
    Bedroom 2 - 1x 1100x450 single/fin/TRV
    Upstairs Landing - 1x 400x450 single/fin
    Hall (front door) - 1x 600x450 single/fin
    Dining room - 2x 600x450 single/fin, 1x 900x450 single/fin
    Kitchen - 1x 1000x700 single/fin
    Downstairs WC - 1x 400x450 single/fin
    Utility - 1x 400x450 single/fin
    Conservatory - 1x 700x700 single/fin/TRV (set to frost-protect)
    Living room - 1x 1600x700 single/fin (cold at bottom)
    Garage - 1x 1100x700 single/fin, 1x 900x500 single/fin/TRV (50mm deep, all other rads are 60mm deep)

    Apologies for the huge wall of text but if anyone has any advice or ideas on what's wrong it would be appreciated!
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