Help with Viessman Vitodens 100W temperature

9 Jan 2022
Reaction score
United Kingdom
I would be grateful fir any advice re the following.
We have moved into a house with a Viessman Vitodens100W, believed around 7 years old, serviced by previous owners’ plumber last June. Separate hot water cylinder.
We noticed that on cold days the radiators were only lukewarm even though room thermostats were calling for heat, so rooms never heated (set to 20.5 deg but only reaching 15-17). I checked the boiler display - water temp 57 deg, so I used the dial to increase temp, up to around 69, which required turning dial almost as far as it would go. Seemed to improve heating. Yesterday, very cold outside, again radiators lukewarm, rooms not reaching desired temp. Boiler showed 63. Temp dial had another mm of travel so I turned as far as poss, temp went to 68, seemed to improve radiator temp. Manual says should be able to set heating water temp up to 80 deg, but clearly that is not poss on this boiler. Does this indicate fault with boiler??? Thanks in advance!!
Sponsored Links
Do you know the power output of your boiler.

How big is your house?

Is the hot tap water surprisingly hot?
3 bed semi detached but quite large (circa 2000 sqft) and old and poorly insulated (at present). Boiler is I believe 17kW. Hot water from taps is pretty hot (not measured temp).
Sponsored Links
We had the plumber round who carried out the service (to fix a leak in the condensate pipe). Seemed very competent (hard though it is to judge). I guess I should ask him the question I posted here (but wanted to get others’ opinions first so I’m not flying blind)
There is the possibility that the boiler maximum firing rate is set too low on the gas control valve. You could try reducing the load on the boiler to see if that makes a difference to its output temperature. See what the maximum temperature you can get is (you've mentioned 68c) Turn off at least half of the radiators that are currently 'warm' - wait half an hour for things to stabilise and again see what the boiler temperature is. If it's gone up a fair amount then it's possible the boiler maximum firing rate is set too low which will require a service technician to correct. He would normally check this as part of a boiler service.
OK so the heating has two zones, upstairs and downstairs. Upstairs has been off for an hour, downstairs still on, so load on boiler reduced. Water temp in boiler with setting dial as high as it will go, is 68deg. The 5 bar indicator indicating how hard boiler is firing (?) shows 3 bars. Downstairs radiators are hot, but not so hot you can’t keep your hand touching them indefinitely. (And room is 3 deg below room thermostat setting of 21 deg. TRVs are on max.). All advice gratefully received.
That gauge is the system pressure and should never rise above 3bar (the point at which the relief valve should lift) If it's at 3 bar then you may have an over-pressurising problem or a failed expansion vessel as normally you'd expect the pressure to be about 1 to 1.5 bar when cold and about 1.5 to 2 bar when warm. You need to get your plumber to check the boiler firing rate. Radiators shouldn't be so hot that they scald.
Do you know the power output of your boiler.

How big is your house?

Is the hot tap water surprisingly hot?
Hi John
Your questions implied you maybe had some light to shed on this. I have answered them in later posts in this thread. Would you care to shine your light? :)
Sorry, I have caused confusion. This is not a pressure gauge showing pressure in bar! It’s a little indicator in the LED display, which looks just like the old wifi/mobile signal strength indicator Shows 1-5 visual bars rather than bars as a measure of pressure

that is the flame size indicator. 5 vertical bars would mean it is burning at maximum power (this is rarely seen unless, say, you have come home to a cold house and it is trying to heat all the pipes, water and radiators from completely cold. IIRC in my house this takes something like 20 minutes before it modulates down, and thereadter provides only enough heat to keep the radiators hot. The fewer radiators you have, the less power this will take. I believe it tries to maintain a 20C difference between flow temperature (hot water leaving the boiler) and return temperature (water coming back to the boiler after passing through the radiators) for economy.

Each radiator should be fully hot, top, middle and sides, but the top rail should feel hotter than the bottom one. Feel them. If they have hot patches and cold ones, the position gives clues. As a rule of thumb, the Flow pipe at the incoming radiator valve should be "too hot to hold" and the Return pipe at the outgoing radiator valve should be "too hot to hold for long." While heating the house, all the radiators should be about the same temperature, until the rooms reach target temperature, and the TRVs (if any) reduce or stop the flow. do you have any radiators that are surprisingly hot?

You say the radiators are only lukewarm, which suggests that the heat from the boiler is not reaching them. If this was due to a sludge blockage, the boiler would quickly reach its target temperature and turn off until it cooled down. If it was due to the cylinder taking all the heat, the tap water would be very hot. If it was due to the boiler having been range-rated too low, the boiler temperature would be low.

Does your boiler fire continuously when you are trying to heat the house? Or does it fire for some minutes, then turn off for some minutes, them back on?

Relevance of the water from the hot taps (you need to buy a thermometer to find out) is that if the cylinder is taking heat from the boiler, even when the cylinder is fully hot, you have a fault which will interfere with the CH.

If your boiler does not have a pressure gauge, go into the loft and look at the Feed and Expansion tank, which is small. Is it hot? How deep is the water? How deep is the mud?

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links