Vailant Turbomax Plus pressure fluctuation and ideal CH temperature

10 Jan 2021
Reaction score
United Kingdom
Hi All, I recently moved to a new place. I am noticing a significant fluctuation in the pressure at operation (given my limited past experience) in the gas boiler and I just wanted to check whether that is ok for this model/seek some advice.

-Vailant turbomax plus
-small 2 floor maisonette
-6 radiators
-boiler downstairs
-no air in the CH, removed all air from highest point in the system
-The "green" area on the pressure gage is between 0.8 and 2.0 and the "red area" is 3.0 (the max).

I noticed the below:
  • When super cold (eg. heating left switched off for a week and outside temperature is 0), the Pressure can drop to 0.4-0.5
  • When operating and CH water is hot, the pressure rises significantly:
    -If I set the CH temperature to 70, the pressure will rise to 2.5 bar approximately at max temp
    -If I set to 64 when it will rise to 2.0 bar at time of max temp and while operating.
  • When CH it reaches desired temperature and stops, the pressure will rapidly drop to the normal range.
  • If I switch the heating off for 10 hours or so, the pressure will still be in the greens, so I believe the pressure is where it needs to be and there is no loss of pressure. If I top the system up to have 1 when cold, it will get to 2.5 when the system is at 64.

1) Is this behaviour normal, given the installation specifics?
2) What is the recommended temperature for CH as a rule of thumb for this boiler? I find sub 60 very slow and incapable of producing adequate heating.
3) What is the maximum pressure I should be having at max CH temperature while operating?

Answers will be greatly appreciated.
Sponsored Links
1) it suggests that the expansion vessel is not working correctly, so it cannot absorb the expansion of the water as it heats. It may just need pumping up with air, or it might need replacing. Your gas engineer can fix it.

2) 60C is usually the most economical temperature for the boiler to run, but this week is unusually cold, so you can turn it up. An important question: Are all the radiators fully hot (top, bottom, sides, middle?) How many cubic metres of gas are you using per average winter's day?

3) around 1.5 is fine but it should NOT fluctuate as much as yours does. It should not go into the red sector. Stop topping it up when cold and get it repaired.
Hi John, many thanks for the super quick response - appreciated.

Re 2) I do not have a thermal camera or something, but (to touch) all radiators seem to be hot across the panels - no noticeable difference to suggest blockage or anything. I have just moved in, so I would not know averages over the whole winter. I did take a note and have had the heating on 24/7 for the past 48 hours - that used up about 9 cubic metres (edit: 9 per day!).

Re 3) I do not top it up to start it, starts ok at sub optimal pressure, and within a few seconds it gets in the greens- any concerns with that? I will keep the part of needing an engineer to assess the expansion vessel situation.
Last edited:

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