I have a Viessmann Vitodens 100-W that heats my house, and I presume delivers hot water too.

I must confess I have been negligent, and never had it serviced. The unit is 8 years old. I will get it serviced soon,provided I can find a licensed service mechanic for it.

Now, I did notice that the pressure gauge reads 13psi, which is pretty much the minimum level according to the manual (the manual does contradict itself by first stating the minimum is 12psi, and later in the text it says 14psi.)

The procedure is not listed in the owners-manual, but is it something that a homeowner could do, add water to increase the pressure?

Is it just a matter of opening a valve somewhere until the pressure gauge shows the correct value, or do I actually have to hook it up to a gardenhose?

gauge of vitodens 100w

UPDATE: Thanks George, I have found something called "Backflow Preventer."

It seems to have a valve both in front and after it? Or is it some sort of other lever, that I see to the left of it?

I am in Canada.


  • We could use some more pictures of your installation. Most boilers have a "make up" water connection with a regulator, it may be turned off. I highly doubt it would be a direct, manual connection to a domestic water system. That's not code legal in the US. What country are you in? Usually there is a check valve or double check valve to prevent potential backflow of boiler water into the domestic water system. ...again, more pics of your system (esp. the plumbing), would help us give you good answers. Jan 22, 2021 at 17:22

1 Answer 1


Lift the lever on top of the pressure regulator briefly, that should allow water to flow in, and raise the pressure.

On most systems, the inlet valve is left closed except for maintenance - they should not normally leak, so they should not need constant refilling. Your inlet valve appears to be open as pictured.

enter image description here

  • Awesome! That worked really well. I'm surprised the manual tells you to call a mechanic, considering how easy that was. First fill up in 8 yrs!
    – Bram
    Jan 22, 2021 at 19:26
  • 1
    Keep an eye on it. It should not be leaking down, and if that repeats (...faster than once in 8 years) you may need to solve a problem before it gets worse.
    – Ecnerwal
    Jan 22, 2021 at 20:04

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