Pressure increase in cold water pipe during hot water heating

  • Erstellt am 2023-04-07 20:55:41

de_mic_ous

2023-04-07 20:55:41
  • #1
Hello everyone,
we have now been living in our house, built in 2006, for 1 year. Since we moved in, I and 2 heating specialist companies have been struggling with the following problem, which we have not been able to solve (please pay close attention to the distinction between hot water (Warmwasser) and cold water (KW):

About the installation:
On the ground floor (EG): cold water line comes into the house—> water meter—> shut-off valve—> pressure reducer, set to 3.5 bar—> cold water line continues…
On the upper floor (OG): heating room with gas boiler Viessmann Vitodens 200 and Vitocell hot water storage tank. The problem only concerns the hot water part, so I will describe this in more detail: The hot water tank has a KFR valve with backflow preventer as well as an 8 bar safety valve on the cold water inlet. Then, of course, the outgoing hot water pipe including shut-off valve as well as a circulation line with (presumably defective backflow preventer—more on this later) and a circulation pump. There is no expansion vessel (MAG) installed for the heating’s hot water part. There is no separate pressure gauge on the hot water storage tank.

About the problem:
Every time the hot water tank heats up, the pressure at the pressure reducer on the ground floor (i.e. on the KW line) rises until the 8 bar safety valve on the cold water inlet of the hot water tank triggers and releases. It is clear that water expands when warmed and has to go somewhere, but it must not get into the cold water line. I should thus not see this expansion (= pressure increase) at the pressure reducer of the KW supply line on the ground floor.

Both heating companies say essentially the same thing: hot water pushes into the KW somewhere when it heats up. Both also say there are theoretically only “2” points where hot water can “pass over” into the KW.
a) At the cold water inlet to the hot water tank
b) In any of the installed fittings
—> otherwise, there would be no direct contact point between KW and hot water in the entire installation.

Previous, unfortunately unsuccessful, measures:
1. The KFR valve on the KW supply line at the tank was replaced. When this valve is shut off, no hot water runs anymore, so it closes properly. Nevertheless, the described problem still occurs even when this valve is completely closed. Thus, at least in my understanding, this point can be excluded as the cause.

2. I have replaced 4 fittings so far, starting with the oldest. These were 3 single-lever mixers, replaced by Hansgrohe single-lever mixers, as well as a surface-mounted thermostat fitting in the shower, also replaced by Hansgrohe. Unfortunately no improvement; the problem still occurs.

Further observations:
If I close both the KW supply and the hot water outlet from the tank, the problem still occurs! From this I conclude that the backflow preventer on the circulation line must be defective, as this is the only route left for the water to go. Unfortunately, I cannot (yet) shut off this line. In principle, I think it does not matter because there shouldn’t be a bridge between hot water and KW here.

Now to my questions:
1. Would you agree with the thesis that it can only be a fitting that allows hot water into the KW?
2. If yes, is there a way to locate the respective fitting?
3. Am I overlooking something? Logical error?

After a year of trial and error, I am really very grateful for any advice.

Thank you very much in advance!
 

de_mic_ous

2023-04-07 21:01:52
  • #2
I should briefly mention why 2 companies are involved: Company A says the pressure increase is not a problem at all for the house installation. The KW pipe easily withstands double the pressure. Therefore, their attempts are rather "half-hearted." I don’t trust the matter, so I also hired Company B. They say the problem must be urgently resolved. And as a layman, I am now stuck in between…
 

KingJulien

2023-04-08 11:51:38
  • #3
Phew, well described, still complicated to understand. I have read it twice now and can only say that I find your conclusions plausible. No idea how to identify the supposedly defective fitting there. Flow noises? (Stethoscope?) Temperature rise? Shut off the hot water and open all hot water fittings during hot water preparation? Just a spontaneous brainstorming. But how is it actually supposed to work? Where is the hot water supposed to normally spread during preparation? If there is no expansion vessel, it can only go somewhere via a safety valve?
 

de_mic_ous

2023-04-08 12:18:03
  • #4
Thank you for the answer! Exactly, the water usually does expand in the KW inlet, but after a short distance the KFR + safety valve come. And there it should release the pressure. But as I said, if everything works properly, I shouldn't see that at the pressure reducer at the house entrance... I guess there is nothing left but to replace/repair the remaining fittings by chance...
 

Fuchur

2023-04-08 12:47:37
  • #5
Turning off the angle valve should be enough for now (where one is present)
 

KingJulien

2023-04-08 13:59:50
  • #6

Right, just close them all first. Then nothing should reach the KW pressure reducer. And then open them one by one, then you will find the defective valve, right?
 

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