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We have a five year old Viessman Vitodens 100-W. We have noticed that this year the temperature of the hot water is not behaving as it consistently did before. There is an issue especially with the shower/bathtub that when you open the tap fully, the temperature doesn't get as hot. If you only open the tap halfway, the temperature does behave as it used to.

The pressure on the boiler is down to the low green end of the scale.

I've been looking at how I can increase the pressure in the unit but our setup does not match any of the other videos/articles that I have found. I would love to avoid having to call in a technician if I can help it.

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I wasn't able to get back to this until recently. I have replaced the faucet cartridge twice, and we're still getting the strange behaviour of the water from the tub faucet coming out not hot (it can be warm/cool, not completely cold).

I've also gone through every tap in the house with a thermometer and every other one is behaving correctly in regards to the hot water. The strange discovery from that is that the bathtub behaves differently between the shower head and the tub spout even though they are connected to the same handle. The shower head gets hot correctly, but if we turn off the diverter, the water coming out of the tub spout quite quickly goes back to being cold-ish.

the view inside the wall the tub/shower setup

mvhancock
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    Just the shower/tub or other sinks also? Problem might be at the shower taps/faucet instead of the heater. – crip659 Dec 16 '22 at 21:43
  • This unit has a five year warranty. Any chance it's still covered? If so, reach out to your installer immediately. – KMJ Dec 16 '22 at 22:36
  • I'd say it's really just the shower/tub and only when that tap is open to full pressure. The sinks in the rest of the house seems to get hotter and stay hotter more consistently. – mvhancock Dec 17 '22 at 18:35
  • Sadly it's over 5 full years. – mvhancock Dec 17 '22 at 18:39

1 Answers1

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The colder water in the bathroom might be a hint for a problem in one or more shower thermostatic faucets, since those faucets are normally the only devices in private homes which allow cold water getting into the warm water net, if the back flow preventer for the warm water connection inside the thermostatic shower faucet is defect, i.e. leaking. Sometimes the BFP does still work at lower throughput, which would exactly match these issues.

Can be easily checked if there is a (ball) valve upstream of the bathroom faucets in the cold water line. If closed, no more cold water can enter the hot water net. Sometimes the thermostatic faucets themselves have small internal valves for both cold and hot water, to be accessible after removing the trim.

See this similar problem: Why does my mixer shower run intermittently cold?

The low pressure at the pressure gauge should be a pure coincidence, since this shows the pressure of the closed heating water loop. Possible reasons are a defect breather (= the small brass cylinder in the middle of the photos) or a tiny leakage or a defect expansion vessel.

Topping up with water to increase the pressure: Sometimes special water is needed according to the system's documentation (demineralized water, anti-corrosion additives etc.).

The pressure should be checked at roughly same heating water temperatures. If the pressure drops again after a few weeks, the system needs to be checked beginning with the mentioned possible causes.

xeeka
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  • I do not believe our shower is a thermostatic faucet. The house is almost 90 years old and the shower and bath fixtures seem quite old. – mvhancock Dec 17 '22 at 18:51
  • @mvhancock Thermostatic valves are available since 1921. F.e., in the US, made by Leonard/Rhode Island. To check if the reason for the problem is the faucet, the cold water line to the faucet should be closed. That way, no unintended mixing with cold water is possible. If there is no possibility to close that water line, other cold water valves can be opened (in the bathroom, kitchen, toilet flush etc.). This would decrease the pressure in the cold water line, making that unintended mixing in the suspicious faucet(s) less likely. – xeeka Dec 17 '22 at 20:39
  • If the problem disappears or is weaker, this would be a very strong indication of a defect BFP in or next to the faucet(s). – xeeka Dec 17 '22 at 20:39
  • Thank you to everyone for your help. I really appreciate it. – mvhancock May 01 '23 at 15:15