My DHC is not heating the water as hot as has been for years. seemed to happen after the hot water was run for a long time accidentally

SOURCE: runs hot then cold

overheat stat call service engineer

SOURCE: Boiler recycling when in DHC mode and water

First check pressure over 1 bar ok then-
1. check flow - is it adequate - a little less than the cold tap ok then -
2. does the boiler initially fire up on low hot flow but alternate as u say
If answers are 1. no but 2. yes then check for a furred up secondary heat exchanger
if 1. yes but 2. no [that is the water not the heating] goes hot / cold and if the boiler goes out when the water goes to cold then it's the diverter valve, either to replace or if it is one which has a internal diaphragm-replace that

SOURCE: The water heater "Stiebel Eltron

I am a free answerer, and I specialize in giving you resources for free
The paid answerers may have a person who knows your product and the problems associated with your model.
I believe they offer a refund if the 'fix' doesn't work.
The manual might also have troubleshooting.

Each day I see 20-25 unanswered questions about tankless water heaters.
And the day before, there were same number of unanswered questions about tankless products.
The number of questions shows the complexity of the machines, the brand-specific-ness of the problems, and the inability of the manufactures to take care of their customers.
If I understand correctly, the manufacturer will refer you to the installer.
There are not many ways a do-it-yourself-er can fix a tankless.

http://www.thetankatwaterheaterrescue.com/forums/forum3/

Kelly

• 3740 Answers
• Posted on Apr 10, 2011

SOURCE: I installed a dhc tankless heater, but when I open

Hi and welcome to FixYa, I am Kelly.

What is happening is that your cold water flow is greater in volume (gals per min) than your hot water. So when you open the cold water it is mixing with the hot water inside the faucett or shower plumbing and actually reverse flowing back into the hot water heater plumbing. The reverse flow slows the water flow sensing turbine inside the heater and it is thinking that there is not any water flow. (There isn't actually due to the equalization of water pressure when you open the cold water faucett) Since this is a safety feature of the tankless heater to keep it from overheating, it shuts off.
Somewhere in the hot water plumbing there is a major restriction. It may very well be the internal plumbing of the tankless hot water heater or the flow detector design.

There are 2 ways to correct this. The easy way would be to restrict the available cold water flow. You can do this by installing another shut off valve inline with the cold water plumbing and just not open the valve all the way. You would have to adjust the setting so that the hot and cold water can be on at the same time and the hot water heater stays on. The hard way is to locate the hot water flow restriction and eliminate it.
It is possible.... if you have a flow restrictor in a shower head or a screen in a faucett areator that they are partially clogged. Clean any flow restrictors and screens then see how the hot water heater performs before you start modifying plumbing.

Thanks for choosing FixYa,
Kelly

Huseyin Huseyin

• 3469 Answers
• Posted on Jun 08, 2016

SOURCE: Why newly installed DHC 8 water heater not heating water.

hi, if this is a major domestic appliance installation issue you might be best served by getting a professional / qualified technician to look at it. There are safety concerns where such installations are involved. Investigating yourself might, in any case, invalidate any warranty you might have on the hardware concerned.
Good luck with this Brian