combi bosch zwb28-3 dhw issues

sam33

Hi,

There was reduced DHW output. Essentially almost luke warm. I cleaned the main exchanger (the one with the burner) which was real bad. That helped some. I replaced the DHW temp sensor and DHW plate exchanger. That also helped some. The sensor had tons of scale on it. The exchanger not so much that I could tell. However, lots of scale throughout the system on the o-rings and seals on the heat side.

The behavior now is if I set the DHW temp to max (140 degrees) and turn on the shower (low flow), boiler DHW temp runs at 129 degrees. Temp at the mixing valve is ~112 degrees (6 feet from boiler). I've got the mixing valve set to hottest which should allow 150 degrees. If I turn off the shower, boiler temp rises to ~145. If I then turn on the shower, that hotter water will flow through the system. Mixing valve temp will increase to ~118 or so initially but then drops. The boiler temp also slowly drops back to 129 degrees. Is this normal operation? Seems to me that with one shower open, the boiler should be able to maintain 140 or close to 140.

I've got a new diverter valve flapper on order and plan to inspect the diverter valve and input DHW filter. But I'm wondering if the scale as fouled the main exchanger. Any thoughts?

thx!

Edit: The system provides ~1500sq ft radiant heat via a Rehau system. I think its Raupex pipe. Two zones upstairs, two zones down. Two bathrooms.

sam33

To add, it's 10 years old and the main exchanger has been cleaned once before.

Here's a photo of the exchanger before I cleaned it.

sam33

Picture of corrosion on temp sensor

sam33

Seemed to be slowly getting worse after all the previous work. I put in a new diverter valve paddle and no improvement although the old one was really scaled.

Intplm.

Have you used something like this to descale the inside of your unit?

As to the temperature of your shower? You do not mention what type or brand of shower valve you have. I'm guessing it's a single handle that is a pressure-balanced valve? They are designed to offer a lower temp to help prevent scalding. Replacing the temp sensing device on that shower might also help but the temp settings can only be adjusted so high. Some are designed so that they cannot be adjusted to a dangerous scalding temp.

sam33

I have not used a descaler, but I was thinking about it. The boiler also provides for radiant heat. I'm wondering if the pex is going to be scaled as well.

It's a pressure balanced valve, but I've tried other faucets as well that don't have any scald protection.

thx

Big Ed_4

Nasty , you need a filter on the hot water inlet first off . There is a fine cone strainer on the hot water inlet . I would replace the flow sensor , it comes with all the seals that you need … Good Luck