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[MTJAG]

I've got a CB5036 installed about four years ago and it has been great.  About two weeks ago, I noticed the DHW wasn't as hot as it had been.  It used to be, you couldn't hold your hand under the hot water for more than a couple of seconds.  Now, the water is about what it is when it comes out of my tankless..about 135 degrees.  Any ideas of what may be the issue?

[mlappin]

I take it when you say heat exchanger your referring to a flat plate?

Do you have a tempering valve installed? Doesn’t happen often but they can go bad.

If a flat plate, when was the last time it was cleaned?

[RSI]

First check boiler water temp after the heat exchangers while running hot water. If it is a huge temp drop then you likely have a flow problem with the boiler water. If it is almost the same temp as before the heat exchangers then the plate heat exchanger probably needs cleaning.

If it is a problem on the boiler side, it could still be the plate that needs cleaning but it would be the boiler side that is clogged. It could also be other things such as the pump, thermostatic valve, etc that is causing the problem.

[slimjim]

Temperature gauges are a great investment for times just like this.

[MTJAG]

First check boiler water temp after the heat exchangers while running hot water. If it is a huge temp drop then you likely have a flow problem with the boiler water. If it is almost the same temp as before the heat exchangers then the plate heat exchanger probably needs cleaning.

If it is a problem on the boiler side, it could still be the plate that needs cleaning but it would be the boiler side that is clogged. It could also be other things such as the pump, thermostatic valve, etc that is causing the problem.

Thanks to all for responding. First, I've never cleaned the plates in the four years since install, but it does have non-corrosive in the system.  Second, the temperature variation I just checked is as follows.  From the OWF, 175, 165.3 going into the exchanger and then 135.1 coming out of the exchanger going into my Rinnii.

[RSI]

Is the 135 degrees the DHW or the boiler water coming out of the heat exchanger? If it is the DHW, what is the boiler water after the plate heat exchanger?

[MTJAG]

Is the 135 degrees the DHW or the boiler water coming out of the heat exchanger? If it is the DHW, what is the boiler water after the plate heat exchanger?

the 135 is after the plate and before the DHW so it is after the plate heat exchanger

[RSI]

Which pipe though? You have a boiler water pipe and a domestic water pipe out of the heat exchanger. I am looking for the temp of the boiler water out of the plate heat exchanger.

[MTJAG]

Which pipe though? You have a boiler water pipe and a domestic water pipe out of the heat exchanger. I am looking for the temp of the boiler water out of the plate heat exchanger.

The boiler water out of the plate is 165 degrees. 
It's definitely the flat plate that's the problem.  My water pressure coming out is about half of what it is going into it.  It's not enough water pressure coming out to turn on my Rinnai, though it passes through.  I tried flushing the flat plate  yesterday with vinegar and I could barely get anything through.  The return line would trickle, then squirt a bit, but it was the same after an hour.  Thinking of going back with a solution of muriatic acid.  Anybody got any suggestions?