Bath fluctuating hot water

[old duffer]

Reporting back as we have now had the plate heat exchanger replaced and boiler is working great and bath hot water stays consistently hot now! Hurray!

With the DHW setpoint at 55C I am getting the following readings:

i6 9.6LPM-10.2LPM
i9 64C-65C
i11 54C-55CC
i14 49C-50C

Previously i9 which is the temp from main heat exchanger was getting up to 88C so now much more sensible temps.

Just wanted to thank everyone on this thread for their help in figuring out the issue, especially Johntheo5 with all the suggestions on tests & investigation, it really is appreciated!

Cheers!

citric acid from ebay *(powder form). Fill the exchanger with the crystals and add hot water until it starts to come out of the holes. Then heat the unit with a blow torch until it starts to boil. The water will turn green. Let it boil for 10 minutes and then rinse.
Repeat until the water colour becomes a weaker green
You now have a fully refurbished heat exchanger.

 

[Johntheo5]

Reporting back as we have now had the plate heat exchanger replaced and boiler is working great and bath hot water stays consistently hot now! Hurray!

With the DHW setpoint at 55C I am getting the following readings:

i6 9.6LPM-10.2LPM
i9 64C-65C
i11 54C-55CC
i14 49C-50C

Previously i9 which is the temp from main heat exchanger was getting up to 88C so now much more sensible temps.

Just wanted to thank everyone on this thread for their help in figuring out the issue, especially Johntheo5 with all the suggestions on tests & investigation, it really is appreciated!

Cheers!

The boiler output is 23.83kw, by calculation.
The increase in primary flow is amazing with the new PHEX, the previous flowrates were ~ 8.5LPM but with the new one because the primary dT is now only 64.5-49.5, 15C and the DHW dT is 54.5-20, 34.5C then the primary flowrate is now 9.9*34.5/15, 22.77LPM, the primary side must have been severely restricted?

citric acid from ebay *(powder form). Fill the exchanger with the crystals and add hot water until it starts to come out of the holes. Then heat the unit with a blow torch until it starts to boil. The water will turn green. Let it boil for 10 minutes and then rinse.
Repeat until the water colour becomes a weaker green
You now have a fully refurbished heat exchanger.

Which side of the exchanger do you find the most fouled??.

 

[old duffer]

The boiler output is 23.83kw, by calculation.
The increase in primary flow is amazing with the new PHEX, the previous flowrates were ~ 8.5LPM but with the new one because the primary dT is now only 64.5-49.5, 15C and the DHW dT is 54.5-20, 34.5C then the primary flowrate is now 9.9*34.5/15, 22.77LPM, the primary side must have been severely restricted?


Which side of the exchanger do you find the most fouled??.

scale is built-up and the fins as they are so thin they easily coat so its the same either side.

 

[old duffer]

The boiler output is 23.83kw, by calculation.
The increase in primary flow is amazing with the new PHEX, the previous flowrates were ~ 8.5LPM but with the new one because the primary dT is now only 64.5-49.5, 15C and the DHW dT is 54.5-20, 34.5C then the primary flowrate is now 9.9*34.5/15, 22.77LPM, the primary side must have been severely restricted?


Which side of the exchanger do you find the most fouled??.

I always run my CH system with citric acid as standard. I even have an injector port to introduce it into the system so I don't need to drain down

 

[Johntheo5]

I always run my CH system with citric acid as standard. I even have an injector port to introduce it into the system so I don't need to drain down

Citric acid to the DHW side only or both sides?

In my seagoing days, Saf-Acid was the holy grail for descaling and had a colour indicator.

 

[Johntheo5]

These might be of interest as it shows the huge difference in flowrates and heat transfer between fouled and cleaned PHEXs. I have assumed a PHEX heating surface of 0.2M2.

 

[old duffer]

Citric acid to the DHW side only or both sides?

In my seagoing days, Saf-Acid was the holy grail for descaling and had a colour indicator.

I use my combi for heat only so its in the CH pipework but will by default clean one side of the exchanger