Puma heat exchanger - scaled

[ChrisR]

I've never come across one of these h/e's in a really bad state. I'm not in a particularly hard water area. BG have stated the boiler needs a new h/e, a figure of £600 has come through, and I have yet to investigate.
Anyone out there had to change/descale one?

Hi Breezer, I remember you did. Was the problem limescale (tap water side)?

 

[Vic Spanner]

Chris..Down here in the south it is a very common problem.We sometimes do 1 or 2 a month. Normal problem is that the flow reduces to a trickle (scales up) or damage by overheating (no modulation, Flow switch sticking).
Price wise i would have thought somewhere between £100.00 - £150.00 for the H/E and a couple of quid for the 4 x "O" rings plus labour.
Straight forward job 2 hours should cover you.

 

[Agile]

I have got to do one on a Lynx which is a very similar HE. Until I have done it I will not know if its the primary or secondary.

I would expect to do it by power flushing it in situ though.

Tony

 

[gazthepottertonengineer]

I have got to do one on a Lynx which is a very similar HE. Until I have done it I will not know if its the primary or secondary.

I would expect to do it by power flushing it in situ though.

Tony

How do you mean primary or secondary its only one crossflow h/exch in these

Also i would check the flow restrictor isnt blocked in the cold water isolation valve as i get this alot

 

[Mikeyboy]

Always check that the AAV . , is operational on the puma type boiler .As Gaz states ,it is a crossflow ht.ex. , and therefore no d.h.w. plate or normal type hydraulically operated diverter valve , as we all know . Sometimes can be an awkward problem to solve .Cold inlet filter , though is a definite suspect .

 

[Agile]

Gas:- """How do you mean primary or secondary its only one crossflow h/exch in these """

Since the DHE coil is in the system water then I conclude its the "secondary" as its definately not the primary!

One of our trainees had a good name for these, a "four pipe" HE !

Tony

 

[chrishutt]

Am I right in thinking that these "4 pipe" heat exchangers must be far more prone to secondary side scaling due to the higher temperatures to which the secondary side is subjected?

As far as I can see there is no mechanism for preventing the DHW water reaching the same temperature as the CH water during CH mode, which might be 82C. Since scale deposition tends to occur above 60C this seems to be asking for trouble. Separate secondary heat exchangers can and do have the temperature limited to below 60C.

 

[gazthepottertonengineer]

Am I right in thinking that these "4 pipe" heat exchangers must be far more prone to secondary side scaling due to the higher temperatures to which the secondary side is subjected?

As far as I can see there is no mechanism for preventing the DHW water reaching the same temperature as the CH water during CH mode, which might be 82C. Since scale deposition tends to occur above 60C this seems to be asking for trouble. Separate secondary heat exchangers can and do have the temperature limited to below 60C.

Thats why these heat exchangers are history i cant think of any current boilers that have them any more

 

[Agile]

Thats a very interesting point Chris H. One that I have never had any need to consider.

I see a lot of Potty Pumas and I have NEVER seen one with a significantly scaled DHW coil. Just the one Lynx which I have to sort out soon but that one is over 10 years old. That Lynx is not serious but just a reduced performance and its for a very nice retired West Indian fellow so we are going to do it for just £15 to cover the chemicals.

There must be a reason and the only one I can think of is that at 82*C maximum the scaling is not any problem.

Similarly think of a Britony II instant and they do not suffer from serious scaling either ???

It is important to differentiate between a HE with 82*C on the heating side and one with 300*C flames onn the heating side, they do deposit !!!

In contrast the Worcester with a four pipe DOES get seriously scalled up and I have seen several of these, one completely blocked!

Tony

 

[ChrisR]

Heat banks scale up their tap water sides, at a bit less hot than a boiler..

 

[chrishutt]

Thats why these heat exchangers are history i cant think of any current boilers that have them any more

How about Ravenheats, Ferrolis and Worcester 28i?

 

[Agile]

Because they are all non-con!