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"Dead" copper HX from overheating?

Hello, I recently worked on a Lochinvar RBL045 (atmospheric with copper fin tube heat exchanger) It has the F9 control (fixed, non modulating).

It has been converted to natural gas and is in a small shop with radiant floor.

It is a bit on the small side, but has worked pretty well the last couple of winters.

The customer called and said it wasn't working. The flame rollout switch was tripped and the HX was very sooted up. We cleaned it all up and it was burning well,  but he said that now his outlet temp doesn't get above about 80 degrees when previously it would get a little over 100 degrees. (this despite the fact that outdoor temps have risen into the 60s) We are running about a 20 degree delta t, have checked the gas pressure and flue. Tech Support told me that if you overheat the HX it can go "dead" i.e. it doesn't transfer heat correctly the way it did originally. This seems to be what our situation is, but I wondered if anyone here has seen that happen and what happens to the copper. I have googled it but can't find anything. Sincere thanks, Kevin


  • Kevin O. Pulver_2
    Kevin O. Pulver_2 Member Posts: 87
    Sorry about the double post

    I don't know how to delete it or I would. Maybe the moderator could make one go away? Please? :-) Kevin
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837

    I am not a metalurgist, but unless teh heat exchanger was run dry and completely annealed, I can't see its thermal conductivity changing THAT much.

    What was your flue gas temperature before the repairs, and what was it after?

    If you pulled the heat exchanger and pressure washed it, you may have changed the surface area by pressure washing it, which would affect the ability to transfer the heat from the hot flue gas to the cooler water. Were there V baffles between the sections of the exchanger? Were they significantly degraded?

    I have seen small residential boilers with CFT heat exchangers that were so limed up, that flow ceased through the heat exchanger, and they melted down and fell into the fire box. THAT is my idea of a dead exchanger...

    So long as you haven't annealed the exchanger, I can't see you changing the heat transfer characteristics tho... But again, I am not a metalurgist.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Kevin O. Pulver_2
    Kevin O. Pulver_2 Member Posts: 87
    Yes Mark,

    Yes Mark there were baffles on top of the fins. I didn't notice any

    degradation though one was missing but apparently had been that way. So

    I'm getting one, which I know will help, but it's apparently not the

    cause of the problem.  I didn't think to check the flue temp before or

    after. (didn't even have my analyzer with me ;-(  but since he had no

    flame when we got there and we gave it a good cleaning, (brushing and

    air only, no power washing) I expected it would be greatly improved.  I

    think it had De-Ionized water with the glycol when it was installed 2 or

    3 years ago but I need to get a sample of that too.  It seems that it

    somehow just got a lot worse all of a sudden, and I'm with you, I can't

    imagine annealing it THAT much without clear running it out of water,

    and the soot should actually insulate it no?
  • hot_rod
    hot_rod Member Posts: 15,028
    what type of radiant tube

    if it is a non barrier I have seen those HX tubes plug up in copper boilers. Which could also cause some of the other problems you are seeing.

    If connected to a cold concrete slab without return protection it could run days at low temperature.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Kevin O. Pulver_2
    Kevin O. Pulver_2 Member Posts: 87
    Thanks HR

    I'm 99.9% sure that it's barrier tube. (If my memory serves)

    And it doesn't have anything for return protection except a bypass.

    I don't think he thought it was condensing and so kept it closed even on startup.

    It was probably condensing and he didn't notice.  I guess I'll have to do a little more checking...
This discussion has been closed.