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Combi boiler DHW troubleshooting

hot_rod Member Posts: 20,826
I got a look at a plate HX from a residential application in the Bozeman area this week.

A new boiler installation, DHW problems less than a year into the start up. Troubleshooter checked pump operation, sensors, control, replaced the control board, per the manufacturers suggestion.

Finally took the HX apart to find the boiler side had fouled. Some of the particles that Dale shook out measured .030. the boiler fill water was from the site, well water, one fill and flush.

We don't often think about the A side on these combo plate heat exchangers, or how hard or high TDS water can compromise them from day one. Same scaling potential inside the boiler I suspect. An expensive troubleshoot and repair!

Here is a graph we put together to show how efficiency and performance drops as scaling accumulates.

Interesting how simple the HX to composite block connection is made with flat washers.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
delta TRxRoyCanucker


  • SuperTech
    SuperTech Member Posts: 2,032
    Very interesting. I imagine a dirtmag would help prevent this type of fouling.
    I've seen a couple of plate exchangers become plugged on the boiler side. They can be a real pain to clean.
  • kcopp
    kcopp Member Posts: 4,305
    They deal with this sort of this over in the UK on a regular basis... They especially have more issue w/ the prevalence of steel panel rads breaking down and plugging up a Brz plate HX. We are just starting to see it here....it is going to get worse.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,645
    Also seems the majority of boiler cleaner and conditioner manufacturers are in Europe specifically the UK.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • kcopp
    kcopp Member Posts: 4,305
    I may be mistaken but it may actually be the single largest boiler market in the world.... until China catches up.
  • clammy
    clammy Member Posts: 3,022
    just had to replace a failed plate on a lars endurance ,the boiler is 9 years piped terrible and the plate had a bulge on the top and was blowing the boiler safety .This was the first water to water plate that i have had fail i have seen others plugged but not fail ,house water pressure was normal boiler limits where set hi .We recommended a new boiler because the the evedence of flue gas recirc and multi fault codes.Never maintained or set up but HO says it s fine lol he did not like the price and complained about it then after he watch and saw what a hassle it was to get out and the new one in and reset the lock outs replace sensor and iginator and a bunch of other stuff he realized he should have listened and replaced it so now he has the new plate same boiler and all the same issues ,we should have passed on that one and let some one else give him the proper price rogering that we usually try to avoid doing to people but we believe in karma and unfortunrly the HO was not even thankful for our efforts and complained about the price we didnt not even make our hourly on that one .I told my partner we should have passed espically since we where not the first to look at the issues live and learn .I did find it strange that the plate failed i fiqured it would get scaled first and underperform before failure .Also know why lars discontinuned it whay a pos peace and good luck clammy on the other side w 175 mbtu on proirity through a plate certainly give ample hot water lol
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • psb75
    psb75 Member Posts: 756
    I pulled a 13 year old Lars Endurance boiler out of a house the same age. It was due to failure of DHW from the plate x-changer. The wiring inside the boiler was awful. Wiring was "crispy" from heat and age. Crazy looking and connections were failing and suspect. Tech support for Lars was "grudging" and difficult to access. Parts were expensive. So...I put in an NTI 80 firetube mod-con replacement boiler. Good price, great support. I put in a Rheem heat-pump 80 gal. elect. water heater for DHW. Great energy savings for the house esp. during this cold winter.
    Now I have a policy: will not work on Lars equipment.
    EBEBRATT-Ed Member Posts: 14,560
    Started up (2) 2,500,000 new Lars boilers recently. I was not impressed, seemed like junk to me and they were brand new.
  • kcopp
    kcopp Member Posts: 4,305
    Many of the Laars combi boilers are simply Baxi Luna units Re-badged w/ Laars stickers. Parts can be hard to find.
  • HVACguyinME
    HVACguyinME Member Posts: 25
    Actually, Laars hasn’t used the Baxi style in a few years. I don’t care what brand, but my opinion on ANY flat plate hexchanger for domestic, are maintanence nightmares making them “the devil.” 3 way valves and exchangers in Maine fail on a daily basis. Yet we still keep putting them in, at least I don’t!
  • hot_rod
    hot_rod Member Posts: 20,826
    I think the issue in this example had to do with the boiler fill water being excessively high mineral content. There is no other way to foul a hx in a closed, sealed, leak free boiler side. Especially in this short of a time period.

    Then, of course the same water is flowing in the B side which will also create problems.

    Plate HX are in themselves not a bad piece of equipment, lots of HX in a small package. They are used in all sorts of refrigeration applications also, but they need to be maintained and kept clean.
    Heat pumps often have plate HXers and they do not experience these same issues.

    The same minerals causing the problem in this case would also foul a fined or even smooth coil indirect, it usually just takes longer, as well as the inside of the boiler itself.

    Pay attention to the boiler installation water spec, I suspect the warranty was voided from day one with this high mineral water installed.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream