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3 way valves in combi boilers

Geosman Member Posts: 25
Just asking those following the main forum if they have experience with Combi 3 way valve failures. I've had to replace one on a Navien that was installed on a black iron piping system operating with a city water feed. The valve got stiff and stuck unable to make domestic hot water. Making a "postmortem" take apart on the valve was not possible. I added Fernox F1 and have not had a repeat issue at this location for the last two years. I've not seen the same issue on combi boilers that operate with a propylene glycol fluid or any that have corrosion inhibitors added to balance the system. I might suspect that possible chlorine in system water as well as "red water" from an untreated system might be the contributor to valve failures.
I'm curious to know what other's experiences have been and if my suspicions may be correct. I'm starting to see more interest in combi boilers for space and functionality now that turndown rates have improved to allow them to more closely match zone loads yet deliver the capacity needed for domestic hot water.


  • Ironman
    Ironman Member Posts: 7,350
    In all fairness to Combi manufacturers, water quality has to be a factor in 3 way valve failures. To what extent, IDK, but you can't connect a mod/con to an old cast system that hasn't been properly flushed and had a good dirt separator installed.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • kcopp
    kcopp Member Posts: 4,395
    This is one of the reasons I use the IBC DC/ SFC combi boiler... no 3 way valve. 2 separate water ways. Much less to fail.
  • Geosman
    Geosman Member Posts: 25
    Thanks for the IBC reference....interesting boiler. I'm bookmarking their titanium pool exchangers for future reference. And yes on the previous post...a clean system saves a lot of aggravation.
  • SuperTech
    SuperTech Member Posts: 2,124
    I've seen my fair share of issues with 3 way valves on combi boilers. I'll be replacing a sticking Honeywell 3 way valve this week on a Rinnai that has been problematic since we started servicing it.  Combis aren't a great option in my area due to the water quality. Indirect tanks seem to be the best option for reliability. 
  • Jamie Hall
    Jamie Hall Member Posts: 22,934
    Please don't blame the chloramines and chlorine in the municipal water -- it's a convenient scapegoat, but it's not the culprit. However, corrosivity is -- and if you are running a system (such as a finely engineered Combi and its valves) you have to control that. Either use materials, such as PEX, brass or bronze, etc., which don't corrode -- or add a good corrosion inhibitor. Depending on pH and dissolved oxygen levels, any iron or steel in the water will corrode, and may -- rather than staying nicely in solution like copper and so on -- form insoluble mineral grains -- which are guaranteed to gum up tight tolerance valves and, for that matter, pumps with ECM motors. In many cases, a magnetic filter may be helpful as well.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Geosman
    Geosman Member Posts: 25
    Just had a very good conversation with Bill Marcho? at Laars in their application support section to discuss early issues with 3 way valves. Bill indicated that early Mascot boilers had 3 way issues when the boiler sat for weeks on end with the valve in its "rest position" (which is for domestic hot water). The situation has been corrected with a software change that cycles the valve/heat call at least every few days to keep the passages and mechanism free and open. It's the same process used on some pumps to exercise them and prevent rotor freeze-up during the off season. Knowing this, along with keeping a clean system with a Caleffi Dirt Mag and good inhibitor package should restore confidence in using the Mascot combi boilers. For older boilers, any thermostat, controller or timer that can initiate a short term radiant heat call on a daily or every other day basis should do the same and avoid issues with 3 way valves no matter whos boiler they are on.
  • PerryHolzman
    PerryHolzman Member Posts: 234
    I have a 14.5 year old Viessmann Vitodens 200 with an indirect installed on a steel piped cast iron radiator Monoflow T system, no filters or magnets (with copper tubing between the boiler and steel pipe, and copper tubing between the boiler and the indirect hot water heater); and have never had a problem with the 3 way valve - other than you can hear it on the 2nd floor of the house when it switches (both my wife and I are now used to it and sleep though it).

    The water was clear at the 14 year point when we partially drained the system to replace the expansion tank (and we added an isolation valve and pressure gauge at the same time, and replaced the pressure reducer fill valve - which had failed and the back flow preventor).

  • psb75
    psb75 Member Posts: 811
    I'm replacing the cartridge inside a Honeywell 3-way valve on a small Rinnai Combi boiler at the suggestion of a field rep. I've been trying to diagnose a lack of hot water at the shower, but not at any sink faucets. Tech support tried to blame household plumbing but homeowners asked why the shower worked before and why the boiler couldn't perform now--for just the shower. So the field rep told me to try replacing cartridge in 3-way valve. Hope it works. I have already done a flush of the dhw heat exchanger.
  • unclejohn
    unclejohn Member Posts: 1,833
    Dirt seperater a must and so are system flush and water treatment on boilers with 3way valves. IMHO.
  • psb75
    psb75 Member Posts: 811
    No shower valve cartridge. Just and old fashioned two handled hot-cold tub spout with overhead shower diverter.