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Replace combi or something else?

JPL941 Member Posts: 51
Hello everyone, recently I noticed my combi boiler, aqua balance Wmb-155c, circulator pumps were rusted shut. I have onix piping so I figured that’s where the O2 came from. I removed all existing black pipe and repiped it with brass. The piping wasn’t really changed from the original layout- when it worked fine. Near boiler piping Long loop with closely spaced tees and the required boiler trim.

The system sat empty for a week until the plumber was able to come over and solder in some new copper changes and a dirt mag.

I noticed that it was hard to get air out of the system since the repiping but it seemed to get better with time (specifically I could hear sloshing in the heat exchanger).

Today I was smelling something weird so I started to investigate the combi boiler and noticed the top collar was warped. I took the front off and noticed the black insulation melted to the rim of the jacket and saw melted plastic above the exhaust flue.

The boiler is unplugged and the gas is turned off.

I’m thinking maybe the rust of the pipes blocked some of the heat exchanger when it was drained of water but I don’t really know.

Anyhow, the combi serviced my DHW But I also have an electric in place in the event the boiler went down. And also the Kitchen with Two kick space heaters rated at 12k a piece and the bathroom at around 6-7k (in floor onix).

I’m debating on what to do.. I can’t sink a ton of money into it because I have already sunk so much into it but I don’t know how to heat those two rooms.

Get rid of combi forever.. add ductless to kitchen, use 40g electric water heater for bathroom and get gas water heater for dhw?

Replace with another mod con?

Use electric hw for those two rooms and get a gas HW heater?

Steam supplies the majority of the heat on the coldest days but otherwise if it’s mild, we use our gas insert and make out just fine.

Thank you all for the assistance.


  • mattmia2
    mattmia2 Member Posts: 7,156
    There are ways to get hot water off a steam boiler, that may be another option depending on how much heat you need and how much your boiler has available. Probably would want to isolate at least some of it with a heat exchanger if you are having corrosion problems.
  • JPL941
    JPL941 Member Posts: 51
    edited December 2019
    I would like to avoid using the steam boiler if possible.
  • Jamie Hall
    Jamie Hall Member Posts: 20,434
    Since you don't want to use your steam boiler -- which might be the least expensive option -- you really are kind of stuck. I'd probably go with a gas hot water heater, perhaps an on demand unit if you have the gas service to supply it, but I don't know what I'd do for space heat, since that has to be a small load. Probably the smallest mod/con I could find -- or possibly even one of the Rinnai through the wall vented gas space heaters.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 7,156
    I would go with a mod/con with a minimum firing rate a little below your smallest zone or as close as I could find, probably something with a 10:1 turndown ratio and an indirect tank. I'd make the indirect bigger if necessary to give you enough hot water with the boiler that matched the heating loads. I'm not sure if it is the most cost effective solution but it is an efficient and durable solution.
  • JPL941
    JPL941 Member Posts: 51
    Hello everyone, thank for your the replies.

    I was thinking has anyone ever used/installed a wall mounted electric boiler? It would only run my kitchen heat, 24k btu at worst and utilize 9kw. Fairly inexpensive to install and a lot less upkeep. Downside is electricity cost.

    The bathroom is currently being heated with a slim style space heater and I may buy a wall mounted unit to get it off the floor and would eliminate the onix tubing.

    Hot water currently supplied with 40 gallon electric tank that my wife runs dry every day. Would consider electric or gas. If gas would need PV or PDV. Any particular brand/model you prefer that is virtually silent?

    Thank you.
    JUGHNE Member Posts: 10,279
    IIWM, I would weigh how many boiler/years I got out of the steamer for X dollars against the combi/years for X dollars.

    Steamer could do an indirect HW tank, somehow the electric WH could be used for the main DWH in the summer. Crank up the temp and install a temp valve.
    Maybe a SS exchanger for the tubing.
    Just ideas as the steamer may be a reliable work horse.