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Adding an Indirect to a combi boiler

Burbman Member Posts: 7
So we replaced the old oil burner and boiler with a new gas unit, a Bosch Greenstar 150k BTU with tankless hot water rated at 4 gpm continuous. Contractor said would have as much hot water as ever needed. I am having a problem with temp fluctuation that nobody can figure out. Both of the upstairs baths have been recently repiped and with new fixtures and both behave the same way....water is cold at first then warms up and gets hot as you would expect (HW temp is set at 140*). Then about 2-3 mins later it gets noticeably colder to lukewarm, then goes ice cold for about a minute then back to hot. I don't know if the cycle repeats since we don't take unusually long showers.

I have searched for an answer and come up empty so I am considering adding a zone and going to an indirect HW heater. Had a 40-gal Buderus in our previous home and it was amazing. Can I just connect the output from the combi HW coil to the indirect tank and then add a heating loop for the circulator? Almost sounds too easy....


    HVACNUT Member Posts: 5,829
    edited May 2018
    Not real familiar with combi boilers, but if it could be done, the DHW circulator will be wired into a zone control relay and somehow the boiler needs to know it's a DHW call so it can bypass the ODR and ramp to high fire. I'm sure there's a way somebody knows.
  • bob eck
    bob eck Member Posts: 930
    Get a digital clamp on thermometer that you can put on the hot water supply leaving the combi boiler. Have someone up in the bathroom and someone in the basement. Turn on the hot water mixed to where you would shower with. Watch the temperature coming out of the combi boiler see if it changes and drops off or it it is the same when the shower goes cold. You might have a cross connection. Some single valve faucets can cause a cross connection.
  • delta T
    delta T Member Posts: 884
    before you go installing more equipment and spending more money, you need to figure out what is going on with your current setup. Something is not working. Does the boiler continuously fire for DHW as this problem happens? Have you ever had someone in the boiler room to check it as has been described above? Sounds to me like the boiler is dropping out for some reason, then re-firing. Was a combustion analysis performed on startup?
  • ScottSecor
    ScottSecor Member Posts: 855
    Could it be something as simple as an undersized gas line feeding the unit?
    delta T
  • Jack
    Jack Member Posts: 1,047
    Is there a tempering valve on the system. Why are you running at 140* supply temp? That is very hot water.
  • Jack
    Jack Member Posts: 1,047
    Perhaps, and I am not sure how the GS would work here, but I know I have seen plenty of tankless units where a unit set at 140* would behave exactly as you are describing your problem. Your supply temp being high reduces the amount of hot water being delivered to the faucets. Keep in mind that if you sit in a bathtub over 105* you are no longer bathing. You are par-boiling. Your thermostatic shower valve is set at a max temp of 112*. Set your supply temp at 120, see how it works and report back.
  • RomanP
    RomanP Member Posts: 102
    Get your tester, manometer, hand tools. Call tech support and troubleshoot the unit. I agree with 140 being too hot.

    Is your unit piped to specs?

    Is your heating side complete and air purged thoroughly, most important... is the air purged thoroughly from your heat exchanger on heating side?

    Here’s is the story... installed Lochinvar Noble in my buddies house. Since I did it myself and had less than 8 hrs available, between draining, disconnecting/removing old equipment and cutting/mounting plywood on walls, rerouting domestic HW and CW inside walls, new gas piping and new vent piping. I was able to connect domestic side, so they could have hot water and primary loop on the heating side, so I could pressurize it. I didn’t have that setup finalized( no purge points, no feeder or spiro vent etc) filled heating loop up and half-**** purged it with what I had. My buddy had those fluctuations in his HW. When I came back to complete the heating side. I was able to purge everything in proper manner. He is a happy camper right now!!! Saved a ton of propane compared to last year, although last years winter was a summer, compared to the one we just had.

    Make sure you have additional expansion tank installed on your heating side. These Bosch combis come with built in ones, but those aren’t working as intended

    Installed couple green star combis before, no complains.
  • RomanP
    RomanP Member Posts: 102
    Contractor decided to use his electrician instead of mine. So he did a lousy job :(
  • Ironman
    Ironman Member Posts: 7,375
    @RomanP Nice looking job.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • RomanP
    RomanP Member Posts: 102
    @Ironman thank you, sir! Trying to take pride in what I do.