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Combination Water heater/boiler inconsistent temps

Hey guys! First time poster here.

I recently bought a home that has a newer Bosch high efficiency tankless combi unit (hot water heater/boiler for our heating system). We have noticed even in the warm weather that the showers start nice and hot, after a minute or two they get luke warm for 30 seconds or so (my wife says "cold"), then hot again, then luke warm, this just keeps repeating. I am afraid to play around with the settings on it. Any thoughts on how to troubleshoot this issue? Thanks in advance.


  • kcoppkcopp Member Posts: 2,609
    These units needs some adjusting. also they do have a flow rate requirement that if exceeded may give you less than great HW. You should get a pro out to take a look.
  • furnacefigher15furnacefigher15 Member Posts: 502
    That symtom is very typical of tankless type water heaters. What seems to work best is to adjust the way you use hot water. Instead of setting the shower valve to adjust to a desired temperature, you should open the shower valve 100 percent, then adjust the water heater set point to the desired temperature, this is typically accomplished by a adjusting a digital thermostat usually at the the heater, but you may need to consult the manual for your unit.

    The issue is that most shower valves are pressure ballancing, this is an anti scald feature (designed for standard tank type heaters). So, when you set the water temp at the shower valve (they way you normally would) then say the toilet flushes, this will cause a pressure reduction on the cold side of the system, and the shower valve reacts by throttling back the hot side to prevent too much hot water flow. This will cause the tankless heater to see a flow reduction, and therefore a heat reduction. At the point of use, what you feel is hot, then cool, then hot again.

    In the tankless world, they call it, "the hot, cold, hot sandwhich"
  • rocky_mtnrocky_mtn Member Posts: 10
    I'm not too familiar with Bosch units, but it sounds to me like it's designed kind of like Eternals were. They sacrifice temperature in lieu of flow. Newer units do just the opposite and regulate the flow to maintain setpoint temperature. Sounds to me like you are just pulling too many gpm's through it and it just flat can't keep up and it's not regulating the flow of water through the HX.
  • bob eckbob eck Member Posts: 518
    Check the installation instructions on the Bosch Greenstar combi unit do you need a mixing valve for the domestic hot water?
  • newagedawnnewagedawn Member Posts: 66
    if a mixing valve is needed that could be failing or got some junk in it
  • j aj a Member Posts: 1,379
    Post a picture of the unit...
  • kcoppkcopp Member Posts: 2,609
    bob eck said:

    Check the installation instructions on the Bosch Greenstar combi unit do you need a mixing valve for the domestic hot water?

    According to Bosch they do not require a mix valve on that unit. However the local inspector may not like that.
  • njtommynjtommy Member Posts: 1,045
    Are you on city water or well water? Does it only happen when Their is a call for heating at the same time or does't it matter?
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