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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: 3,606
    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: 514
    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: 13
    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: 922
    Check the installation instructions on the Bosch Greenstar combi unit do you need a mixing valve for the domestic hot water?
    newagedawnj a_2
  • newagedawnnewagedawn Member Posts: 587
    if a mixing valve is needed that could be failing or got some junk in it
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • j a_2j a_2 Member Posts: 1,796
    Post a picture of the unit...
  • kcoppkcopp Member Posts: 3,606
    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: 1,105
    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?
  • RoninPh_dRoninPh_d Member Posts: 2
    Hey everyone, I'm preparing to replace a 20+ year old heatmaker combo domestic hot water/ baseboard boiler. I usually use sidearms, but space limitations require a combo unit. I've been researching the web but was hoping for some insight from installers/service techs as to quality brands. Any suggestions would be appreciated.
  • kcoppkcopp Member Posts: 3,606
    You're going to get a myriad of answers on this one... It depends on how good your supply house is at getting parts and how good your tech support is. I have installed a few different brands. I have come to really like the IBC DC unit. Specifically the 20-125. I prefer that to other units w/ brazed plate HX in them as it has less parts to break.

    What are your needs? # of people and bathrooms.
    Usually combis are only good for small homes w a bath or bath and a half.

  • RoninPh_dRoninPh_d Member Posts: 2
    1full bath, 1100 sq.ft. A couple and occasionally a guest. It's in the mountains outside of Denver so I have multiple supply houses which can generally satisfy my needs. I'm not familiar with IBC DC. I generally install Weil-Mclain, Burnham, veissmann boilers with a sidearm. I've been limping the heatmaker along for a few years and for what it is I've been mildly surprised by it. The owners are friends and I've been preparing them for the financial hit and it is overdue as I've done as much service as I care to at this point.
    Thanks for any input.
  • rocky_mtnrocky_mtn Member Posts: 13
    Sounds like a perfect match for a Triangle Tube Excellence model. Has a 14 gallon buffer tank, and you'll never run out of hot water. Current model PEA110 with the ACVMax control. I'm in Castle Rock if you need any help with design or retro, hit me up. I work at the supply house and am the heating guy.
  • gschallertgschallert Member Posts: 170
    Viessmann has a new Vitodens 100 combi (B1KA 35) rated 21 - 125 for heating and 21-149 for DHW:
  • bob eckbob eck Member Posts: 922
    Take a look at the Lochinvar Noble condensing combi boiler. Fire Tube SS heat exchanger with plate heat exchanger. Three models that deliver 2.6 GPM , 3.8 GPM and 4.8 GPM of domestic hot water with a 70 degree F temp rise (incoming water temp 50 deg + 70 deg equals 120 def F going out to your faucets)
    If you need the larger output domestic hot water install the larger BTU input boiler for domestic hot water needs and the contractor can down fire the heating BTU output of the boiler to meet the heat loss of the home. This boiler has a 10 to 1 turndown.
  • JackJack Member Posts: 1,045
    Does the Bosch unit require a flow restrictor in the cold water inlet? Some combi's do in order to eliminate the condition you are seeing

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