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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.
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.
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Comments
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"
What are your needs? # of people and bathrooms.
Usually combis are only good for small homes w a bath or bath and a half.
Thanks for any input.
similar to
http://www.triangletube.com/us/customer/product/V7YBWioAAEgERkgc/challenger-combi-cc125h
http://www.viessmann-us.com/en/residential/gas-boilers/condensing-boilers/vitodens-100-b1ka.html
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.