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Bosch Greenstar combi 131 won't maintain domestic hot water temp
I had a brand new combi 131 installed to replace a 30 year old boiler and traditional gas hot water heater at the end of May 2016. Since then we have been dealing with fluctuating water temperatures from the faucets and shower. The boiler temperature is set at 122 degrees. When the shower is running it steadily climbs to 137 and then drops incrementally down to 117. The boiler installer and I were able to figure out that if the shower and two other faucets were running simultaneously, it seems to hold the water temp. It is only when there is one water source running that there is a problem. Trouble is, we don't usually have multiple things running at the same time.
When the installer called Bosch their comment was that the boiler was trying to heat to maintain a minimum flow rate which was more than the water running from one faucet/shower. Their suggestion was to install an external mixing valve on the piping and control the water temperature coming out of the faucets using that.
It seems illogical that a very expensive boiler is not able to maintain a set water temperature with one faucet running and needs an external thermostat/mixer. Also concerned about losing the energy efficiency and longevity of parts if it is constantly cycling between a high temp and heating to a high temp and then turning off.
Any thoughts with regards to the suggested approach or what else might be the problem?