Bosch Greenstar combi 131 won't maintain domestic hot water temp

Mike25 · July 2016

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?

Timothy77 · July 2016

I think there min btu is 36k which could cause some problems. Did they say what the minimum flow rate was?

Timothy77 · July 2016

Some models you can put a remote aquastat under your kitchen sink which fixes the issue.

Firecontrol933 · July 2016

The external mixing valve will work, but only to a point. If the minimum flow of water that you are using from a single given faucet at any time is less than what the boiler can "match" with it's firing rate then the boiler is still going to shut it's burner down and then fire right back up. The mixing valve will help a little to smooth out the water temps you see at the faucet, but can't fix a (less than) water flow issue.

Very close attention needs to be paid when selecting the proper size of these units to make sure the real world usage is used to determine things instead of the once in a great while scenarios.

Sometimes if it is just one fixture that is the problem with low water flows, then it might just be a matter of modifying or replacing that one fixture to increase it's flow to something above the minimum flow that the boiler wants to see to not cycle on and off.

If it's a whole home problem then more involved and sometimes expensive measures are needed to make the new boiler perform to your expectations and in a way that you can live with.

Bottom line is: until you find a way to get the required minimum flow through the boiler it will cycle on and off

kcopp · July 2016

You really should have a good quality mixing valve on the hot water piping. Most all codes require that...even if the combi boiler manufacturer says you don't need one.
I then raise the water temp on the boiler to 140 and mix it down to 125-130F.

4Johnpipe · July 2016

Had a similar issue with a Greenstar. It turned out that the transfer valve inside the unit was fouled with debris. I called in the manufacturers tech service and even he was surprised by the amount of build up. We also ended up installing a whole house sediment filter due to the fact that the water mains were being replaced after Super storm Sandy and I could not afford another call back.

DaveV · August 2019

The problem is not your boiler, sort of. Your shower head is limiting the flow of water and the boiler is cycling on and off since it is producing more water than you are using when fired. This is giving you the temperature fluctuations. Find a showered that produces greater than 2.5GPM and your problem will be solved.

Bosch Greenstar combi 131 won't maintain domestic hot water temp

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