My hot water seems to run out sooner in the winter than in the warmer months. This has been the case for the 5 years I’ve lived here. Last year seemed worse and this year though seems like even more so. We’ve had no changes to our living situation. I had a plumber come and he checked the efficiency on the water heater, said it’s all good but that the installation was done improperly. That it needs a mixing valve and a pump, we turn the temp up and the tank will last longer and recover faster. He wrote up the estimate with “boiler not piped with priority and no mixing valve. Bandaid repair install mixing valve on tank and set outlet temp to 120 degrees and raise tank temp.”
Does this sound legit or is he just working an extra $900 out of me for plumbing work only to turn around and tell me I need a new water heater afterward?
Thanks. 
Best Answer
Looks like you have a hydronic boiler that is heating your house through multiple zones as well as heating your domestic water via a heat exchanger within the tank to the right. If so, this would be referred to as an indirect fired hot water heater.
During the warmer months, the boiler only needs to heat the domestic water as the zone control valves for the heating zones would be closed. This allows for 100% of the boiler’s output to be used to heat the water.
Now in the colder months, the boiler must satisfy all zones, including the domestic water tank. Therefore the output of the boiler is being shared to each of the zones as they call for heat and the zone feeding the water heater will no longer be getting 100% out the output of the boiler.
There could be many potential reasons that your’s is not keeping up with he demand such as:
A schematic diagram of your setup would be helpful in reviewing for potential issues. However, being that this seems to have always been the case, it might be that your boiler is undersized.
Regarding the addition of the a mixing valve and pump: where were they suggesting them being installed?
If you have turned up the tanks temperature too high, there is potential for scalding to occur at the plumbing fixtures. However it is often more efficient to run the water heater at a higher temperature and in these cases a mixing valve is installed on the outlet of the domestic hot leaving the tank to bring the temperature of the water as it leaves back to below scalding.
The only place I see that a pump might help would be on the heating zone that heats the domestic water tank. Similar to this setup:
However, a better understanding of your setup would be required to determine if feasible.