Do I need a mixing valve in my boiler/water heater system?

Question

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.

Answer 1

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:

• The boiler is undersized and cannot satisfy all the zones during peak demand
• Improper balancing of the zones
• Something piped incorrectly and causing a reduction in efficiency

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.

Answer 2

A mixing valve can be installed and the temperature raised in order to get the hot water to last longer. As for the recovery rate it will vary depending on how much water you use and how hot it has to be reheated to. I don't know why the plumber wants to install a pump however.

Answer 3

A mixing valve will allow you to run your water heater at a higher temperature and mix cold water in to get to the desired temperature at the tap. It isn't required.

I can't quite make out the piping configuration, but if you are getting hot water already, it should be fine.

I don't know what the plumber did, but I would check the dip tube to make sure it isn't cracked. I would also look into adding a heat trap loop to the hot and cold (not required). That looks like a Triangle Tube indirect water heater and that's in the installation instructions. :)

Answer 4

install a taco zone control panel with a priority, this will shutdown heating to the house and prioritize the heating of the indirect hw tank, once tank is satisfied it will switch back to heating home, usually 10 minutes

Answer 5

(a) A mixing valve on the potable hot water output from the IDWH will allow the water in the tank to be heated to 145F instead of only 120F, and this will prevent - even kill - the growth of pathogens inside that tank. 120 degrees is just not hot enough for health protection. 145 degrees in the IDWH tank will also allow the difference between the cut-in and cut-out temperatures on the water heater to be increased to 25 degrees, instead of just 5 degrees, and that will mean the IDWH will not have to be heated by the boiler as frequently each day - especially if you install simple heat traps and a timer [see (b) & (c) below]. The mixing valve provides the added benefit of ensuring against scalding injuries and normally providing hot water at virtually the same temperature.
(b) A timer could also be put on the IDWH to prevent heating of the domestic water in that tank when no one will be using it for bath or shower or dish washing.
(c) Simple heat traps on both the hot and cold potable water lines where they connect to the IDWH would reduce heat losses from the potable hot water tank during all the many hours that no hot potable water is being actually used in the house 24-7-365. That can add up to a lot of wasted heat. (d) A boiler control which prioritizes the IDWH will prevent the boiler from trying to heat both the house and the domestic hot water tank at the same time. It just means that during the time that the boler is sending heat to the IDWH it is prevented from also sending heat to your space heating system. If you have anything like normal - even greater than normal - residential use of hot water - or less - you won't even notice the lack of space heating for the ten or 15 minutes it will take to heat the domestic water tank. And controlling the boiler water distribution this way - by prioritizing the IDWH over the space heating - will end any speculations by salesmen and the uninformed that your boiler is "undersized".