Why use a mixing valve?

**Terry Beck** · 03-09-2011, 06:45 PM

See pic. Electric 65gal water heater with mixing valve between the cold and cold supply. What is the advantage to this? (why would a plumber install one).

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James Duffin · 03-09-2011, 06:53 PM

Normally it is done so you will have a safe temperature at the sink faucet. It is sometimes done so you can have 140 degree water at the dishwasher and 120 degree at a sink but in this case I don't see a hot water line leaving the heater except for the one going to the mixing valve.

**John Arnold** · 03-09-2011, 06:55 PM

Not positive I understand the set-up, but could be so the water heater temp could be very high to reduce chance of micro-organisms (Legionella, etc.) in the tank, but then tempered to protect against scalding?

James Duffin · 03-09-2011, 07:13 PM

Just curious if the water heater is used for space heating along with domestic usage?

**Terry Beck** · 03-09-2011, 07:24 PM

first, excuse the typo in my initial question - the valve is between the incoming cold water supply and the outgoing hot water feed.
The temp setting on the upper thermostat was set about what I would expect to produce 120 degree temps, the lower thermostat was set a bit higher. I took hot water temp measurements at kitchen and master bath (nearest and furthest points from the water heater): 121 and 123 degrees

In this case, to me it just seems to be a waste of energy. Why heat hot water, then cool it down? I 'm having trouble understanding the logic of the plumber that installed it.

- - - -

Though as a side note, this same builder also installed two 4" exterior fresh air vent connections from the roof to the main trunk of the HVAC return air duct in the basement ("make up air"). I am all in favor of proper ventilation of a home, and love vanEE air exchange systems, but I didn't understand his logic of on this feature either.

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**Terry Beck** · 03-09-2011, 07:26 PM

James, to answer your question, no. Just a regular hot water heater. The heat system is a forced air furnace.

I see the point of your question though, because normally I only see mixing valves, or tempering valves, on the return side of hot water boiler heating systems (radiant or baseboard).

**Terry Beck** · 03-09-2011, 07:32 PM

John,

Might be but I doubt it. In any case, as I just mentioned, the themostat setting weren't high enough for that anyway.

House is on a well, so we often test for bacteria in the water supply. When I find repeated contamination then usually we recommend some sort of biological filter (UV). I've never seen concerns about contamination of the hot water tank.

**Jim Robinson** · 03-09-2011, 08:47 PM

Last year or so, there was some buzz about Legionaire's disease, and that you should run the water heater to 140, and use a tempering valve. It didn't get much traction, but maybe that is part of it?

**Matt Fellman** · 03-09-2011, 09:02 PM

Originally Posted by Jim Robinson

Last year or so, there was some buzz about Legionaire's disease, and that you should run the water heater to 140, and use a tempering valve. It didn't get much traction, but maybe that is part of it?

This is what I've heard.... basically a one side of the argument is against scalding so it must be at 120... the other side says that's prime bacteria breeding ground. So, a whole house mixing valve fixes it.

I'm actually not seeing too many of the whole house ones but I am seeing them regularly for soaking tubs (usually in the master) where there's a mixing valve under the sink vanity.

I suppose it makes sense since a tub is the only place you can get water straight from the tank on your whole body. Presumably, the mixing valves at the tub/shower and stall showers are set to a non-scalding temperature.

Mark Ramos · 08-21-2011, 06:19 AM

If the home is in an area that gets humid and the main feed is very cold, the mixing valve could be used to prevent sweating pipes and fixtures.

**Ron Hasil** · 08-21-2011, 07:42 AM

Lots of plumbing codes state that you can not use the gas control or the electric thermostat for precise temperature control. Which if you follow the letter of this code you will be required to install a tempering valve.

In Illinois they have added that on tankless water heaters that a tempering valve be installed. ALso since there are hybrid heaters coming out they address that as well.

TITLE 77: PUBLIC HEALTH
CHAPTER I: DEPARTMENT OF PUBLIC HEALTH
SUBCHAPTER r: WATER AND SEWAGE
PART 890 ILLINOIS PLUMBING CODE
SECTION 890.1220 HOT WATER SUPPLY AND DISTRIBUTION

Section 890.1220 Hot Water Supply and Distribution

7) Indirect, External, Submerged Coils. Indirect, external, tankless or submerged coils used in heating water shall be equipped with a thermostatic mixing valve or valves when not connected to a storage tank. A pressure relief valve shall be installed on the cold water inlet of the tank. A properly sized temperature and pressure relief valve, based upon the energy input rating of the coils, shall be installed on the tempered line with the temperature sensing element immersed in the tempered water line as close as possible to the mixing valve.

8) Direct Fired Instantaneous Heaters. (Storage tank of more than 64 fluid ounces.) Direct fired instantaneous water heaters shall be equipped with a thermostatic mixing valve or valves which conform to ASSE 1017-1999. A pressure relief valve shall be installed on or adjacent to the heater. A properly sized temperature and pressure relief valve, based upon the energy input rating of the heater, shall be installed on the tempered line with the temperature sensing element immersed in the tempered water line as close as possible to the mixing valve

So I would go with meeting a plumbing code.

**H.G. Watson, Sr.** · 08-21-2011, 10:34 AM

Originally Posted by Terry Beck

See pic. Electric 65gal water heater with mixing valve between the cold and cold supply. What is the advantage to this? (why would a plumber install one).

That valve has been installed to control the temperature of the hot water supply system, and is at the hot water outlet from the water heater. The valve is futher supplied with cold water, sharing supply with that of the water heater (cold water intake side). The plumbing systems are not properly supported or protected, this appears to be in-progress construction, not yet completed.

Montana uses the 2009 UPC., prior to that it utilized the 2006 UPC (not the ICC plumbing codes/chapters). The UPC addresses requirements and has for several code cycles. Is applicable for homes other than those upon farms supplied by a private water supply system.

I would suspect that this home has more than one tub, tub-shower, or shower compartment in the home, more than one toilet, and that likely they are on different levels or stories than that of the 65 gal. electric storage-type water heater and likely laundry room, planned automatic dishwashing machine, etc.. I would further suspect that there are employed other devices "downstream" regards to shower compartment controls and/or tub fillers.

Water heater thermostats were never intended to provide precise temperature controls for hot water systems. The UPC expressly prohibits reliance or use of same for temperature control for hot water systems.

Theoretically, for the phenomenon known as "stacking" or "thermal layering". The hot water is less dense and rises to the top of the hot water tank. The cooler water drops to the bottom of the tank. Stacking or layering occurs when hot water rises to the top of the heater due to recurring short duration heating cycles caused by a frequent number of small quantity hot water uses. This phenomenon can occur in any type of storage water heater and generally is more significant in vertical heaters.

At the top of a water heater that is theoretically set for 120 F to prevent scalding, the temperatures can easily reach 150 to 165 degrees Fahrenheit. These extremely high temperatures will cause third degrees burns and severe scald injuries in an instant upon contact with the skin.

This is why it is highly recommended to install the proper type of an ASSE 1017 thermostatic mixing valve on the outlet piping of a water heater to limit the hot water distribution temperatures to a maximum safe delivery temperature of 120 F. If high temperature hot water uses are required in a building, it is recommended to install an ASSE 1070 thermostatic mixing valve on the local branch piping serving a fixture or group of fixtures. The mixing valve can then reduce the hot water temperature to a safe temperature.

ASSE 1017 - Temperature Actuated Mixing Valves for Hot Water Distribution Systems. Thermostatic mixing valves complying with ASSE 1017 are designed to control temperature from +/- 3 - 7 F, depending on the size, when flowing at the required flow rate. It should be noted that ASSE 1017 has no test for compensation during pressure fluctuation, so in order to minimize pressure fluctuations between the hot and cold water lines; the mixing valve needs to be located at the hot water source. If there is a circulating pump in the system, care should be taken to make sure a mixing valve is piped properly. The tempered water return pipe should split after the tempered water circulating pump and one line should be routed to the cold water inlet of the water heater with a balancing valve to throttle this flow. The other valve should be routed to the cold water inlet of the ASSE 1017 thermostatic mixing valve. This piping arrangement allows the mixing valve to mix 100 degree return water with 140 degree hot water to get 120 tempered water deliver to the tempered water system when there is no flow from any of the fixtures. If there is no flow, no cold water can mix with the hot water to temper the hot water. If the tempered water return pipe is only routed back to the water heater, and there is no flow from a fixture, the circulating pump will force hot water to leak through or blow by the clearances in the mixing valve causing the tempered water line to rise to the full water heater outlet temperature setting. If the tempered water return line is only routed back to the cold water inlet or mixing valve return inlet, then the cooler tempered water return water will leak through or blow by the clearance in the mixing valve and the tempered water system temperature will eventually drop to ambient temperature. This would provide an opportunity for thermal shock as the hot water arrives at a shower after the system has cooled down.

Additional controls, such as individual pressure balancing and thermostatic controls can be installed downstream such as shower compartments, tub fillers, etc. to handle thermal shock and outlet temperature. (ASSE 1016, ASSE 1062, ASSE 1066, ASSE 1070, or even 1069 for a shower "room", for example).

**Dave Hill** · 08-22-2011, 06:40 AM

I too was wondering about these the first time I saw one. So I went to the manufacture's web site. The one I saw was an Apollo mixing valve. The web site explained that with the mixing valve, in effect allows the extra piping in the house to "store" hot water, making hot water available quicker.
I have no idea how this can work, but that's what I took away from the description. You'll have to google it, I don't have the web site handy. I don't recall any discussion about it being used for height or to control organisms from growing. I have only seen them in brand new homes here in the desert around Phoenix - no altitude issues here. This is just my $0.02 on the subject.

**Ralph Holden** · 08-22-2011, 09:21 AM

Another reason to use such a tempering valve is to increase the amount of hot water available for use. The hotter the tank water, the less it takes to mix with the cold to produce the desired temperature. Another issue I see with the pictured installation is the lack of a "heat trap" in the hot water line. The hot water supply from the water heater should go down (usually a minimum of 18") before entering the tempering valve. That will keep hot water from migrating up to and through the valve and creating a slug of very hot water in the piping above the water heater.