Mixing Valves - How effective are they?

[Jdavis37]

I have another thread I started with regard to our water heater questions, and once we finalize our decision and installation I'll finish it out. This is a 25th hour question so here goes.

Mixing valves (with selectable thermostatic controls). Do they really extend the amount of hot water coming out of a given faucet, or do they serve primarily as a scald control? Intuitively it would seem they should extend the water heater given the water is mixed at the tank outlet. But in effect every faucet is a mixing valve, just further downstream of the water heater.

I was set on my final answer up until this afternoon but now am seeking perhaps a less expensive solution. At end of the day I just want hot water without spending a fortune if I don't have to ($4.99K for hot water to me is expensive). Tub is 75 gallons to fill line (minus the person). If a mixing valve can extend a hot water heater at least somewhat effectively, I may change my prior answer!

 

[Terry]

A true thermostatic mixing valve on a water heater allows you to turn up the heat on the water heater and then mix it down to 120 degrees.
That does give you more usable hot water from the tank.

https://terrylove.com/forums/index....c-mixing-valves-water-heater-tempering.45651/

 

[Sponsor]

 

[Jdavis37]

A true thermostatic mixing valve on a water heater allows you to turn up the heat on the water heater and then mix it down to 120 degrees.
That does give you more usable hot water from the tank.

https://terrylove.com/forums/index....c-mixing-valves-water-heater-tempering.45651/

Thanks Terry. With our aging and soon to be replaced water heater, I have had to turn the WH thermostat up greatly in order to fill the tub (during winter when inlet water temp is between 50 and 55 degrees on average). Now I wonder how much more effective it would have been to have installed a mixing valve at the WH outlet. I now fill the tub using only hot water and make up the remainder with cold once the hot runs out. Often in winter when it does turn cool there is just no easy way to fill it. A new 55 gallon water heater with mixing valve may work and be much more cost effective than the other solutions we have been given!

 

[hj]

You can "turn the temperature of the tank' up to its maximum anytime you want to. A "mixing valve" gives you more SAFE TEMPERATURE water when you do it, however, rather than depending on the user to modulate the temperature by mixing more cold water with the hotter water.

 

[elb01]

Hello,
Is this mixing valve add 50-50 hot and cold water automatically?
Or it does because of thermostats?

 

[Phog]

There are several types of mixing valves.

A plain tempering valve (which is increasingly uncommon) mixes cold into hot without paying attention to pressure or temperature. If pressure changes on one side (for example someone flushes toilet and the cold water pressure drops) the out mixing % will change too.

A pressure balanced tempering valve will maintain the same pressure ratio when mixing the output, aka someone flushes the toilet and now the valve adjusts so mixing % stays the same & temperature out does not change. However if the temperature of the incoming hot or cold water changes (for example water tank temp drops from 140 to 125 deg as it gets drawn down) the outgoing water will also change in temperate.

Last, a thermostatic mixing valve senses the temperate of the outgoing water and adjusts the mixing % to meet a specific temperature setting, the pressure and temperate incoming don't matter anymore, you always get the same temperate water out. This last type of valve is what is being discussed above.

This one installed for a bathtub, placed under the bathroom sink for the tub.

 

[Jadnashua]

The tempering valve best used on a WH output does sense the outlet to the home, and adjusts the amount of cold water needed to lower the higher tank's temperature.

I wish my wintertime inlet water stayed at 50-55...I've seen mine reading 33-degrees after a cold spell! I run my tank at 140-degrees, and temper it down to 120. Note, those valves do eventually fail, either by getting jammed up with mineral deposits, or the sensing device (often a bimetallic strip but might use a wax device) failing.

 

[Tuttles Revenge]

Anyone have a calculating formula for the math illiterate to determine the water heater stored capacity to gain a desired capacity.

Say I'm replacing an 80 gallon tank that was intended to provide water for a soaking tub but will be replacing that tank with a 50 gallon but will want to ensure I have the stored BTU to still fill that tub. In the Pinned and locked thread mentioned above Honeywell mentions that a 40 gallon tank would have the capacity of a 60 gallon tank.. but it doesn't mention the temperture of the stored 40 gallon tank..

 

[Jadnashua]

If you don't raise the tank's storage temperature above the outlet temperature of the tempering valve, the valve will not increase the amount of 120-degree water you can draw through it.

Your tank may have a maximum recommended value. The one I have lists a maximum storage value of 140-degrees F. some might allow 160-180-degrees or maybe more. Code calls for the maximum residence water to be 120-degrees F, except for a direct feed to things like a dishwasher or washing machine, but every faucet is limited to 120-degrees. the higher the storage temperature in the tank, the more it will cost because the higher the temperature, the bigger the difference with the room, and that drives the heat out into the room faster. Newer WH are required to have better insulation to meet energy star requirements, so that can help. If you can't or don't add insulation your hot (and cold if you're using recirculation) water pipes, then your losses will be higher.

Using a unit that only runs the 'hot' to 95-degrees will only run for a couple of minutes, then shut off, then not come back on again for maybe 15-minutes or so, less often in the summer than in the winter.

Used with a timer and insulated lines, it may not cost more when you account for the water down the drain. The convenience IMHO, is worth it.

To calculate the BTUs stored, figure about 8#/gallon of water (water weighs more, but the cooling as you add replacement water, that will give you a closer value) by the number of gallons by the storage temperature. Without a tempering valve use 120-degrees. With a tempering valve, use whatever storage temp. That will give you a relative quantity of stored energy. The real total energy would use a different number, but you're interested in the relative value, not absolute differences.

Standard issue for garden tubs in a home. Normally found in the bathroom sink cabinet.

 

[Tuttles Revenge]

If you don't raise the tank's storage temperature above the outlet temperature of the tempering valve, the valve will not increase the amount of 120-degree water you can draw through it. **Shocking**



To calculate the BTUs stored, figure about 8#/gallon of water (water weighs more, but the cooling as you add replacement water, that will give you a closer value) by the number of gallons by the storage temperature. Without a tempering valve use 120-degrees. With a tempering valve, use whatever storage temp. That will give you a relative quantity of stored energy. The real total energy would use a different number, but you're interested in the relative value, not absolute differences.

I'm just interested in a quick, down and dirty estimate. Something I can easily explain and therefore sell to a customer. Somewhere I had read that bumping the temp of the tank to 140 and tempering it down gained 20% in stored energy.. But that doesn't Jive with the value given in that honeywell brochure if the temp were set at 140.. their claim is that a 40g tank can hold the same btu as a 60g tank which is a 50% increase. I've always just done it knowing it works and how it works, just not the more intricate math on how to calculate it.

I will see if I can get your calculations to work for what I'm doing. I appreciate the explanation.

 

[Reach4]

Anyone have a calculating formula for the math illiterate to determine the water heater stored capacity to gain a desired capacity.

Say I'm replacing an 80 gallon tank that was intended to provide water for a soaking tub but will be replacing that tank with a 50 gallon but will want to ensure I have the stored BTU to still fill that tub. In the Pinned and locked thread mentioned above Honeywell mentions that a 40 gallon tank would have the capacity of a 60 gallon tank.. but it doesn't mention the temperture of the stored 40 gallon tank..

If I did it right...

So if my coldest mixing water is 60 degrees, and I used to set the old 80 gallon tank to 13o, and I set the the new WH to 160 degrees. I would need a 56 gallon tank to give the same amount of tub-filling ability as I had with the old WH.

 

[Tuttles Revenge]

I looked up a calculator similar, but it didn't have size/volume as a function.. That looks exactly like what I'm looking to do.

 

[wwhitney]

If I did it right...

I checked the middle column, looks good to me.

I'd write the formulas like this:

C = cold water temp
T = target temp
V_T = target volume
S = storage temp
V_S = storage volume

Then "heat" in the tank = V_S * (S - C), and "heat" required for the "target" is V_T * (T - C). In general you'll want to equate those and solve for the unknown.

If you're comparing different size tanks at different temperatures, then S and V_S represent one tank, T and V_T represent the other tank.

If you're calculating tank size to fill a tub, V_T would the tub fill volume (rule of thumb for that as a fraction of total tub volume to overflow?) and T would be the initial tub fill temperature (rule of thumb for that?) . Also, V_S would more properly be the volume you can draw from the tank at full temperature S; can you really get 80 gallons of 140F water out of an 80 gallon water heater set at 140F?

Cheers, Wayne

 

[Reach4]

I have attached the spreadsheet which generated that table.

Feel free to modify.

 

[Tuttles Revenge]

Playing around with that now!

 

[MikeQ]

I have another thread I started with regard to our water heater questions, and once we finalize our decision and installation I'll finish it out. This is a 25th hour question so here goes.

Mixing valves (with selectable thermostatic controls). Do they really extend the amount of hot water coming out of a given faucet, or do they serve primarily as a scald control? Intuitively it would seem they should extend the water heater given the water is mixed at the tank outlet. But in effect every faucet is a mixing valve, just further downstream of the water heater.

I was set on my final answer up until this afternoon but now am seeking perhaps a less expensive solution. At end of the day I just want hot water without spending a fortune if I don't have to ($4.99K for hot water to me is expensive). Tub is 75 gallons to fill line (minus the person). If a mixing valve can extend a hot water heater at least somewhat effectively, I may change my prior answer!

Back in the day it was common to set water heaters to 150-160 degrees. This helped prevent the tank from running out of hot water during periods of high usage and provided hotter water to the furthest reaches of the system more quickly (considering the initial cooling that happens in a long run of copper pipe). As the water at the tap warmed up we would start to bleed in some cold water to keep the temperature as desired.

Then the energy crisis happened and people started setting their tanks to 120-130 degrees to save money and energy. This worked fine for most people (scary media stories about Legionnaires disease notwithstanding). Large families left their water heaters at the higher temperature so they didn't run out of hot water on the third consecutive shower. If the shower water felt uncomfortably hot, we stepped out of the shower stream. Smaller families saved money by reducing the standby losses of a high setpoint.

Now, most people set their water heater to 120 degrees but the reason given is not to reduce standby losses and save money, it's to avoid scalding. I don't get it. Did peoples skin get thinner in the last 50 years? Last I checked, humans had a built-in scald protection - if it feels too hot your hand jerks back.

 

[Sylvan]

"Do they really extend the amount of hot water coming out of a given faucet, or do they serve primarily as a scald control? "

Many people think a tempering valve (mixing) is considered scald protection IT IS NOT reason being it is slow acting so if someone is in a shower and the washing machine is on or someone flushes a toilet there is a pressure drop and loss of cold water volume and the person in the shower can be scalded

Even Holby which is the only tempering valve I use .. The best one made stated it is not considered an anti scald device

Temperature control and/or pressure-balancing devices or a combination of devices conforming to ASSE 1017, ASSE 1070, ASSE 1066, or ASSE 1062 can be used.

Thermal shock

Pressure-balancing, thermostatic or combination pressure-balancing/thermostatic valves conforming to ASSE 1016 or CSA B125.1 are designed for controlling thermal shock and scalding in showers and tub/shower combination units. ASSE, ASME and CSA recently published a harmonized standard for shower valves