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Community and Q&A

Should my indirect hot water heater have a mixing valve?

dtgriscom | Posted in Mechanicals on

I have a 2300 square foot colonial, built 1960 in Massachusetts (Zone 5A), with three zones of fin tube baseboard. There’s a 75 gallon whirlpool tub which we almost never use; the two showers are fairly low-flow. Heat loss done by was 55,840. I use town water, which is very good quality (Quabbin reservoir). I’ve accepted a bid to replace my heating system with a Lochinvar WHN-85 boiler and SIT40 indirect water heater.

The plumber is recommending against using a mixing valve on the SIT40 output due to reliability concerns (e.g. scaling). But, I’m concerned about Legionella (although I’ve never had a mixing valve before, and have never heard of local problems), and would like the flexibility of being able to raise the storage tank temperature.

Any thoughts on this? If I do ask for a mixing valve, the plumber won’t have a lot of experience with installing one; any specific valve recommendations?


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  1. GBA Editor
    Martin Holladay | | #1

    The type of valve you are talking about is usually called a tempering valve. This type of valve requires hot water input and cold water input; the valve mixes the two streams to achieve the desired temperature setting, which is user-adjustable, and the adjusted water is sent to the hot water valves at sinks, showers, and tubs.

    Here is the advice given by a U.S. government agency (OSHA) website:

    "Can Legionnaires' disease be prevented? A. Yes. ... Specific preventive steps include: ... Maintain domestic water heaters at 60°C (140°F). The temperature of the water should be 50°C (122°F) or higher at the faucet. ...
    "Q. Do you recommend that I operate my home water heater at 60°C (140°F)? A. Probably not if you have small children or infirm elderly persons who could be at serious risk of being scalded by the hot water. However, if you have people living with you who are at high risk of contracting the disease, then operating the water heater at a minimum temperature of 60°C (140°F) is probably a good idea. Consider installing a scald-prevention device."

    You have to decide which approach makes sense for your household.

    I don't worry much about Legionnaires' Disease, which is rare.

  2. dtgriscom | | #2

    The quoted OSHA recommendations make sense to me; none of us have an elevated risk of catching a disease, and as I said our water supply is very good, so I'll take the disease concern off the table. However, although none of us have an elevated risk of scalding ourselves, I would still want to limit the tap temperature to 120F if I run the DHW tank hot for another reason.

    What about other benefits/concerns? I know that I could get more hot water out of the tap for a given tank size if I run the tank hot and mix it back down, but I'm not expecting to run out of hot water with this system (again, we almost never use the whirlpool tub). I really don't want to add another maintenance item, so if a mixing valve is likely to seize up every few years then I'll punt.

    What about the boiler system functioning? I know our baseboard will generally need higher water temperatures than would, say, large cast-iron radiators; will running the DHW at 120F conflict with that in any way? Yes, they'll be on separate circuits, but will there be problems if they both call for heat at the same time?

    Finally, would it be better for the condensing boiler's efficiency if I run the DHW at 120F? That's the only thing the boiler will do for half of the year, so it's a concern.


  3. GBA Editor
    Martin Holladay | | #3

    Most systems with an indirect water heater put the indirect tank on its own circulator, and set the tank up as a separate zone. The temperature of the water in the indirect tank is controlled by an aquastat connected to the circulator. So the temperature of the indirect tank can be different from the boiler water temperature, which is likely to be higher in order to take care of your space heating requirements.

    As far as I know, the lower the temperature at which you set the aquastat on the indirect tank, the lower the energy use.

  4. dtgriscom | | #4

    Makes sense to me: no mixing valve needed.

    Thanks for all the detailed help,

  5. Expert Member
    Dana Dorsett | | #5

    Massachusetts code (and the IPC) requires tempered water for any distribution plumbing to bathing or hand washing taps, but allows untempered water for laundry or dish washing equipment. You can't simply lower the aquastat setting or use a mixing valves such as shower or sink mixers at the taps for code compliance on this front, as it is expressly dis-allowed as a substitute in the language of the code.

    An inspector would condemn this system for use until the requisite tempering or mixing valve is installed between the indirect and the tub.

    That's not to say you can't set the aquatstat, thermostatic mixing valve/ tempering valve to whatever you want once it is installed.

  6. dtgriscom | | #6

    Well, that's an overriding concern. I do have faucets (and the whirlpool tub) which can run straight hot water, so that means I have to have a mixing valve present at the DHW tank output, or it won't pass MA inspection?

    If so, any recommendations on mixing valve manufacturers/models?


  7. JonathanRupp | | #7

    What i was told by a friend who has a solar hot water company is that the main losses / inefficiencies in indirect fired hot water tanks (with a high mass boiler) are in the summer when your boiler has to heat itself up to (and above) the temperature of the DHW tank. In the summer, it then sits idle, and looses heat for most of the day, before the next firing, when it has to re-heat itself before it starts to warm the tank. For summer operation then, you want to minimize the amount of time the boiler needs to be hot, so you want to only fire the hot water tank 1ce a day, which then requires a large tank, or high temperatures w/ a mixing valve, so you dont run out of hot water.

    What is interesting then, is for winter operation, your boiler is already going to be hot all the time, so there isnt a big loss of firing the tank (beyond heat losses in the tank circulation loop) the main losses are then through the DHW tank walls. Here then you want to lower the tank temperature so to minimize tank losses.

  8. user-2890856 | | #8

    Take the time to read the following papers . Setting a water heater lower does not save energy except for maybe about the first cycle which equates to about .02 NG . In fact increasing the storage temp and using an ASSE 1017 device can actually provide the following benefits , Scald protection , by the way codes state that the gas valve on a water heater shall not be used as the limiting device , The hottest water stratifies at the top of a water heater and is what comes out of the faucet first . The sensor which tells the heater ti turn on and off is located at the bottom of the tank where the cold incoming water is mixing with hardly heated water and the stuff up top can be REALLY HOT by the time the sensor shuts down the operation . While Martin states that legionellosis is rare , I can safely state that 25-30% of cases diagnosed as Pneumonia are in fact Legionellosis . Research it through CDC . The EPA just conducted another study of the public drinking water in the nation and tests determined that iver 50% of those supplies contained Legionellosis bacteria . Once an indirect is up to temp you can actually lengthen the time between calls for heat from the boiler by setting the differential to say 30* below setpoint . This will also make your new boiler run a nice long EFFICIENT cycle instead of short cycling it will also make your 40 - 50 gallon indirect provide usable hot water like it was an 80 Gallon so you may actually be able to decrease the size and first cost . A mixing valve gets set to a certain setpoint temp for delivery to the faucets , now since water pressures vary and the water from the tank can vary in temp how do you think the valve makes sure the water is a consistant temp for delivery ? Jeopardy music! It moves allowing different rates of hot and cold in to the mixed temp desired . Now ,what do you think happens when the water coming out is too hot ? It closes protecting you and your family . Some think that mixing valves and legionellosis are just something we plumbers came up with to increase the bottom line , I assure you the threat is real and this is not a gambling hall so don't gamble . And for God's sake why would you ask a question which requires specialty knowledge by a LICENSED individual on a green building site ? This is not an insulation, air sealing , wall assembly issue this is a whole different ball game that , no disrespect intended Martin , should be presented in a different forum and answered by folks who take the Credo "Protect the health of the Nation " seriously and whose business and responsibility it is to know such things as second nature . Hope this helps you choose wisely .
    Recommendations . Taco 5000 Series mixing valve for DHW . Set the indirect storage temp at 140* and to call for heat at 110* , Set the temp on the mixing valve no higher than 115* (106* gets uncomfortable) for the kids . Enjoy your new lower gas bills and don't sweat getting sick or burnt by your new system .

  9. GBA Editor
    Martin Holladay | | #9

    Thanks for your comments.

    At GBA, we get all kinds of construction-related questions all the time, and our entire community chips in to try to help out questioners. Your expertise is appreciated. Reader contributions are one of the reasons that the GBA site works so well as an information clearinghouse.

    The information I provided on Legionnaires' disease came directly from the OSHA website. OSHA recommends that water heaters be set at 140°F, but also notes that in homes without a tempering valve -- that is, most U.S. homes -- such a high setting might pose a risk to small children and infirm or elderly persons.

  10. Expert Member
    Dana Dorsett | | #10

    While most homes may not currently have a tempering valve, they will (or should) have one when any new water heater is installed, to comply with the IPC. This requirement has been in the IPC for at LEAST 8 years (probably slightly longer, but I don't want to look it up, OK :-) ), and most states are at IPC 2006 or newer.

    I'd be a bit leery about any plumber who recommends skipping the tempering valve. A licensed bonded plumber in MA should definitely know better.

    You don't gain any efficiency running longer burns at higher temp with a low mass condensing boiler like the Lochinvar WHN-85. Anything you may gain due to the longer burn time gets eaten up by lower combustion efficiency due to higher-than condensing temperatures plus higher standby losses. If it were a cast iron boiler it would be worth setting it for a very high differential to max out the burn times, but with a low mass burner like yours even 2-3 minutes would be plenty. It doesn't take a high differential on 40 gallons/334lbs of water to get that with a boiler that modulates down to 17,000 BTU/hr, but you'd have to set it up to run something other than highest-possible fire when serving that load. There are some tradeoffs to be made, but you can still get there without storing water at 160F+.

  11. user-2890856 | | #11

    Protecting equipment from short cycles while providing the required function of the system is always my concern Dana . While there may be a marginal efficiency gain from this strategy there certainly is a penalty for short cycling the equipment . Agreed ?

  12. Expert Member
    Dana Dorsett | | #12

    Sure, short cycling always exacts an efficiency penalty, but how short is "too short" depends a lot on the thermal mass of the boiler and associated zone plumbing. A 2-3 minute minimum burn for the indirect using a 4-6 plate cast iron boiler would be an efficiency disaster, but not so much for a low-mass mod-con like the Lochinvar WHN85.

    Even full-on it's output never exceeds 80,000 BTU/hr (1333 BTU/minute), with a differential of even 10F on the 334lbs of water in the tank that's 2.5 minutes of burn. With the burners cranked to the max it wouldn't actually hit it's 95% efficiency. You'd be looking at something closer to 88-90% unless you backed it off to something in the mid-firing range serving a tank kept at 130-140F. But either way, the abandoned heat at the end of the burn in the WHN85 is a small fraction of what you'd be abandoning in a high mass boiler.

    If you're storing water in the indirect at 160F+ it means even with heat-purging controls on a high-mass boiler you would be abandoning significantly more heat in a high mass boiler than if you were keeping it at 140F, but it's less than the hit incurred by short cycling the thing multiple times on long hot water draws.

  13. dtgriscom | | #13

    Well, the new system is in. A Lochinvar WHN-85 boiler with a Lochinvar SIT 40 indirect water heater. And no: there is no mixing valve on the water heater output.

    My reasoning:

    This plumber has installed a whole lot of heating systems in my town, and obviously hasn't run into problems with the local building inspector
    I'd rather he did what he has experience doing, rather than something I'm pushing him to do
    We have plenty of DHW capacity, and don't need the extra given by a mixing valve
    Cooler temperatures in the HW tank allow the boiler to run cooler and more efficiently
    I trust the boiler's control of the HW temperature, and even if that fails my family isn't overly at risk for scalding
    Not every law and regulation should be followed (e.g. what percentage of drivers drive the speed limit on the highway?)


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