good tempering valve for electric on-demand?

archibald tuttle

when i'm dealing with my own house, I have taught my wife about the vaguries of smaller electric ondemand (e.g. bosch "tronic") and in my case I installed the 9 KW watt which is marginal for cold weather shower flow so I didn't want a tempering valve that wouldn't close all the way. now the cost and convenience of these units can suggest them as replacements where single apartment supply is needed and the electric service can manage but I'd be looking at the 12 KW unit and a tempering valve for low flow because these don't regulate output temp like the modulating gas units. (although the installation instructions for the 12KW seem to indicate it may actually have some rudimentary modulating that would make this less of a problem than on the 9KW, i'm on hold to with tech to see if this is so.)

over time, i've never been particularly impressed by the standard tempering units from the old tankless coil days but maybe i expect too much life and performance out of them, and i never have really emperically tried to get them to go from high coldwater mix to low to no cold water to see what kind of range of adjustment they can offer out of the box. brands? models? experience? anyone. thanks

brian

PS - even our 9 KW unit blew out 3/4 pex on the hotwater outlet–given the unfinished state of our place (the cobblers children and their shoes problem),wasn't as much of a disaster as it could have been. But low flow outlet temps on these are high (not like baseboard temps that we standardly expect pex to sustain but at higher pressures. I have never seen a blowout of pex like this and think it was no coincidence it was at the hottest point of outlet only a few inches from the unit so i've taken to adding 3 or 4 feet of copper if space is available on the outlet and if there were a good tempering valve would be possible to place it in the hard piping (even with shorter hard piping) so that the pex did not see high temps up toward the thermal cutout are not presented to the pex. (installation instructions . . . oh yeah those, recommend copper but don't really prohibit anything else so i'm kind of splitting the difference for the time being. they specifically prohibit pipe dope (who knew). I learned the hard way and had to take the flow switch off and clean it several times (vs. the takagis have always been bombproof regardless of what I put on the connections . . . .).

They don't want a check valve or vacuum breaker on these so I don't think these are circumstances of raised pressure beyond typical domestic water pressures that you could see on a tank style system requiring an expansion tank (vs. takagi recommends a check if you get nuisance triggering of their on demands). Not only isn't the system limited by a check but you are only heating on flow anyway so there is no stored hot tank that could expand to over pressure the system while there is no water delivery.

archibald tuttle

so, not unexpectedly, customer service and reps for bosch have no idea how the US12 regulates output temperature.

although this phrase from the submittal:

"Water heater shall use the signal from an internal flow switch to activate the correct number of elements to reach the target temperature set on the external dial"

suggests to me that it isn't infinitely variable but is capable of self-regulating the number of elements called (I assume there are 2 although that isn't clear and i'm guessing based on my experience with the US9 and the internal physical similarity albeit differing controls. ) so basically, I think they mean it can operate at full power or half power in which case I don't know why the specs don't just indicate 12 kw or 6 kw operation that would easily answer the question.) That ain't super sophisticated, but its way better than US9 and they are similarly priced albeit you need more amps and up a wire size to run the thing.

they sort of seemed to say to me that the water temp sensor is actually incoming (which makes sense from a seasonal adjustment perspective although I don't know why you wouldn't just monitor output result . . . ) and then there is a temperature adjustment screw that maybe affects the setpoint at which the choice of 1 or 2 elements is made.

anyone who has actually messed with one of thes US12s i'd be interested to hear the results.

thanks

brian

yellowdog

ive never messed around with a bosch on demand but we do install steibel eltron electric on demands as well as rinnai propane ones. they, along with all conventional tank installations, all get caleffi mixing valves and we have had great luck with them for years.

hot_rod

The 520 is a fast responding thermostatic, based on a design for tankless water heaters. While it is not a listed 1070 valve, it does have a shutoff function if you lose hot or cold and it is unable to mix. Give one a try, they get installed on tankless coil boilers and get good reviews.

https://www.caleffi.com/sites/default/files/file/01266-22-1_na.pdf

archibald tuttle

@yellowdog score one for caleffi. @hot_rod what are the parameters of flow adjustment for the caleffi tempering/mixing valve for DHW? do they go so far as no cold water add in depending on the setting? @yellowdog you have installed a lot of these in the field. are there service parameters half a dozen years out. have you had them apart?

i have not had much need for tempering valve. the gas fired on demands we use have pretty good modulation and target temperature delivery so i haven't been tempering.

hot_rod

All thermostatic mix valves need temperature differential between hot inlet and mixed outlet to work properly 

I think it is 12 degrees on that model. So if you want to mix to 120 outlet, hot inlet needs to be 132

Some thermostatic valves need 25 degrees or more differential 

yellowdog

@archibald tuttle we have to put them on everything in vermont and new hampshire. usually if there is a complaint and its the mixing valves fault, we just replace it. i think they are serviceable to a point, but we have never had any long term success by opening them up and trying to clean them or anything.

archibald tuttle

@yellowdog I can appreciate that caleffi focused on quick response for ondemand installs, but I can't understand where on demands having reliable output temperature control would need a tempering valve ( I just got interested in less sophisticated on demands that don't provide such control for circumstances that need a bargain solution, ergo my present interest in tempering).

A system design that needs 10 or 12 degrees above the desired delivery temperature to operate seems counter intuitive in an age all worried about energy. At least gas condensing units can still condense based on input temps but, for those of us sensitive to the amp load added to services from electrics, we are going to be chary to go for the additional capacity needed to raise the output another 10 degrees–nevermind the utility sensitivity to system loads.

I get It isn't a 1 for 1 trade off as you can heat a consequently smaller volume to the higher temp but my instinct rebels against making the water hotter than needed in a day and age of sophisticated control (I'm the first one to take issue with over control and I am using the less sophisticated electrics to some extent to avoid the cost of controls and then essentially adding one downline in the form of a tempering valve so I have to look in the mirror and see if i'm really felling OK).

OK, california will just put out bulletins asking you not to charge your electric car they made you buy, or take a shower on certain days. That will fix it. (To be fair, California would probably insist that you get a heat pump hot water heater for almost 10 times the cost of an entry level on demand and write you a check with other ratepayers money to encourage the 'right' choice).

My idea for tempering electric on demands is kind of the reverse of the apparent industry standard (that could be the standing headline on my posts). I envision a tempering valve that adds no cold water at high demand–as the 9 KW units I have can just keep up with a shower at full bore. That could be a significant design headache relative to current technologies as described by @hot_rod and maybe I should just yield to the resistance :-) and face facts that resistance heat is a dinosaur and no-one is going to fill that market niche to the extent it even exists . . . but I can dream.

I could also close the ball valve 90% of the way and get output temps up without added amps and ride the line near the overtemperature cutout and try the existing caleffi, but that doesn't really seem like a solution for repetition elsewhere but my own experimental palace. And maybe I should just anticipate replacing rather than repairing the valve when inevitably i replace the hot water unit, whatever technology is chosen. No disrespect intended to caleffi. Anything operating in DHW service is going to be challenged with corrosion over time and virtually all peripherals i'm aware of, the major service parts cost as much as a brand new unit–given that design changes over time can require stocking of multiple generations of repair parts and that the cost of manpower and operating a small business leave contractors and customers perhaps better served by remove and replace strategies.

And I'll definitely try the next step up in electrics that apparently can run at 'half-throttle' if not having more finely stepped or infinite output control which, in my addled mind, means you don't need (vs. require) a tempering valve.

25¢ more worth of good Gulf. damn can't find that commercial on youtube.

hot_rod

A group of manufactures got together to suggest tankless water heaters could meet or exceed the temperature control performance of a listed 1017 mixing valve. They did get two new standards developed for this, ASSE 1082 and 1084

So in theory if you buy a tankless or combi that has listed to either or the new standards, you should not need an additional mixing valve.

I’ve yet to see this standard on any tankless or combi. I’m sure there is a cost involved for manufacturers to list and be able to apply the shield to equipment. And I suppose the AHJ would need to be onboard with its acceptance.

https://www.asse-plumbing.org/media/23753/asse_tankless_water_heaters_white_paper.pdf

archibald tuttle

@hot_rod I'm not really sure how much energy savings are at issue, given that you can substitute less water at higher temp for more water at a lower temp coming out of the same heater albeit everybody and their brother seems to think you can save energy by running lower temps even where condensing is not at issue. But my real world experience has been that the modulating fire on demands have been as reliable on temp. control over the almost 20 year arc we've had 'em in service as any tempering valve over that period (i will try a caleffi next needed install, but then i gotta wait 20 years to see if I can take that back :-).

The install instructions for takagis only specify tempering if you are using them dual purpose for heating, albeit i understand those are manufacturers recs and not the code demands, but there is no reference even to tempering per code anywhere else. The ASSE seems to also be industry best practices rec. I get how the code derives it approach to on demands from the old tankless coils that could be sitting at really high temps in a boiler and maybe whatever the real world headwinds to certifying under the 1082 or more likely the 1084 standards even if they perform reasonably along those lines.

I'm not out to run down good tempering hardware but really to (knock on wood) say how impressed I am with the longevity of the internal regulating mechanisms on the takagis (I don't have experience with other brands but glad to hear about long term performance from any who do). my interest in tempering is with less sophisticated units like those i was discussing.

but at root, i thought this is why we have anti-scald shower valves which are intended to prevent scalding and thermal shock (i.e. the cold version of scalding) as well. I guess you could get scalded at the sink in theory because there aren't balancing valves on the vast majority of faucets; but in the shower you don't have the same option to quickly remove yourself from the water flow while correcting which is why the shower is the focus.

hot_rod

There are several states that require mixing valves, point of distribution, ASSE 1070 on all types of DHW appliances.  I’m not sure if those states with that requirement are looking at or accepting tankless that list to the new ASSE standard. 1082, 1084?

I think all single handle shower valves need to be 1016

There are both  T temperature and  P pressure compensating shower valves