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Thread: Stiebel Eltron Tempra 29 Plus, issue with shower

  1. #16
    In the trades Dana's Avatar
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    Quote Originally Posted by Atticweb View Post
    I'm not predicting anything. Just going by what I read, which is often contradictory and/or irrelevant to my situation. Why do you recommend the thermostatic? This is the kind of suggestion with fact I'm looking for. Valve looks a bit pricey. I see you're in California, pretty sure you won't have an issue with temp rise at full flow.
    Especially with a GAS FIRED tankless, which is less flow restrictive (= less of a pressure delta between the hot & cold sides at the mixer) and has much higher BTU/hr output.

    Not that it's going to help this situation, but an electric tankless will be marginal at mid-winter incoming water temps in Buffalo, even after you've fixed the mixer balance problem. The Tempra 29 is going to be pretty much maxed-out at normal shower flows at the end of January when incoming water temps drop under 40F. The deep subsoil temps in Buffalo are under 50F, so as we go into the real winter it doesn't take long for the water temps to drop- it's not much in the way of passive geothermal heating from below. You could perhaps buy more showering capacity with a drainwater heat exchanger, but that wouldn't help batch draws (like tub fills) a bit. With the Tempra 29 you'll have to schedule other hot water use for when nobody is using the shower, at least in the winter. For a 1 person household it's not too tough to manage, but for a 3-4 person household it's a bit of a pain. (Been there, done that.) That would not be the case with a 199KBTU/hr gas-fired tankless.

    The IPC hasn't allowed either the temperature control on the water heater, the temperature controls at the shower mixer, or the anti-scald at the mixer as acceptable alternates to pre-tempered water distribution at least since 2006, and probably before then. Anti-scald valves on showers is still explicitly required, even with the tempered water distribution. (See section 607 of IPC 2012 )Routing untempered water to dish and laundry is allowed, but not to bathing/lavatory. Most people with tanks put the tempering valve at the tank, and only distribute tempered water in their hot water plumbing. State wide NY is still under IPC 2006, but some municipalities may have upgraded. You'll note the verbiage regarding tempered water distribution and anti-scald valves hasn't changed much.

  2. #17
    DIY Junior Member Atticweb's Avatar
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    Thanks, I've been reading about the thermostatic and still don't get it. The fact is my Stiebel Eltron Tempura 29 Plus *controls* and delivers the exact water temp it is digitally set to, by reducing flow if necessary. So, purchasing a valve whose main attribute is to do that also not only doesn't make sense to me, it seems, like I've said before, that it would possibly interfere with the pressure balancing and temp control that is inherent to the way the SE unit works. Again not predicting that, just trying to make sense of it all to choose the best option for an electric tankless that is built to always deliver the same temperature, always adjusting it for incoming and other factors.

  3. #18
    DIY Junior Member Atticweb's Avatar
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    Thanks Dana, your post came in while I was replying to another. I used the temp rise charts from SE and an independent source to choose the 29 Plus (some saying I was borderline 24+ but I went for more), realizing that probably no other use while a shower is taken would be SOP. That's no problem for us empty nesters; venting a gas unit is (for various reasons in this 160-yr-old house) not an option. Thanks for the code I'll have a look at, very helpful. I assume it was written before units such as mine that make it impossible to scald oneself. You seem very knowledgeable; adhering to code and understanding my situation, which valve replacement type would you recommend and why? Even brand if you don't mind.

  4. #19
    DIY Senior Member lifespeed's Avatar
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    Quote Originally Posted by Atticweb View Post
    Thanks, I've been reading about the thermostatic and still don't get it. The fact is my Stiebel Eltron Tempura 29 Plus *controls* and delivers the exact water temp it is digitally set to, by reducing flow if necessary. So, purchasing a valve whose main attribute is to do that also not only doesn't make sense to me, it seems, like I've said before, that it would possibly interfere with the pressure balancing and temp control that is inherent to the way the SE unit works. Again not predicting that, just trying to make sense of it all to choose the best option for an electric tankless that is built to always deliver the same temperature, always adjusting it for incoming and other factors.
    Most people have an expectation of being able to adjust the temperature inside the shower rather than walking to the water heater control panel, although in your situation you will face an upper limit imposed by your water heater and other factors. I have not seen the issue with water heater temperature control and thermostatic or PB valve interaction you are theorizing. Really, this is a basic water heater design requirement. An inability to maintain temperature over a range of flow rates would be a failed design. I doubt you will see this, I know I don't in my setup. You could test with a thermometer while you varied the flow at a hot tap. Should stay fairly constant.

    Edit: down the road as incoming water temps change you may like the option of setting your water temp to 110F or higher for various reasons. A thermostatic valve makes the shower temp constant regardless of hot water temperature (so long as the minimum input temp is met).
    Last edited by lifespeed; 12-10-2013 at 10:57 AM.
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  5. #20
    In the trades Dana's Avatar
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    Electric tankless water heaters and others with precise settable temps existed long before IPC 2006 was drafted. Having a thermo-mechanical tempering valves are designed to protect even when there is a fault condition in the hot water heater or the water heater is cranked way up, and anti-scald valves are designed to protect when there are large or sudden pressure differences between hot & cold. It's the anti-scald features that interacts with the high pressure drop of an electric tankless at full flow.

    Even though I understand the nature of the problem, I don't have sufficient familiarity with the array of mixers & electric tankless units out there to make a particular recommendation on the mixer here.

    There are any number of thermostatic mixing valves that could be put on the output of the Tempra 29 to make it code-compliant, and if you crank up the temp on the heater, then dial it back down with the valve, you would have somewhat less pressure difference between cold/hot at the mixer.

    In much of the world the electric water heater for showers is at the shower head, and there are no separate hot/cold feeds, but that isn't the US standard.

    Were this day-1, before up upgraded the power and installation of the electric tankless I might have recommended going with a tank type electric hot water heater (or maybe a hybrid heat-pump version if sufficiently subsidized in your neighborhood.) Tankless hot water heaters have these issues, and sometimes it's just not worth the aggravation, but the reasons for going tankless vary. Space savings is one common theme, never running out of hot water is another, then somewhere down the list the higher efficiency aspects are real (if pretty small with electric tankless). But electric tankless hot water heaters work better in locations with warmer incoming water temps than in the cool northeastern US.

  6. #21
    DIY Junior Member Atticweb's Avatar
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    OK, I feel like you're just not reading my posts, or I'm not getting through well enough. There are numerous reasons I had to go non-vented, non-24/7 heated tank, but I'm already writing a tome here. The unit is working perfectly, just as advertised, in some pretty adverse conditions. It controls itself, without help. I've done ALL the experiments and YES it's a busted ol' mixer that I need to replace. I'm just trying to figure out the best solution to get a *@^&$@ hot shower (excuse the language). I am getting one now -- with the cold supply (and upstairs toilet!) turned off. I *should* be able to get this with the latest and greatest valve unit, just would like to know what it is, what y'all recommend, one that will totally shut off the cold water so I can get what the unit is set for. This just doesn't seem like rocket science; matterafact, just the opposite. Do I really need to go 20 years in the past for a solution"? So, what is the best code-compliant faucet/mixer whatever that will let my unit do what it's doing so well on it's own?

  7. #22
    DIY Senior Member lifespeed's Avatar
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    To provide feedback for your application on the particular model of Toto thermostatic mixing valve I use I did the following:

    Turned on the shower and confirmed the shower water matched the 104F setpoint with 130F incoming hot water supply. Bathroom sink faucet (standard two-handle) hot water was above the range of the thermometer.

    I then set the water heater to 105F and confirmed the water at the bathroom sink faucet was 107F. I turned on the shower with the temperature control at the same nominal setpoint and measured the water temperature at 100F. Increasing the mixer valve temperature did not result in an increase in water temperature.

    Based on these observations I conclude that some temperature "overhead" is required for (at least this model) thermostatic valve to regulate the temperature. It will not provide the same temperature at the showerhead as supplied at the hot inlet to the mixer, but will be a few degrees less. Of course I can't speak from experience about other brands of thermostatic valve or pressure balanced valves.

    If your heater configuration is unable to provide 5 - 10 degrees higher temperature hot water supply than the desired shower temperature (104F is considered typical, YMMV) then you are correct to look carefully at mixer valves that can provide "full hot". This may not be a common request, so you will want to confirm this ability before soldering it into your wall and burying it behind tile, sheetrock, or whatever.

    Good luck!
    Last edited by Terry; 12-11-2013 at 06:16 PM.
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  8. #23
    DIY Junior Member Atticweb's Avatar
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    Thanks lifespeed for the experiment. Helpful, hear you on confirming and that seems to be the issue. Again thanks much.

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