Tankless woes

[fullfins]

I have installed a Stiebel-Eltron Tempra 24 Plus and have two issues: the water temperature fluctuates at the shower (+/- 7 degrees) and the water pressure also fluctuates very strongly. The fluctuation in pressure is definately new. We don't have thermostatic valves. We have 2.5 baths, washing machine, dishwasher and a kitchen sink, but rarely are used at the same time. I installed the unit about three months ago and have had more of an issue of late as fall has set in. We always had a minor temp issue but it has worsened. Need to know if I can do anything to improve the situation. I have looked at changing to the thermostatic valves but that is very pricy and requires getting into the walls as far as I can tell. Any sugestions are welcome.

 

[Redwood]

Just wait until January...
You ain't seen nothing yet!

I wouldn't install one of them north of 30 degrees latitude myself....

 

[Sponsor]

 

[Jadnashua]

Don't know how yours adjusts the outlet temp. Some do it by modulating the burner, some do it with a mixing valve. If the thing can't modulate continuously (i.e., it adjusts in larger steps rather than linearly), it might be turning on and off. If yours has a mixing valve, it might be sticking. I'd give the tech support people a call - they (hopefully) know their unit well and can advise you if this is 'normal', and if not, how to fix it. Some of them void the warranty unless installed by a certified installer.

 

[Dana]

Uh, Jim, the Stiebel-Eltron Tempra has no burner to modulate, being an electric tankless and all...

Most electric tankless unit modulate the immediate power level applied to keep the output temp constant, but I suspect the bandwidth of the response is intentionally damped electronically in it's control circuitry. If there are other issues creating pressure/flow oscillations that the control can't track or have an unfavorable phase relation with you could easily get big temp oscilations around the setpoint. One potential for starting an oscillating interation would be the response time of the shower's anti-scald valve, which would also exhibit a detectible oscillation in flow. The fact that it has come on now that the incoming cold water temps are falling may support this theory.

Does the same flucuation occur when running the hot at high flow at say, the kitchen sink, or is it just at the shower?

Try setting the temp at the heater low enough that at the shower you're running hot-only or with but a minor mix of cold, see if that changes (or fixes) the symptom.

 

[Redwood]

Maximum temperature increase above ambient water temp.
@ 1.50 GPM 92
@ 2.25 GPM 73
@ 3.00 GPM 54
@ 4.50 GPM 37

 

[Jadnashua]

Doesn't matter whether it is a gas or electric burner (element), if the modulation is either slow or non-linear, it could raise/lower the temp before it can respond fast enough. It gets worse if you are near the minimum flow, and the thing might be turning on/off entirely. Some anti-scald shower valves can NOT work well with different pressure between the hot and cold. Some tankless systems have a flow restrictor, some don't. Many can exhibit lower pressure on the hot side, potentially shutting down the shower valve.

 

[Dana]

Maximum temperature increase above ambient water temp.
@ 1.50 GPM 92
@ 2.25 GPM 73
@ 3.00 GPM 54
@ 4.50 GPM 37

In most of NC a typical winter low temp on the incoming water wouldn't be much lower than 45-50, given that average air temps are in the 40s, and the subsoil temps considerably higher:

To take a 105F shower with 45-50F incoming water, methinks they'd be fine, if the rest of the symptoms can be put to bed.

Jim: Min flow on electric tankless units are WELL under those for fossil burners since they can modulate the output much lower with reasonable accuracy (it's basically a dimmer with temperature feedback.) IIRC for the Stiebel-Eltron Tempra series the min for turn-on is ~0.5 gpm, but it'll continue to run down as low as ~0.3gpm once it's been kicked on. They're fairly low-head too with less than 2psi drop at max rated flow, if memory serves, unlike their fin-tube heat exchanger fossil fired cousins.

If the water pressure is unusually low there could be some micro-boiling sizzle on the heat exchanger causing some issues, but if you have 15psi+ you should be OK. I don't know if they spec a min-pressure, but realistcally it's probably over 10psi

 

[Zl700]

That model does modulate with using 2 elements and also has a water flow adjustment valve

 

[Redwood]

I agree it probably is the mixer's pressure balancing that is the issue.

However, I see flow being a problem for tub filling and certainly when water is run elsewhere.
IMHO electric tankless is terribly anemic...

 

[Dana]

I agree it probably is the mixer's pressure balancing that is the issue.

However, I see flow being a problem for tub filling and certainly when water is run elsewhere.
IMHO electric tankless is terribly anemic...

No argument there- living with an electric tankless will almost always require some water-use scheduling, even well south of the Mason-Dixon.

Before ripping it out and replacing it with something beefier, bear in mind that in showering applications a drainwater heat recovery heat exchanger comes close to doubling the apparent capacity (as well as the apparent efficiency.) It won't change the rate of a tub fill, but it may save your marriage should the washer/dishwasher get used while your spouse is showering. Unless your heating hot water only with off-peak rate electricity at less than half the national average per kwh, it'll pay off on power savings too (much faster in MA/CT than in NC.) It's not an option for first-floor shower with slab-on-grade construction though.