More Q&A Spotlight
Pat Beurskens has been happy with his heat-pump water heater, but he fears the time is coming when it won’t be able to keep up with demand.
He foresees the day when his young daughter will be using more hot water, and Beurskens is finishing out the basement of his Seattle home to be used as a rental. That, too, will increase hot water use.
Against this backdrop, Beurskens’s GE Geospring, a model that has been discontinued, is showing signs of fatigue. “Lately,” he writes in a post at the Q&A forum, “the water heater has been maxing out after two showers. I’ll be retrofitting a 1.5 [gallon per minute] adapter, which will help, but I’m still nervous about having a tenant and running out of hot water.”
In order to squeeze more capacity out of his system, Beurskens is considering a solar water heater and an electric tankless water heater.
He’d prefer using the heat-pump water heater most of the time because of its high efficiency. He doesn’t want to replace it with a larger model because of the expense and because he’s not eligible for another rebate.
He asks, “My main question is: Would it work to put the tankless electric heater in a series in front of the heat pump tank so that the tankless kicks on only when the tank starts dropping in temp?”
That’s the question for this Q&A Spotlight.
Avoid electric tankless heaters
The best option would be to avoid an electric tankless water heater altogether, argues Dana Dorsett.
“An electric tankless isn’t as sensitive to incoming water temps as fossil burners,” he writes, “but they are really abusive to the grid infrastructure, and one of the least green options there is.”
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9 Comments
If you limit flow to 1.5 gal/min (I use < 1 gal/min), then a secondary heater can be less than 35A (and usually 0A), with a low correlation to neighbor's use. Save the standby losses and space and use an small electric tankless.
Dan Cautley's study while interesting doesn't appear to have studied a good analog for a single family or 2-3 family home with, much shorter water distribution & drain runs than the 26 unit apartment building or a 6 suite student housing, where the distribution losses are much higher. I'd be curious to see what his 36 home NAHB Research Center study had for particulars back in the 1990s, but I'm basing my performance WAG on multiple Canadian studies from 2005 or later (one of which was in Vancouver, not too far from Pat Beursken's location.)
My temperature numbers are based on in-situ meaurements at my house (though not measured to NIST standards of accuracy, and no data logging over time :-) ). The heat exchangers themselves and code requirements for pipe insulation have improved quite a bit since the 1990s. The new gravity film units from EcoDrain even leave Renewability's PowePipe series in the dust, and those PowerPipe units beat anything available in the 1990s by comfortable margins, inch for inch.
As Dan stated:
>"In general, thermal mass will reduce performance — think of taking a shower using 12 gallons of water, where it takes 3 gallons of flow to heat up the drain line and heat exchanger before reaching steady-state performance."
No kidding!?! To get all the way up three gallons of water takes 150' of 3/4" pipe between the water heater and shower! That's certainly not my house, and probably not yours (or Pat's) either. But it's a good point- volumes/distances matter.
At my house there is about 15' of (insulated) copper plumbing from the water heater to the shower mixer, most of which is half inch, and well under one gallon total volume. YMMV- but it's easy enough to estimate your ACTUAL distribution volumes (and losses) rather than try to infer it from a much larger multi-unit residential situation, or unspecified 36 houses from the 1990s study.
Cast iron vs. plastic drain pipe would make a real net performance difference too, as does vertical vs. horizontal drain. With vertical drain pipe there is a lot greater loss due to the greater surface area contact from the gravity film, whereas with horizontal drain it's at most 20% of the surface area in contact with the water, even with 2" drain, whereas with vertical drain it's nearly 100% surface contact (which is the very feature gravity film heat exchangers are exploiting) Most newer small scale residential houses are using plastic, which has both lower thermal conductivity and less thermal mass than cast iron. In my house there is about 10' of 4" cast iron drain between the shower and the drainwater heat recovery unit, 8' of which is horizontal.
Again, YMMV, but Pat Beursken's case is more likely to be more similar to my house than either the student housing or a 26 unit apartment building.
>"But this doesn’t mean it’s necessarily cost-effective."
From a net present value of the installed cost MERELY against lifecycle energy costs it varies with local rates and the efficiency of the water heater. An essential part of the cost-effectiveness in Pat's case is the avoided/traded cost of installing a second water heater (tankless or tank) or an oversized heat pump water heater or some other solution. The avoided/traded cost is a large fraction of the cost of installing drainwater heat recovery unit right up front. The present value of the cost DIFFERENCE varies with both the electricity rates and the estimated cost & efficiency of the alternative solution.
Would it make sense to look at Water Heater Booster at the top of the tank to increase the apparent capacity?
https://www.rheem.com/innovations/innovation_residential/water-heater-booster/
It seems like the homeowner is mainly concerned about edge use cases, not daily use, so adding a bit of capacity for just those events seems appropriate. The devices are only a few hundred dollars and I'd assume he has the proper electrical in place since it's heatpump unit today.
The only time the booster would kick on is when the pump could not keep up with demand so no standing losses to worry about.
That's an interesting product, not sure why I didn't see those in the past. Since it runs only on a 30 amp circuit, I wonder if the affect on the grid that Dana mentioned is still a concern.
The current solution of a drain water heat recovery pipe and flow restriction seems to be working for now. I may consider a water heater booster if/when our demand increases.
Just curious - have you put you're water heater into hybrid mode?
One other thought - if the existing heat pump water heater’s output is falling off there is a good chance the efficiency isn’t what you think it is either. Might want to get it serviced, may be low on Freon
How do you test the efficiency of a GE heat pump water heater? I know I can connect mine via wired Internet. If I do that, does the unit maintain any statistics that can be viewed if I point a web browser to the IP address of the water heater?
The eithernet port on the water heater is for the wireless adapter which is no longer supported since GE sold out the heat pump water heater. Be careful plugging any thing into the port as I blew my control panel out trying to get the wireless to work. ( just plugged it in)
What is the impact on hot water capacity by increasing temperature and adding a mixing valve?
Is there a simple rule to estimate it? I've heard "50% increase" and "10 degrees equals 10 gallons" (increasing the water heater temperature by 10 degrees is equivalent to having a tank that's 10 gallons larger).
For a 40-gallon tank, what's the effective capacity at 140'F or 150'F?
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