Electric hot water + net zero?
I am working on a spec project and I will install a solar P.V. system. My goal is to make this a “pretty good house”. I would like to move the systems towards net zero but will not be seeking any certification.
Since it is a spec project the actual use of the house is an unknown but typically where the house is located it will be bought by a second (or third, fourth…) home owner. Therefore, the usage may be sporadic to very seldom.
I am using electric for the cook top and heating. Does it make sense to use electric for the hot water also? I am limited on space so I am not going to be able to have a solar hot water with a storage tank.
What I am considering for electric hot water is a tankless unit. Something like the Stiebel or the Rheem which seem to be the best on quick scan of reviews for something that can handle more than one shower at a time.
If the electric tankless are not the way to go I would revert to a gas tankless.
Thanks,
Alan
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Replies
An electric tankless is an abomination, putting an extreme but random & intermittent load on the local grid distribution infrastructure. If residential demand charges become the standard (it's currently rare, but may become common for Net Zero houses as a means of paying for the grid), a tankless electric is a liability, since a single 10 minute shower can more than triple or quadruple your average draw. Residential demand charges for houses with PV is more than a mere theoretical possibility:
https://www.greentechmedia.com/articles/read/massachusetts-legislature-could-overturn-eversources-new-demand-charge-for#gs.d7rdlq
Any resistance electric water heating would also add a kilowatt or three to the size of the PV array needed to hit Net Zero, which isn't cheap.
A heat pump water heater is the other end of the spectrum, with very low peak draws, providing some latent cooling even when the sensible cooling loads are low, reducing the size of the PV needed to offset the energy used for heating water.
A gas tankless isn't doing the world/climate any favors, and would incur the initial expense of hooking up to the gas grid, usually not free, and plumbing the gas lines in the house is more expensive than wiring for a heat pump water heater.
In most cases going all-electric with heat pump HVAC and heat pump water heaters is going to be the greener path forward, easier and more affordable to do in new construction rather than retrofit.
https://www.vox.com/2016/9/19/12938086/electrify-everything
https://www.greentechmedia.com/articles/read/what-does-it-take-to-electrify-everything-in-your-home#gs.d7sequ
^^^Note, these folks went with an electric tankless for space reasons, and it may eventually come back to bite them far harder than what it cost to upgrade the electrical service to support that load.
http://www.natethehousewhisperer.com/blog/electrify-everything-resources
Thanks. That clarifies my potential directions. Electric tankless is off the list.
Alan,
Dana gave you good advice. To summarize:
1. If you are aiming for net zero, you need to have an electric water heater, not a gas water heater.
2. The usual solution is a heat-pump water heater.
3. If you can't find an appropriate place to put the heat-pump water heater, install a conventional electric (tank-style) water heater.
4. No matter what kind of electric water heater you install, balance your annual load with a PV system sized for your needs.