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I'm in the market for a new water heater and had some questions.

Current unit is an 50g Envirosense Power Direct Vent heater (Model No. 6G50100N full specs). The water heater is part of a hydronic system meaning it's also the source of heat for my home, I don't have a standalone furnace.

Despite being less efficient, I'd really love to switch from natural gas to an electric as, per Saul Griffith, I'm trying to replace all natural gas appliances I have with electric models to do my part against climate change.

(Note that this only works because in Ontario, where I live, 96% of electricity is provided by zero-carbon sources like nuclear and hydroelectric so electrifying this appliance doesn't end up netting more carbon.)

The issue I have is that I'm not sure I can use an electric unit.

Current model has a recovery rate of 116.36 US Gallons / Hour. Electric models I find online seem to have recovery rates that barely break 25 G / H. My concern is that, since this is part of a hydronic system, my current recovery rate is needed to be able to heat my home in the winter (I live in Canada) as well as supply regular hot water needs (showers, dishwasher, etc.).

Am I stuck getting a natural gas unit to match that recovery rate? Are there any engineers who could help me with a ballpark calculation I could do to determine the "minimum recovery rate" I need, given that I have a hydronic system?

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  • Can you get us a 99% winter design temperature for your area? Also, are you limiting yourself to a single 240V/30A circuit, or are you willing to throw more "grunt" at the problem? Finally, how much space do you have for a larger hot water tank? Apr 23 at 3:21
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    Shhh... @GeorgeAnderson don't point at the elephant in the room, that's not nice. When "going green", electricity just magically appears at the outlet. Never mind the fact that he'll have to have 5 electric heaters (@ 25 G/H) to do the work of 1 gas heater (@ 116 G/H).
    – FreeMan
    Apr 23 at 11:51
  • @GeorgeAnderson, while I appreciate the snark, I live in Ontario and the source of our local electricity is 96% carbon-free (60% nuclear, 26% hydroelectric source). Your points about efficiency are accurate, but Ontario's electricity sources are "clean". Apr 23 at 13:09
  • @FreeMan Thanks for the comment. Renewable power isn't a panacea. Solar panels last about 20-25 years and can't be recycled. Wind turbine blades, at the end of their useful life also can't be recycled and are cut up and buried: bloomberg.com/news/features/2020-02-05/… And the rare Earth metals needed to manufacture solar panels are sourced in countries with very lax environmental standards, and often use child labor in mining operation. Apr 23 at 13:09
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    +1 for the nukes! Very efficient, many new plants can use waste from old plants and/or bomb making, thus reducing/eliminating the storage requirements. Many new designs fail safe, not fail Chernobyl. But... but... nu-clear is soooo scary!!! </soap box>
    – FreeMan
    Apr 23 at 13:25
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The revolution you're talking about won't be powered by well-wishes or symbolic purchases. That's actually worse than nothing, because it results in failed installations, wasting resources and disillusioning people about the new technologies.

No, it requires actual knowledge and skillful application of craft.

Start by identifying the equipment you do have

You don't have a water heater. You have a hydronic system that is gas-fired, and that system both heats the house and heats your water. Hydronic systems are able to use radiators or baseboard water-fed heaters. However it also might have an air handling system which circulates heated/cooled air throughout the house. Since hydronic systems cannot provide air conditioning, the house must have a separate A/C approach, and that would need air handling because air conditioning doesn't work with radiators.

So, status quo ante, you have three systems that either are interconnected, or will be:

  • Water heating
  • House heat
  • House air conditioning

So far, your proposal is to rip out only the water heater, and convert it to resistive electric heating. That will leave a mish-mash of scattered tech behind, including 2/3 of a gas hydronic system and That 70's air conditioner. And I'm guessing that proposal is because that's the only thing you know so far.

The math on resistive electric heating

OK, so let's take a look at that water heating requirement and do some math. A "BTU" is the scientific unit for the energy it takes to raise 1 pound of water 1 degree F. (the output of various furnaces and the like are called "BTUs" but are actually BTU/hour).

Suppose our recovery rate is 1 gallon per hour (bear with me). 1 gallon of water weighs 8.33 pounds. Taking cool 50F water off the street and raising it to a bacteria-safe 140F requires 90 degrees rise. Well, 8.33 x 90 is 750 BTU. 1 watt-hour is 3.41 BTUs. So 1 gallon per hour needs 220 watt-hours per hour... or more simply: 1 gallon per hour recovery needs 220 watts.

25 gallons per hour recovery needs 220 x 25 == 5500 watts. Which happens to be exactly the watt rating of your common everyday electric water heater. It works out to 23 amps. (which is derated 125% to 28.75A, and that just fits on a 30A breaker. Imagine that.)

Now, your oddly specific 116.36 GPH recovery rate, you need 25,600 watts, or 106.66 amps. Derated, that takes 133.33 amps, leaving little remaining room on a 200A service. Realistically, that won't fit on a 200A service with your other stuff. You'll need a "Class 320" 400A service to power this and your other electric ambitions, which is real money. By the way, the hydronic system goes in the trash in this scenario. Electric can provide heat directly in the same locations as the radiators.

But even so, this is yesterday's news. And you will not like the electric bill if you do it this way.

Method 1: New style heat pumps

Air conditioning is the wildcard here. What's maddening about North American practice is air conditioning systems is almost a heat pump - which is an essential aspect of the next-gen tech that you want.

This is a superb explainer, and you have to watch it. OK?.

I bet you didn't know air conditioners only needed one more valve to do that trick.

Modern heat pumps are stupidly more efficient than classic air conditioners, and that too is why they're part of the solution. Heat pumps let you tick two boxes: a) heating and b) air conditioning.

The usual heat pump involves using air handling (not radiators) because the air conditioning mode requires air handling.

That crosses off hydronic systems, but we can still score a win with water heating, because They make "heat pump" water heaters too. The normal heat pump water heater takes heat from the utility room, so you'll need to heat that room to effectively put that heat into the water.

Or method 2: a ground sourced heat pump

This is a special type of heat pump that eliminates the big evaporator/condenser unit outside... as well as most of its noise. It is not interchanging heat with the fickle ambient air... but rather, the more stable earth temperature.

Here is an explainer on that.

This happens one of two ways: Either you use a well to pump water out of the earth (and return it some distance away via another well)... or you bury a bunch of coolant loops and run antifreeze through them.

This works more efficiently because the ground source is cooler in summer and warmer in winter than the ambient air.

...And keep your hydronic system

They make heat pumps specifically for hydronic systems. You can get ones which interchange with ambient air, or that interchange with a ground source.

So, you replace the "gas boiler" component with a heat pump unit, and nothing else changes.

A hydronic, ground-sourced system is the "best of all worlds (but one)" system. It is the only system that allows the hot water heater to source energy externally.

What's more, because ground-sourced systems are efficient year-round, there simply is no need for expensive resistive "emergency heat", and that means you don't need 400A electrical service.

However, again, you need an air handler to make air conditioning work.

They solved that problem

Older heat pumps had a problem: they could not work in the very cold, and so they required those expensive resistive "toaster heaters" to heat the house during those times. A friend in the northern US has an older heat pump system that used 30A for the heat pump, *but 140A for the "emergency heat" resistive heating. Yikes! That reason alone forced my friend into 400A service. (you may recall this happened in Texas; all those heat pump systems calling for emergency heat made it impossible for the electric grid to recover.)

Newer heat pumps solve this one of two ways:

  • Ground-sourcing, where the ground is always temperate; or
  • A much wider operating range, due to better refrigerant choices, and better electronic controls, which make it easy to have a a "self-defrosting" cycle.
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  • Harp, i usually agree with most of what you say here but you are really on a soap box on this one and I take issue with the following: 1) geothermal heat pump loops are very expensive to install and depending upon soil conditions, you end up topping off the ditches for years....hard to have a nice lawn that way. And you need a LOT of space for them, a small city lot doesn't have a chance. And how many people could do a well to heatpump to well? Most are on a city/county/community water supply. And where I live it's not even legal to do that. ,....continued in next comment.... Apr 23 at 12:48
  • The Texas grid failed mostly because the natural gas delivery equipment froze, not because of back up resistance heat usage. Many generators went offline bc of no fuel. My previous home had a heat pump and I believe the backup resistance heat power requirement was 30 amps. 140amps is a ridiculously high number unless it was for a mansion. Your answer is largely biased and opinion based, rather than "How do I do this". Apr 23 at 12:56
  • So much good stuff in this response, thank you! I went deep on heat pumps a while back and indeed was surprised they work in climates like Canada these days though to your point, it's usually recommended you go with a ground sourced version as our winters can be pretty brutal. Apr 23 at 13:20
  • Re: "they make heat pumps specifically for hydronic systems", are there any vendors you'd recommend having a look at? Apr 23 at 13:21
  • @GeorgeAnderson There's no soap-box here, it's a review of the available tech, which is what OP asked for. You're in way too much of a hurry to do what amounts to naysaying. Ground-source is just an option on the menu. Your first complaint (cost), watch the video. Your second complaint (illegal wells) reflects a misunderstanding of why those anti-well rules exist, nothing about them prohibits heat pump process water as long as you don't dump it inappropriately (e.g. by stealing city sewer or draining the aquifer). Apr 23 at 17:50
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Don't count on an electric heater necessarily being better for the environment unless your area is mostly powered by renewables. Either way, you're burning something, but to generate electricity you burn it in the wrong location and some of the heat is not turned into electrical energy. The electricity travels on transmission lines and some of it is lost in distribution and as volt drop on the wires in your house. With natural gas you need only worry about the efficiency of your burner and heat exchanger. Where I live, your plan would result in a substantial amount of coal being burned instead of much cleaner natural gas. It may also be more worthwhile to switch if you use a tankless heater depending on your use case, although I believe gas versions of those exist.

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  • Over time, the generating plant mix will shift. So the percentage of fossil today in the energy mix does not reflect the fossil percentage 5-10 years hence, which is certainly likely to be better, given Canada's commitments to that goal. Even easier for Canada, since solar is getting stupid cheap and Canada is glutted with hydroelectric power, which works very well as a storage method for solar power (pumped storage). Apr 23 at 9:20
  • @Harper-ReinstateMonica true, but in many areas the balance and distribution of energy types is political, and at least where I live, there's enough resistance to change and life left in our coal plants that renewables won't dominate in the near future. The point is not that it is impossible to have a lower impact, just that it is not guaranteed and you could make things worse.
    – K H
    Apr 23 at 9:23
  • Appreciate the call out here on this, it's an important point for others to read if they happen upon this question later and something I checked before considering this. In Ontario, where I live, 96% of our electricity needs are met by zero-carbon sources (60% nuclear and 26% hydroelectric source). Apr 23 at 13:15
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    @ThreePhaseEel thanks! According to that doc, 1 F or -17 C. Apr 24 at 22:42
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    @ThreePhaseEel air handler (AirMax if it matters). Apr 24 at 23:31

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