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The majority of your savings are from switching to gas from electric. Gas is significantly cheaper per BTU. Gas tank heaters are still pretty efficient. Especially if insulated.

It's easy to understand.

First, a gas-fired power plant converts about 40% of thermal energy to electricity, discarding 60% to further heat the planet. Then a few more percent are lost in transmission lines and voltage-lowering transformers. Only then that electricity is used to heat water or air in one's house.

If you just directly burn that same gas in the house to produce heat, it's nearly 3x as efficient.

Gas-fired power plants are already known to be particularly low-efficiency. They're not representative of electricity production in general.

I wonder what kind of thermal electricity generation achieves efficiency above, say, 45%. Efficiency of thermal engines, even when you recycle the waste heat of the gas turbine exhaust with another cascade using a low-boiling substance and another turbine, is limited.

I only mentioned the gas-fired power station to make it an apples-to-apples comparison. It would be hard to compare e.g. to solar panels, even though direct hearing by concentrated sunlight might be feasible.

https://www.ge.com/gas-power/resources/articles/2016/power-p... is over 62% efficient.

If you use the electricity to run a heat pump you can get a couple more Joules out of the gas. Ideally your gas-plant is also connected to a district heating network, so the 60% heat are not completely lost.

A thought occurs to me just now: Why not also use that 40-60% waste heat to drive district cooling during the hot season? There's the absorption refrigeration cycle[1], which requires only a a heat input to drive the cycle to pump heat from the cold side to the hot side. Is the difference between the coefficient of performance between the more common vapor-compression cycle and the absorption cycle such that it is more cost effective to use the generated electricity to pump heat out of conditioned spaces during the hot season?

1. https://en.wikipedia.org/wiki/Absorption_refrigerator

Combined cycle power plants do use the waste heat:

https://en.wikipedia.org/wiki/Combined_cycle_power_plant

Both cycles are limited by the same fundamental value (the Carnot coefficient) so I don't think that it would be a huge difference between the two cycles. Rather, I suspect that there are not all that municipalities that have (or need) both district heating AND district cooling. I expect that in the future we'll see a lot more industrial use of waste heat from power plants though, and part of that might take the form of on-site absorption chillers.

The modern gas power plants archive over 60% efficiency. It is with coal that one gets at best 45%.

Wow, TIL! Thanks.

In a post parallel to yours, lmm says that gas-powered plants are low efficiency. Can either of you supply some documentation or references for your respective positions?

I already posted to another sub thread, https://www.ge.com/gas-power/resources/articles/2016/power-p... - gas power plant with 62% efficiency.

The best coal plants can archive 47% efficiency, https://www.brighthubengineering.com/power-plants/72369-comp...

Using gas generates fixed additional costs though, apart from the energy price. You have to pay an additional provider fee, the annual maintenance of the gas heater, regularly change the CO detectors, etc.

It's worth it for a larger volume though. Not so much for a small flat.

The problem is the cool water in the pipes.

A typical solution is recirculating the water so that it stays hot.

(I have a gas tank heater and the faucets far away take a bit to warm up, because of the water in the pipes)

Are recirculating systems safe? Those have to be designed to prevent microbial build-up. I'm thinking of Legionnaire's disease.

They are not closed loop, they just push some water from the hot water line back down the cold line, where it will wind up back in the water heater and be used. They are no more prone to disease than you existing system.

We have a solar (parabolic vacuum pipes) water heater which provides 95% of our annual hot water needs, backed up by gas. The heat is stored in a layered and heavily insulated 300 liter tank. The water delivered to the tap isn't actually from the tank, but it's just fresh water heated instantly and on-demand through a heat exchanger which is part of the storage tank. No need to worry about stale hot water with such a system.

I had a friend who spent months measuring the power output of all his appliances and lights trying to figure out where half his electricity bill was going. He finally realized it was the hot water recirculator.

We have an under-sink unit that has hot water ready 24/7. After hooking it up, I noticed it increased our monthly bill by about $15.

I bought a used Wemo plug for $10 and set it to only be on for 3 hours in the morning and 3 hours at night. Now we have hot water at any of the times when we actually would want it, and we aren't wasting money/energy to keep hot water on tap 24/7.

You probably only need to run the pump for 1 or 2 minutes every hour. The water in the pipes will stay warm for a while, no need to run it constantly.

Make sure to set the temperature really high if you don't run it continuously or you risk breeding Legionella.

Appreciate that; it's practically boiling.

> He finally realized it was the hot water recirculator.

The problem is using devices that are either timed or (even worse) just runs 24/7.

Ideally you'd want one that runs on demand when triggered by a switch: hit the button, it runs for 30-60s while you undress, and then we you turn on (e.g.) the shower the water is already hot.

The hot water supply for the shower should be the same as the sink, and so when you go to shave or brush your teeth, that's then already primed as well.

Do a search for "Gary Klein", who has written a lot of designing good hot water systems over the years.

* https://www.garykleinassociates.com/videos.html

Its worth noting that you can combine an on-demand (tankless) hot water system with at tank. The on demand system heats the water and stores it in the tank (which is not a water heater). The tank itself, since it has no heating elements, can be _very_ thermally efficient. You get the benefits of both systems (though it does cost more to install, obviously)

There are electric instant hot water systems available. The sink at my office only has a cold water line plumbed to it and a small black box heats the water right there. I've seen some in kitchens for making things like soup or coffee real quick. Not sure why they're not more common, maybe reliability/flowrate/other cost issues?

They need power and produce some noise. They take up space which may be at a premium. They require yearly-ish maintenance (running a cleaning solution through them with a small pump).

One heater per tap.

Heat pump water heaters mentioned in the article are better for efficiency. They typically use half the energy of a normal water heater.

And they cool the air around them, so if you live in a hot area they save more.

And if you live in a cold area, you get no efficiency advantage, pay more, and probably eventually leak coolant with a GWP of 2088.

A system like Sanden CO2 or Chiltrix will heat your water with outside heat, so it's always 2x+ the efficiency of resistance heat. If your climate is predominately hot then thermal collectors are far more efficient, but interior hot water heat pumps would be an okay choice where that's not possible.

Leaking coolant is a major problem. The standard for most small heat pumps is r412a and it is problematic.

To make the installs cheap, these systems use flare fittings that almost all seem to leak. Couple that with the fact the 412a systems run at high pressure and you get a lot of leaking refrigerant.

No HVAC techs I’ve met like the 412a systems and most seem to be hoping for more sanity from r32, though that will have its own set of problems (flammability)

I live in a cool place. This winter we had extended periods of -20C. Heat pump efficiency drops, but still stays above 2. SCOP (for year round average for heating coefficient of performance) units sold now are better than 5.

Also, if you are worried about the refrigerant leaking, you can sacrifice a little bit of efficiency and buy a unit using CO2 as a refrigerant with a GWP of you know: 1.

I think the biggest problem here for leakage are random construction dudes cutting the pipes when they demolish or remodel homes. If only they could gather the gas in the outdoor unit before, all of it would be saved.

If the hot water heat pump is a single unit (which is the vast majority of residential installations) and indoors, it is just stealing heat from your regular heating system. You're paying for that heat, so you aren't getting COP > 1.

You're only paying for that heat in the winter though.

Hence the preface of "cold area", but my heating season is over 6 months. The ROI is already long in HWHP even assuming the best numbers, and often negative once you account for reality in areas which get cold.

Who has that, a "indoor heat pump"? That's just stupid. Sure, exhaust air heatpumps "use" the (excess) heat from indoors, but that would just get expelled anyways.

https://www.hotwater.com/water-heaters/residential/hybrid-el... this is what people are talking about when they reference hot water heat pumps without qualification, because they are the vast majority of the market.

There is no excess indoor heat in winter in 99.9%+ non-passivhaus homes.

Right, I stand corrected. I had never seen such a heatpump (based in the nordics, that concept would never fly), and I'll agree with you that in homes heated by "regular means" (resistive/electrical, fossil fuels) they add little to no value, simply adding complexity. In heating season they would just siphon heat energy from the rest of the house...

(They would however de-humudify the space they are in, but that's another thing...)

I'd argue that every house that has any sort of ventilation, even naturally aspirated ones, have excess heat in that any amount of air leaving the house "unreclaimed" while there is any heating need contains wasted energy.

Heatpumps using that energy are very prevalent in the Nordics and Scandinavia, even with a majority of the time being heating season here.

Nah. It does not necessarily have to be an air-water heatpump either, colder climates more often use geo-thermal or ground-water loops as the energy-well. Then the surrounding climate have much less of an impact.

Refrigerant leaks are still an issue though, but moving to hydrocarbons and CO2 will mitigate that. Even R32 is a step up in that regard.

My parents got a heat pump water heater (electric) a few months ago and it's great. Definitely the priciest option, but it sips electricity and also cools the area around it (basically operating as an "inside out refridgerator").

In this same remodel I also convinced them to get heat pump / AC combo unit to replace the old gas furnace. Again amazingly cheap (even compared to gas), but also higher upfront cost.

This is in a coastal California region, so I'm not sure how well these heat pump solutions will work in colder places (ice would likely form on the heat pump near freezing, limiting its capability), but if anyone is looking to upgrade either a hot water heater or an AC unit, I cannot recommend the heat pump option enough (if you can get through the higher up front cost).

Definitely works in cold areas. Most heat pump manufacturers offer separate models for cold climates below zero.

The worst case is efficiency approaches resistance heating at 1.0, maybe with backup resistance heat strips. But a well designed system to match worst case load should rarely see this.

For some reason they've really only been widely available in the US in the past few years.

> For some reason they've really only been widely available in the US in the past few years.

I think it's cheap energy. In places like Japan they've been using heat pumps a long time

> But a well designed system to match worst case load should rarely see this.

Well actually it'll see it quite often. Every time a heat pump goes into defrost mode it'll use resistive heating inside to hold over while it's working like a traditional air conditioner to thaw out the outdoor coils.

Check rebate programs with your utility. Mine gives up to $1000 rebate for buying a new heat pump water heater