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I have a tankless hot water heater system in my new home that runs on a city natural gas line. It’s super efficient and thus my energy bills are very low compar

I have a tankless hot water heater system in my new home that runs on a city natural gas line. It’s super efficient and thus my energy bills are very low compared to what I used to pay for the traditional electric hot water heater in my previous home, however it’s not a silver bullet. I find myself running the water for a ridiculously long time in order for the water to start getting hot, sometimes for more than a minute, depending on where I am in the house (obviously the farther you are from the heating unit, the more time it takes for the hot water to travel to its destination). Our water consumption is therefore quite a bit higher, thus I am not confident this is a good ecological or financial tradeoff. I have used similar systems in Europe and they seem to be a little quicker, but that could be from higher water pressure. They seem to suffer from the same problem though. Perhaps a good solution would be to have a small (4-5 gallon) booster tank that is periodically heated like a traditional tank-based system, so that when hot water is run, it can immediately be delivered while simultaneously engaging the on-demand system. Forgive my layman’s terminology, I’m not a plumber or HVAC engineer, but hopefully the idea is clear.

vel0city 5y ago
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?
- vorpalhex 5y ago
  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).
maxerickson 5y ago
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)
- analog31 5y ago
  Are recirculating systems safe? Those have to be designed to prevent microbial build-up. I'm thinking of Legionnaire's disease.
  - sgc 5y ago
    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.
  - eliaspro 5y ago
    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.
- krrrh 5y ago
  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.
  - gnicholas 5y ago
    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.
    - fy20 5y ago
      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.
jdsully 5y ago
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.
- nine_k 5y ago
  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.
  - lmm 5y ago
    Gas-fired power plants are already known to be particularly low-efficiency. They're not representative of electricity production in general.
    - nine_k 5y ago
      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.
      - fpoling 5y ago
        https://www.ge.com/gas-power/resources/articles/2016/power-plant-efficiency-record https://www.ge.com/gas-power/resources/articles/2016/power-p... is over 62% efficient.
  - adrianN 5y ago
    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.
deviledeggs 5y ago
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.
- coryrc 5y ago
  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.
  - coryrc 5y ago
    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.
  - abakker 5y ago
    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)
  - jnsaff2 5y ago
    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.
hyperpallium2 5y ago
One heater per tap.
RHSeeger 5y ago
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)

vel0city 5y ago

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?

vorpalhex 5y ago
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).

maxerickson 5y ago

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)

analog31 5y ago
Are recirculating systems safe? Those have to be designed to prevent microbial build-up. I'm thinking of Legionnaire's disease.
- sgc 5y ago
  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.
- eliaspro 5y ago
  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.
krrrh 5y ago
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.
- gnicholas 5y ago
  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.
  - fy20 5y ago
    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.

jdsully 5y ago

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.

nine_k 5y ago
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.
- lmm 5y ago
  Gas-fired power plants are already known to be particularly low-efficiency. They're not representative of electricity production in general.
  - nine_k 5y ago
    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.
    - fpoling 5y ago
      https://www.ge.com/gas-power/resources/articles/2016/power-plant-efficiency-record https://www.ge.com/gas-power/resources/articles/2016/power-p... is over 62% efficient.
- adrianN 5y ago
  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.

deviledeggs 5y ago

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.

coryrc 5y ago
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.
- coryrc 5y ago
  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.
- abakker 5y ago
  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)
- jnsaff2 5y ago
  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.

hyperpallium2 5y ago

One heater per tap.

RHSeeger 5y ago

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)