Tankless water heaters are terrible....

[nwdiver]

This cropped up on my Facebook feed... not sure if it was intentional but it looks like they're fishing for prospective model 3 owners...

What really irritates me is how they take advantage of general ignorance around water heaters. 99% efficient sounds good to the lay person but that's a COP of <1 which is TERRIBLE from the perspective of a modern water heater. Heat Pump Water Heaters START at ~2 and go up from there.

Besides low efficiency... tankless heaters are BRUTLE on the grid. No one is going to schedule their shower around off-peak hours. You can use a heat pump water heater to store energy and schedule it to heat water during off-peak hours. Some tankless heaters can pull up to 18kW when heating water. It doesn't take many simultaneous showers to really start adding up.

 

[BluestarE3]

What is COP?

 

[nwdiver]

What is COP?

Coefficient of Performance. COP of 1 means 1w of heat per 1w of electricity. 2 means 2w of heat per 1w of electricity.

 

[brkaus]

Applying electric current to the water to generate heat? No shock hazard there...

I do agree, as far as electric hot water goes, a heat pump hot water heater is going to be much more efficient. And electric storage tanks don't have the same heat loss problem that a traditional gas tank water heater would have. So when your electric, the "tankless" isn't as much of a benefit.

 

[dhanson865]

+1 I'd personally never use a tankless water heater.

The only case for them is long runs where the water faucet would be far away from the water heater and the faucet is rarely used. Instant hot water at the faucet and theoretically you can run one cold water pipe to the location (if it isn't a retrofit).

But it's expensive and not grid friendly. For weird one off uses only, not a good idea for the whole house and not for every house.

 

[Derek Kessler]

Instant hot water at the faucet and theoretically you can run one cold water pipe to the location (if it isn't a retrofit).

The cost of running a second pipe for hot water when you're already running one for cold is negligible.

 

[Az_Rael]

I guess I will jump in because I love mine and will install them in all future homes I own.

Mine is gas heated and our electricity bill even with the car is less than we were running when I lived in Texas with our traditional water heater. (Most of that is probably because we are in new construction built in 2010 with good insulation, thermal windows, radiant barrier, etc - its efficient enough that solar doesn't make $$ sense for us. My house in TX on the other hand was built in 1983.)

But honestly I don't worry about any efficiency standards for our tankless water heater - I love the endless hot water it provides. So it doesn't matter how many overnight guests we have and if everyone is taking a shower in the morning or if the dishwasher and laundry are running. Everyone has hot water.

And I got extra storage space in the garage in the nook where the traditional water heater was designed to go.

 

[BluestarE3]

What about gas tankless water heaters... are they terrible as well?

 

[BrettS]

I have a whole house tankless water heater and honestly, I love it. I live alone for most of the week, so I don't have a lot of demand for hot water most of the time, but I usually have my kids on the weekends and often my parents come to stay with me too. If I had a storage water heater that was big enough to allow 5 people to shower in quck succession on the weekends when everyone was getting ready at once I would be wasting a lot of energy to keep all that water hot the other 90 or 95% of the time when I am alone.

The tankless water heater only draws as much power as it needs for the current demand, so if I'm just showering alone it will draw about 8kW. This is certainly more than a standard storage water heater, but it's not a ridiculous amount and only draws that much power for the few minutes I'm in the shower. Other than a bit here and there for washing dishes or doing laundry it draws 0 power for the rest of the day. If two people are showering at once and the dishwasher is running then it can get up to close to 18kW, but that doesn't happen frequently.

I use a brultech power monitor, so I can see exactly how much power my water heater (as well as other appliances) are using over time. When I replaced my standard storage water heater with the tankless my water heater energy consumption went down by a full 33%.

Obviously we could try to stagger our showers a bit more and get by with a smaller water heater, but a heat pump water heater has a very long recovery time and that wouldn't always work out well, especially when we don't have a lot of time to get ready in the morning before we go out and spend the day somewhere. Compared to a conventional electric water heater we might be looking at up to 18kW for 40 minutes while everyone cycles through the showers (it won't be a full 18kW for that whole time as showers are turned on and off between users) or 3kW for 3 solid hours as the conventional water heater turns on and keeps going until the tank is hot again. In the end it will use the same amount of power. There is something to be said for convenience.

Perhaps I have an unusual use case, but for me the tankless option saves a considerable amount of power and still allows everyone to shower whenever they want.

 

[dhanson865]

The cost of running a second pipe for hot water when you're already running one for cold is negligible.

In open space that is true*, but for some situations the ability to drop the second pipe makes it easier. Like I said I personally wouldn't use one. It's only useful in my eyes for oddball situations.

* Just as it's cheap and easy to add wiring, data cables or anything else when the walls aren't finished, space isn't a concern, you have access...

 

[McRat]

When do you use tankless heaters? If you have uninsulated hot water pipes in the ground a long distance from the main tank. Not only must you waste several gallons of water needlessly, the thermal losses add up as well. You need to run your tank hotter to get the same performance. Even in some 2 story houses, you could benefit by only heating water for immediate use, and save water to boot.

If you want a sink with hot water in an outbuilding that is not frequently used. Pool shower, detached garage, etc. No energy wasted when not in use.

We use tankless electric for outbuildings, but natural gas for main house.

 

[mikevbf]

I have a hot water heat pump and solar panels, but I will admit the hot water heat pump has a very slow recovery rate. Add to that that I sometimes host 20+ guests for a week or so in the summer and you can see how this might be a problem. My solution was to keep my old inefficient electric hot water tank and put it inline after the hot water heat pump. When I know the demand is going to go way up for hot water, I turn on the second tank. I am not sure it was necessary, but I also put a bypass on the old hot water heat tank for when I do not use it so I can fully turn it off at the breaker box.

 

[SageBrush]

+1 I'd personally never use a tankless water heater.

The only case for them is long runs where the water faucet would be far away from the water heater and the faucet is rarely used. Instant hot water at the faucet and theoretically you can run one cold water pipe to the location (if it isn't a retrofit).

But it's expensive and not grid friendly. For weird one off uses only, not a good idea for the whole house and not for every house.

Not for the shower, either.

*maybe* for the kitchen sink if as dhanson says the pipe run is long in order to save on heat losses. I get the impression that tankless heaters got their start in places where space is at a premium or as a plumbing retrofit for slum landlords. The UK comes to mind. Anybody who can install a tank (or two) is much better off.

And by the way, tankless gas is also offered for sale but it typically requires expensive pipe upgrade to supply the high power required.

 

[BrettS]

I get the impression that tankless heaters got their start in places where space is at a premium or as a plumbing retrofit for slum landlords. The UK comes to mind. Anybody who can install a tank (or two) is much better off.

I totally disagree. A tankless heater will be more efficient than a tank heater simply because you have no storage heat loss. You can insulate the tank as much as possible, but you will still lose some heat over time. The tankless heater uses a lot of power (or gas) during the short time that hot water is being called for and no energy when there is no demand. Without the heat loss from the tank the tankless water heater will save energy over time.

Exactly how much energy depends on hot water demand. If you are continually using a lot of hot water and the water doesn't have a lot of time to sit in the tank, then you won't be losing much heat. If you only use hot water occasionally and the water heater is just sitting there keeping the water inside it hot then you will see more energy savings. As I said above, I am using only 66% of the energy to heat water with my tankless heater than I was with a conventional tank heater, and I know this because I have a power monitor on my hot water circuit, so this was a direct comparison of my tank water heater's energy use of 60 days with my tankless water heater's energy use over 60 days.

Frankly, because of the additional cost of tankless heaters I think most slum landlords will just throw the cheapest tank water heater they can get in. They don't want to pay extra for a tankless water heater.

 

[nwdiver]

The tankless water heater only draws as much power as it needs for the current demand, so if I'm just showering alone it will draw about 8kW........ 3kW for 3 solid hours as the conventional water heater turns on and keeps going until the tank is hot again. In the end it will use the same amount of power. There is something to be said for convenience.

The comparison I was making was tankless electric vs heat pump hot water heaters not obsolete resistance electric tank heaters. The difference in terms of efficiency is YUGE! I agree that the efficiency difference between resistance tank and tankless is negligible... that was part of my point. If you're looking for efficiency the heat pump hot water heater is the clear winner. It uses ~70% less electricity. My heat pump water heater only pulls ~400w.

8kW may not sound like a lot... but it adds up fast. My house shares a 50KVA transformer with 4 other houses. A tankless heater is each home would use >50% of the capacity. People tend to shower in the mornings and evenings... evenings in particular is when kW are most precious.

 

[nwdiver]

A tankless heater will be more efficient than a tank heater simply because you have no storage heat loss.

Absolutely not true... you don't lose that much heat from a tank... especially if it's used daily. The aggregate efficiency of a tank heater is >90%. A tankless by definition cannot be >100%. Not much room for improvement. Heat Pumps in contrast are typically >230% 'efficient'. Using 70% less electricity than tankless.

 

[BrettS]

The comparison I was making was tankless electric vs heat pump hot water heaters not obsolete resistance electric tank heaters. The difference in terms of efficiency is YUGE! I agree that the efficiency difference between resistance tank and tankless is negligible... that was part of my point. If you're looking for efficiency the heat pump hot water heater is the clear winner. It uses ~70% less electricity. My heat pump water heater only pulls ~400w..

Yes, that is absolutely true, but the long recovery times make heat pump water heaters inconvenient at best or even unusable in some cases. There is definitely a trade off in terms of power usage and convenience, but I disagree with your assumption that tankless heaters are terrible. I think there are definitely use cases for them and I still maintain that they are more efficient and much less terrible than a conventional tank heater.

But yes, if efficiency is the main goal then a heat pump or solar water heater is absolutely the way to go.

 

[nwdiver]

Yes, that is absolutely true, but the long recovery times make heat pump water heaters inconvenient at best or even unusable in some cases. There is definitely a trade off in terms of power usage and convenience, but I disagree with your assumption that tankless heaters are terrible. I think there are definitely use cases for them and I still maintain that they are more efficient and much less terrible than a conventional tank heater.

But yes, if efficiency is the main goal then a heat pump or solar water heater is absolutely the way to go.

Every heat pump water heater on the market is a 'hybrid' design. They have resistance elements for backup. I agree there are niche applications where tankless heaters are more appropriate but in general any application where a traditional water heater is in use a heat pump can offer a replacement with no change in overall convenience. What bothers me is when tankless heaters are marketed as 'more efficient'.

If unlimited hot water is the goal then ideally you could put a tankless heater downstream of a heat pump heater... best of both worlds

 

[brkaus]

What about gas tankless water heaters... are they terrible as well?

There is a fundamental difference between a typical electric storage tank and a gas storage tank water heater.

Electric - An electric storage tank type water heater has an electric element inside the tank. The tank can be insulated pretty darn well as to minimize heat loss when the water is sitting in the tank unused.

In my opinion, for electric hot water I would prioritize [1] heat pump (due to 2-4x COP), [2] tank (due to less peak electricity demand), [3] tankless. Tankless electric does have less loss than a tank. But if you have lots of points of use of hot water, an electric tankless could take a significant amount of current. For example, in my house there was not enough capacity in the panel to put in an electric tankless that would meet the hot water needs.

Gas - A traditional gas storage tank type water heater has a burner under the tank and a flue pipe up through the middle of the tank. The heat transfers from the burner to the water through the bottom of the tank and up through the flue pipe. The downside of this is when the burner is off, the heat will also radiate out the same path. This heat escape path just cannot be insulated. If the water heater is in a cold attic, it will lose heat pretty quickly... So, with a gas tankless it is minimizing the heat loss compared to a tank. It does require a larger gas line than a tank heater, but in most cases the existing gas line is adequate. A tankless burner can also be a 95+% efficient condensing burner. So, yes, in my opinion a tankless gas water heater is significantly better than a tank gas heater.

There is a second type of gas water heater called a hybrid tank gas heater. In this case, they use a fully enclosed/insulated tank (no flue in the middle) and a tankless style burner to circulate water through it. So, minimal heat loss in the tank and a higher effeciency burner.

As far as cost goes (at least in my area) - cheapest to most expensive - Natural gas -> Electric -> Propane

 

[ecarfan]

What about gas tankless water heaters... are they terrible as well?

I have had a Takagi gas tankless water heater for over 5 years and it has worked well and reduced my NG use. I can't provide any hard numbers.

In the future I plan to replace it with either a heat pump or an electric tankless but haven't done much research yet. This thread is interesting.