Tankless Water Heaters

How do tankless water heaters work? The only information I can find is that the water is heated “instantly.” I want to know how the water is heated instantly. Does the gas just fire up to an uber-hot temperature to heat the water as it passes by? If so, wouldn’t using more gas to achive a temperature high enough to “instantly” heat the water negate any savings by not having a tank?

From my research (pamphlets, brochures and the like) it seems like if you’re talking an average family of 4.2 people who use hot water regularly, then the savings are there and can be significant depending on the volume of the hot water used. When I was looking into it the only option on the market were electrical hot water heaters.

Keep in mind you’re not heating and re-heating water to maintain a specific high temperature, say, overnight or during the work/school day when you’re unlikely to use water.

Since it was just myself and my daughter, it was more cost efficient at the time (both initially and long-term) to get the gas-heated water tank - but it’s been a few years and there may be better options out there now.

Since both myself and my daughter like to take loooong showers, though - there are times I think the always-available instant-hot water would have been much nicer…

Just a few sources:

Tankless Water Heaters by SETS - distributor of electric tankless water heaters.
www.tankless-water-heater.com/

EREC Brief: Demand (Tankless or Instantaneous) Water Heaters - … As with many tank water heaters, even the largest whole house tankless gas models cannot supply enough hot water for simultaneous, multiple uses of hot water …
www.eren.doe.gov/consumerinfo/refbriefs/bc1.html

e-Tankless Water Heaters - distributes electric tankless water heaters.
www.e-tankless.com/

Niagara Industries Inc. - offers products for residential, commercial, and industrial use.

Controlled Energy Corporation - distributes gas and electric point-of-use water heaters.
www.controlledenergy.com/

Tankless Water Heaters and Space Heaters by Low Energy Systems
www.tanklesswaterheaters.com/

CET: Gas Tankless Hot Water Heaters - … 5.2 gallons per minute. Gas Tankless Hot Water Heaters, T-K2 In Stock. … 6.9 gallons per minute. Gas Tankless Hot Water Heaters, T-KD20. …
www.cetsolar.com/tankless.htm

They work simply by having the water pass through multiple small pipes that are heated by gas or electicity. The pipes are sufficiently hot, and the flow through them sufficiently low, that the water is heated as it passes through.

By “instant”, they mean only as opposed to heating over a period of hours as would happen with a storage system. They do not mean to imply anying miraculous.

There would be a degree of inefficiency in the process, but there is a high degree of efficiency (when compared with a storage system) because you do not expend any energy keeping the water hot while waiting for it to be used.

I’ve stayed in houses that have them, and have generally found them to be a bit painful (sometimes literally). Yes, they are efficient. Yes, they take up very little space. Yes, they can produce hot water until you run out of gas (:)), but their temperature control leaves a lot to be desired, in my experience.

They have to have (presumably) fairly sophisticated feedback controls to ensure that the water comes out at a particular temperature, even when the flow is varies. In other words, if you turn the hot tap on full, the heater presumably has to be going full blast to produce the required temperature. Then if you turn the tap down to a trickle, the heater has to reduce its output, otherwise it would over-heat the water.

While the more modern designs seem to cope OK (ish) you can tend to find that when you first turn on the hot tap you get a blast of near boiling water, followed a varying temperature until the heater settles down. And of course if you then turn the hot tap up or down, varying temperatures may result until the heater finds stability.

Our house had one until we replaced it with a regular water heater. The tankless heater has some sort of coil heater inside, plus a VERY small tank. What tends to happen (so the plumbers tell me) is that the coil gets mineral deposits and loses its ability to heat effectively.

In Asia, almost all water heaters are this type. I’ve used them for years. almost all are gas powered and you can hear the gas “ignite” when you open up the hot tap.

Yes, they are standard in Japan. I have a modern one in my apartment with electronic controls and love it. I set the temperature to 40 C and use pure hot water for the shower instead of mixing it with cold water to adjust temperature.

Temperature stability used to be an issue on older models, but not on those made in the past 10 years in my experience.

I had a much older system that could be discribed as a tankless hot water system. the boiler, in addition to creating hot water for the heating of the house had a sepperate coil running through for hot water. On demand the gas would fire up and heat water through the coil. The temp would be dependant on the flow rate. Very primative version of the tankless models of today.

For a while after getting a conventional hot water tank we had it set up so if we ran out of hot water we could turn 2 valves and run the water through the old tankless system.

We have this type of heater at home. It was fitted just over a year ago and replaced the old tank type system. The new heater is compact (oh so important in a small european house) and means that the tank has gone giving us space for a new, larger bathroom when we get around to it.

The water heater does domestic hot water and the central heating (hot water) system. It runs from gas piped into the house. Everything is in one small unit - heat exchanger, expansion tank, pump etc.

The controls are quite sophisticated. The temperture of both the domestic hot water and central heating water can be controlled by dials on the heater such that you can avoid scallding hot water and radiators (important when there is a baby in the house).

The only down side is that the heater controls (limits) the flow rate of the domestic hot water so that it comes out at a consitent temperature. This is not a problem except when filling a bath - it takes significantly longer than filling a bath from a tank of stored hot water. On the other hand one never runs out of hot water - there is no waiting around when everybody wants to use the bath one after the other.

Just to clear things up the tankless heaters being refered to here are quite different than the old tankless water heaters like the one I have in my house. (While I was typing this the last two posts refer to this type of system, but the one that G. Cornelius has is probaly way more efficent than the old fashioned one I have)

With a steam or forced hot water heating system there is water in the boiler that is kept hot at all times during the heating season. An older style tankless water heater has a coil that runs through this water and is heated indirectly from the boiler water. Since the burner for the boiler is sized to heat the entire house it has no problem producing enough heat for endless hot water. Since the furnace is running anyway in the winter this system can be more efficient.

However in the summer months you must keep the boiler water hot even when it is not used for heating the house. This is probably not to much different from having heated water in a tank like a traditional system except the boiler must be kept hotter than the water temperature that you want because the transfer is not 100% efficent. Also most older boilers are not insulated as well as modern hot water tanks.

Another common problem is (as ratatoskK mentioned) is there tends to be buildup of minerals inside the coil that effectivly insulates the water to be heated from water in the boiler. Mine is at this point now and I must keep the boiler at a high temperature to get hot water. But I can get lukewarm water forever :rolleyes:. One solution is to run an acid through the coil to disolve the minerals. However if there are any weak spots in the coil the acid will eat through them.

I was considering getting a traditional hot water tank for my house (propane) and using the tankless heater to preheat the water. I had never heard of this new type of tankless heater though and will look into it.