DIY "instant" hot water heaters

[zug]

Instant hot water heaters save a ton of money in energy, but are pretty pricey.

I just moved out of an RV and it had a 6gal electric hot water heater under the sink. It was perfect - the water lasted just long enough to take a shower or do dishes. There was only a few feet of plumbing between the hot water heater and the sink/shower, so it never took more than a few seconds to go from cold to hot, saving water.

I didn't look all that closely at the hookups, but it seemed like it ought to fit in a bathroom or kitchen. Anything I'm not thinking of?

[MooreBonds]

Depending on where you live, if you plan on having a professional install this with a permit, there are several potential local plumbing code regulations which may hamper that plan (expansion tank on the inlet to the heater, the T&P relief valve piped to near a floor drain, etc.).

If this is a DIY project, it would be an interesting concept - as long as you realize that you can't take a 20 minute shower, and that as soon as you use up a fair amount of the storage, it will take quite a while to replenish (rule of thumb is approximately 4 gallons per hour recovery per kW of heating element size). Another rule of thumb is that only about 80% of the storage capacity is available hot water, due to colder incoming hot water mixing, stratification, etc.

[Tennis Maniac]

@MooreBonds:
I always wondered if there was a way to supplement existing hot water supply with an under-sink electric hot water tank.  My theory was to use the hot water feed from your existing hot water tank as the input to the under-sink electric hot water tank?  Theoretically, you would then get the benefit of instant-on with the backing of your main hot water tank for those 20 minute showers.  Depending on how far away the sink is from the main hot water tank, there could be a few gallons of cold water getting mixed into the under-sink hot water tank before hot water from the main tank.  With this solution, you might get away with a smaller tank under the sink, reducing costs.

Maybe someone here knows if this is a viable solution or can work out the temperature changes in the output water as a function of time, distance to main hot water tank, size of under sink tank, etc??  Maybe there is some other variable that derails this whole idea.

[Bakari]

I actually discovered, when my 6 gallon gas/elec RV water heater got a hole in the tank at 15 years old, that getting a simple gas powered instant water heater actually cost less upfront AND used much less gas than it would have if I replaced it with another RV 6gallon tank.
Found it on ebay.  Same connections, fit in the same space, was very easy to install myself.  And now the hot water never runs out.

@MooreBonds:
I always wondered if there was a way to supplement existing hot water supply with an under-sink electric hot water tank.  My theory was to use the hot water feed from your existing hot water tank as the input to the under-sink electric hot water tank?  Theoretically, you would then get the benefit of instant-on with the backing of your main hot water tank for those 20 minute showers.  Depending on how far away the sink is from the main hot water tank, there could be a few gallons of cold water getting mixed into the under-sink hot water tank before hot water from the main tank.  With this solution, you might get away with a smaller tank under the sink, reducing costs.

Maybe someone here knows if this is a viable solution or can work out the temperature changes in the output water as a function of time, distance to main hot water tank, size of under sink tank, etc??  Maybe there is some other variable that derails this whole idea.

I would think that if the tank was very far away from its primary point of use, the system would benefit from a point-of-use heater as close as possible to the faucet.  They make little electric plug in ones, which might not be powerful enough to go from ground water cold to shower hot, but would probably be enough to go from an existing water water tank turned down to its lowest setting to shower hot.  But I'd use an instant, not a tank.  I don't see what the point would be of having a small tank after the main tank, the water in each would get cold when not in use independently, and each would need to keep its own supply warm.  The main tank would only be feeding the little one when the water is flowing, at which time the little one wouldn't have time to heat the incoming water before it passed out again.

[MooreBonds]

@MooreBonds:
I always wondered if there was a way to supplement existing hot water supply with an under-sink electric hot water tank.  My theory was to use the hot water feed from your existing hot water tank as the input to the under-sink electric hot water tank?  Theoretically, you would then get the benefit of instant-on with the backing of your main hot water tank for those 20 minute showers.  Depending on how far away the sink is from the main hot water tank, there could be a few gallons of cold water getting mixed into the under-sink hot water tank before hot water from the main tank.  With this solution, you might get away with a smaller tank under the sink, reducing costs.

Well, you have to be careful with those instantaneous heaters (speaking both as a plumbing contractor and plumbing engineer). Not only do the instantaneous electric ones require a HUGE amount of power (dont' have experience with gas-fired instantaneous ones) , but the cheaper electric models blindly step up the water by a certain amount. You have to spend a bit more to get electric instantaneous ones that have a thermostat to regulate what the discharge temperature is.

In other words...if you have a tank water heater upstream of a cheaper instantaneous one, the cheap-model instant one will step up the water by a set amount of degrees (like 50 degrees), whether the incoming water is 50 degrees or 120 degrees...which means you could be getting 140 or 160 degree hot water when you first turn on the faucet! You can buy a thermostatic mixing valve to keep the water no hotter than a certain amount - but by the time you do all of that, you'd be better off spending the same amount for a thermostatic instantaneous heater without the tank-type heater.

[Bakari]

Well, you have to be careful with those instantaneous heaters (speaking both as a plumbing contractor and plumbing engineer). Not only do the instantaneous electric ones require a HUGE amount of power (dont' have experience with gas-fired instantaneous ones) , but the cheaper electric models blindly step up the water by a certain amount. You have to spend a bit more to get electric instantaneous ones that have a thermostat to regulate what the discharge temperature is.

In other words...if you have a tank water heater upstream of a cheaper instantaneous one, the cheap-model instant one will step up the water by a set amount of degrees (like 50 degrees), whether the incoming water is 50 degrees or 120 degrees...which means you could be getting 140 or 160 degree hot water when you first turn on the faucet! You can buy a thermostatic mixing valve to keep the water no hotter than a certain amount - but by the time you do all of that, you'd be better off spending the same amount for a thermostatic instantaneous heater without the tank-type heater.

I was specifically thinking of the cheaper, raise temp by a set amount, type (the very smallest of which can plug into 120V) to go downstream of a tank heater, because the tank heater has a thermostat.  So you have a consistent temperature going into the point of use instant heater (until the tank runs out of hot, at which point the temperature drops, but point is it will never raise above your initial set point).

So, if the instant raises the temp 30 degrees, and you want 140 degree water at the tap, you could then lower your main tank heater from 140 down to 110, there by substantially reducing losses through the tank and pipes (the rate of heat transfer is directly related to the temperature differential inside vs outside the tank).

[Blindsquirrel]

 If you are in a house, a GE Geospring heat pump water heater may be cheaper yet. We have no natural gas option and it was $860 after I sold our old water heater for 100. Electric bill dropped about $ 18 a mo. (had a power sucking 80 gallon beast.)