Topic: Not very exciting and way off topic - tankless water heaters

[tsflstb]

So after another cold shower this morning, I'm getting serious about replacing my water heater.  I have a 40 gallon unit in the attic.  It's not working very well because the tank is fouled up with calcium deposits dropping out of the water.  It's also 8 years old, which is about like 80 in human years.

Been thinking about using one of those tankless "on demand" units that just heat the water as it flows.  I have natural gas and realize I might have to upsize my vent and service lines.  I know these are pretty common in Europe and I like the idea of lower energy use, plus not having a big tank of water that could possibly leak in my attic.

There's a $300 tax credit for installing one of these high-efficiency bad boys, but the cost is about $700-$1000 above a replacement tank.  They're supposed to last a while, and are easily serviced.  Anybody have experience with it?

[DammitDan]

I've seen those in action on This Old House...  Very cool little pieces of equipment, and I'm curious if the savings in electric bills would offset and eventually pay for the extra cost?

[AHuff]

No first hand experience, but my folks have 2 units so far, with a third on the way in the next few months. As far as I am concerned tank units are a thing of the past.

Pros: Size, efficiency, no tank to fail and flood

Cons: Upfront cost

What kind of delay do you have before hot water gets to your faucet now? Would you mount a tank-less in the same spot as your old unit? The only issue they have had is one of their bedrooms is at the opposite end of the house from where the unit is, and its a 2-3 minute delay before hot water gets there.

[Bob Wessner]

I think Dan may be confusing two different devices. The Rheem unit replaces your hot water tank in the basement, or wherever. As AHuff points out, you would still have a cold pipe run from the unit to the faucet(s) unless, you also installed the small electric units right at each faucet until the hot water arrived. A neighbor across the street installed the Rheem about two years ago and loves it. Next time our hot water tank is due for a swap-out, we plan to go that way and regain some real-estate in the basement to boot.

[DammitDan]

I think Dan may be confusing two different devices.

?

The one I saw on PBS (though I don't remember the name) was mounted on the wall in the same place as the old water heater, and connected to the same water lines that the previous heater had used.  It was very efficient and cost effective.  Is there another type of tankless on demand water heater that I'm getting confused with?

[Bob Wessner]

There are other smaller units (use electricity) to instantly heat a small amount of water local to the faucet until the hot water from either a tankless unit or traditional hot water tank reaches the faucet in use. Between uses, the water in the pipes between your basement and the faucet gets cold and you have to run the cold water out of the hot water tap until the hot water reaches it.. that's what we do in out house. Of course the basement unit can either be heated electrically or via gas depending on what is available to the homeowner.

[DammitDan]

Ah, I see now.  My parents' house has the same problem of cold water out of the tap until the hot water catches up (though they are still living in the past with their huge 50 gallon water heater ).  An electric heater closer to the exit point would be nice, but the amount of cold water they have to pull to get hot water is probably less expensive (and more eco-friendly) than running a full-time electric heater in each of the bathrooms.

I on the other hand live in a tiny apartment where it only takes 2 seconds for the hot water to hit my faucet.  Of course I only have a 10 gallon tank, but hey who needs to take long showers? 

[Bob Wessner]

.. but the amount of cold water they have to pull to get hot water is probably less expensive (and more eco-friendly) than running a full-time electric heater in each of the bathrooms.

I agree. Now when water gets really expensive and the Great Lakes have been drained down to puddles, it might be time to rethink the equation. 

[DammitDan]

When water gets really expensive?!  It's already one of the most expensive liquids available to the general consumer! 

[Bob Wessner]

Particularly the bottled varieties intended to line the pockets of Pepsi and Coke shareholders. Sadly, my wife is among them, though not habitually.

[Uncle Ernie]

Around here, house painters use tap water to remove old paint.  I installed filters for that.  Evian spelled backwards... Coincidence?

The ONLY reason I haven't installed an intant heater, aside from the cost, is that if we have some kind of disaster, there will be drinkable (potable?) water in a big tank down stairs.  Kind of stupid, but there have been a couple of times we had to make sure there was water on hand.

[Chris Schneiter]

I put a small unit in my business that runs to two sinks....make sure if you get one that you get a large enough one. Mine runs on 220, and the amount it heats depends upon water flow...If too much water flows through, it doesn't have enough power to heat it all ...for instance if two sinks are on at the same time, or if you turn the water on too hard at the faucet...I even had to turn down the feed line to it so it would heat. For what I put it in for I guess it works fine, but don't think you can put a cheaper one in and run a household. You need a unit that will be able  to keep up.

[tsflstb]

Good point Ernie.  Although my family's disaster plan includes looting the local liquor store to stay hydrated.

The other thing I just realized is they rely on a circuit board to control the burner, so even a gas heater needs electricity to work.  During the last big hurricane (Rita) we lost power for 5 days.  I guess that's not a huge deal, but something else to think about.

There are some sizing charts that give you an idea of what to get based on your usage.  I'd be using a gas burner, so I wouldn't have to mess with 220.  I may be overly conservative, but I figured flow based on the "perfect storm":  two showers going at the same time as the washing machine and dishwasher.  That puts me into a commercial grade unit. 

Not sure how often that scenario would play out, but I've got two young daughters that I can see using a bunch of hot water before they move out.  And I would rather not spend the next 18 years hearing "Dad why were you such a cheap ass with the water heater?"

[nickjtc]

My experience with these was when I was a lad in England. Usually they were small, wall-mounted, gas heated, and right next to where the hot water was required. Unlimited hot water was always a nice thing, but in reality I seldom used it more than was necessary. In the places I lived where they were installed I suspect that the primary reason for that type of hot water heating system was to save space.

The ones I experienced all had a small pilot light which was on all the time, so there was no electrical connection to the unit. The only real disadvantage was that when the wind blew from a certain direction through the venting the pilot light would get blown out!

As we see the price of natural gas and electricity spiral upwards I can imagine that this kind of set-up will become more and more cost effective.

[tsflstb]

I wonder how the older units managed the burner intensity?  The circuit board I was talking about senses the water flow and adjusts the burner to compensate and keep an even temperature.  I did see one unit with a water-powered generator and battery that ran the electronics, so no AC power was needed.

[csendker]

Tankless heaters are rated to raise the temperature of a certain flow of water through them a given amount (GPM vs DT).  But it's a function rather than a set quantity.  If you open too many taps at once, the GPM will rise, subsequently the DT will drop.  There's limits, of course.  Typically, these replace the existing hot water tank and tap back into the existing cold water feed and hot water discharge lines.  You size them based on the number and type of plumbing fixtures you have in the house.  Normal stuff leads to normal heaters.  Whirlpool tubs with monster fill spouts can make for big tankless heaters.

If you had to wait a while for the hot water to arrive with the old tank, you'll have to wait with the tankless.  This is because the water in the line between the heater and the sink cooled off, and you have to evacuate the cold before you get the hot.  This takes longer now with the low-flow fixtures than before when faucets resembled Niagara Falls.  This is a whole different story (which recirculation systems solve).  Depending on your house layout, you can sneak a cross-connection at the highest sink with a service valve cracked open and set up a gravity recirc system (hot water rises, cold water drops back down to the tank in the basement, if it's there).

The dedicated underdeck electric point-of-use heaters are usually for super-hot water, but they can be used for normal temperatures too.

And gas vs. electric will be a function of the utility costs in your area.  Here in Buffalo, we pay an insane $0.12-$0.14/kwh making electric anything expensive.  Natural gas isn't cheap, but more cost effective than electric.  The tax credit will help, but they're still expensive.  The savings you get are from eliminating the heat lost from holding a hot tank 24/7.  I got a 50 gallon tank when mine blew 5~6~8(?) years ago because the tankless weren't popular back then.  My kids do their best to empty it, but it's been holding it's own.  I put an extra insulation jacket on it to minimize the heat loss.  And if you go down and drain a couple of gallons out the drain valve a couple times a year, it will last a lot longer because the dropped solids won't be able to collect.  Actually, what happens is they collect on the bottom of the tank and form an insulation.  Unfortunately, the bottom of your tank is also the heat exchange surface, so insulting it will prevent the heat from getting to the water, heat up the metal and burn a hole in it, thus the ubiquitous leaky tank.

[tsflstb]

So with the heater in the attic, I'm out of luck for a gravity-recirculation system right?  I like the idea though.

I guess I could get a small electric pump to do that job.  Maybe set it on a timer to come on in the morning for instant hot water at the tap?

[csendker]

Yup, hot water rises is the gravity concept, and you already have it up there.  You can use a dinky pump with either a timer (simple) or a thermostatic controller (more complex-turns on when the water temp drops below say 100 and turns off when it's above 115 or so).  Just make sure you get one rated for potable water - a tiny plastic one would do it, you just want to circulate a little bit as far along your hot water piping system as you can to minimize the cold 'slug'.  Keep the pump real small or better yet run a dedicated line back to the heater or you risk heating up your cold water line.

[Bob Wessner]

Chris,

You seem to know your way around this stuff. I noticed the temp ranges you mentioned (100-115). I recall reading something about a year ago wherein they have determined some organisms in pipes survive temps up to 120F and recommend folks not set water temps below that. Did you ever hear of such a warning?

[csendker]

Nasties can grow in dark, hot, wet places (tanks, pipes) up to around 130 something.  That's why commercial set-ups use 140 as a set-point (commercial kitchens use 180 for sanitation purposes).  Unfortunately, 140 will also scald the heck out of you, so the commercial systems use mixing valves to drop the delivery temp down to about 110, losing another 5 degrees on the way to the faucet where it comes out at a nice, state mandated 105.  (Degrees F, that is, for all you international folks.) 

Mixing valves are expensive things, so you don't see them on residential systems.  120 is kind-of hot, and definetly in potential scald territory.  Hot water is worse than say, a hot radiator.  You touch a radiator, get burned, pull away fast and say 'ouch.  Done deal.  You stick you hand or body in scalding water, get burned, jump out and continue to get burned by the film of water on you.  You say 'ouch' a whole lot longer, even with much lower temperatures.

I can't say that I've ever heard of a "nasties-growing" issue in residential systems, but that's not to say it never happened.  I suppose you could raise the temp once or twice a year.  Burn everything out, then clean the pipes with the super-hot water, drop the temp and then let the tank recharge to the normal temp.

You can also cheat the size of your tank by raising the temp.  It takes less GPM at 120 to mix with cold water to make your hot water, effectively making your tank 'bigger'.  But at the risk of burning.  If you're ging high on the tank temp, I'd recommend "Pressure-balancing" shower valve ($) and maybe even sinks ($$).  These maintain the desired water temp even with big, momentary changes in water pressure (can you say "flush"?).

[Bob Wessner]

I'd recommend "Pressure-balancing" shower valve ($) and maybe even sinks ($$).  These maintain the desired water temp even with big, momentary changes in water pressure (can you say "flush"?).

The balancing valves are code here whether new construction or any retro work on showers. Now the "flush" just drops the pressure temporarily.

[nickjtc]

I wonder how the older units managed the burner intensity?

There was no 'burner intensity'! You had the choice between hot, hot or hot; and since there were no mixer taps in those days the units would have not worked in a shower situation.

[Uncle Ernie]

I'm learning a lot here!  I always thought people in England didn't take showers- they just dab at body parts with a damp hanky, or some such.

[DammitDan]

I'm learning a lot here!  I always thought people in England didn't take showers- they just dab at body parts with a damp hanky, or some such.

It's not a damp hanky, it's a moist  hanky!  (just because moist sounds even more disgusting)

[Rocking-M]

We have on demand hot water, but the heating unit may scare most of you off.
I heat with a hot water wood furnace. It also heats our hot water for the house through
an on demand heat exchanger. Super too, hot showers last as long as I want.
The temp is 180 of what ever I set the wood stove to. The furnace also
heats the shop

The draw back is the furnace was made in Canada.