General Question

ibstubro's avatar

What's your personal experience with an 'on demand' hot water heater?

Asked by ibstubro (11243 points ) December 12th, 2013

I getting ready to re-plumb my house, and I think I want an on-demand hot water heater.

If you have personal experience with one, I would appreciate your positive or negative comments.

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38 Answers

SecondHandStoke's avatar

On demand heaters are best in an original application.

Refitting doesn’t always make for the better.

The fewer users the better for a household situation.

snowberry's avatar

When I was in France for a summer, we had one. It was a very simple affair, and attached to the back of the shower. It was so simply made, a handyman could easily construct one in his spare time, so I’m confused by the exorbitant price by the ones you buy now.

I loved it too, by the way, and wished the concept would make it to the states soon. Sooo, 40 years later it has!

My biggest concern for a newly installed one would be buildup of hard water deposits inside the pipes as they heat if you didn’t also have the house plumbed for soft water.

ibstubro's avatar

@SecondHandStoke it will likely be a total re-plumbing of the entire house and only 2 users. Thanks.

Thanks, @snowberry. I have a water softener, but I’m not convinced of it’s efficiency. My house was built in the ‘60s and used a sand point for water until last year. Hard water deposit in the pipes would be the reason for the re-plumb.

JLeslie's avatar

I am going to get it in my new house, I don’t have experience with it, but I was warned that our water is so hard here the smaller lines where the water heats up can get blocked up and that getting a water softner is a way to prevent it. It might not be an issue where you live.

ibstubro's avatar

I have the water softener, but unsure how well it works, @JLeslie, so that’s a concern for me, too.

I also have a vague notion that you have to back-flush the on-demand once a year. That’s a concern as to where you backflush to and who remembers to do it.

That was part of what prompted my question.

I hope you learn something that helps you out here, too.

JLeslie's avatar

I was really glad to see the Q. :)

stanleybmanly's avatar

I have a question, but first I’d like to comment that the build up in copper pipe occurs only in hot water lines and is completely different from the stuff in galvanized pipe. I was wondering if on demand heaters can be used in conjunction with a traditional water heater, such that you would not have to waste gallons of water waiting for the hot water to arrive. I know that on demand heaters lack the capacity for continuous hot water flow that is assured with a 30 gallon or above hot water heater

Skaggfacemutt's avatar

We tried it in the 80’s. Maybe they have improved since then, but the one we had was useless, even though we paid three times more than a hot water heater would have cost us. You had to run the water at a trickle to get hot water. The faster the flow, the colder the water became. It would take forever to run a bath, and the dishwasher and washing machine ran too fast for it to keep up. Totally useless.

ibstubro's avatar

I hate to hear that, @Skaggfacemutt!

Of, so @stanleybmanly what you’re suggesting is an on-demand at the site of water that gives way to the tank? Never thought of that! Interesting, but expensive, idea!

stanleybmanly's avatar

Skaggfacemutt’s point is still valid. You won’t be able to obtain the same rate of flow from an on demand heater that can be obtained from the standing water heater, and you will certainly not be able to run a washing machine and shower at the same time. The solution would appear to boil down to an on demand heater at every hot water outlet. ( talk about expensive). You have some research ahead of you turbo, and I’m interested in what you find out. Is there a possibility of relocating the big heater in a central location that would be closer to the major outlets (kitchen bathroom laundry)?

ibstubro's avatar

Well, @stanleybmanly, the on-demand would have to be centrally located, anyway. If the tank was between the water source and the on-demand, then the on-demand would stop heating as long as the tank was providing water @ temp.

LuckyGuy's avatar

I had one in a house I lived in overseas. Endless hot water… nice. It was in a relatively tropical climate so the unit was outdoors. However…. when that thing kicked on, you knew it. The system was quite noisy, like a laundry room dryer. Also the ignition system made enough electromagnetic interference it affected the radio and TV.
My data is 15 years old. Hopefully they are quieter now.

snowberry's avatar

The house I was in had the plumbing all located close to the water heater. Kitchen on one side of the wall, bathroom on the other. That would make a difference. And as @LuckyGuy said, it sounded like a jet engine. Sure was nice though. Maybe the extra cost in the ones offered now is due to sound deadening and technology that would not allow electromagnetic interference.

WestRiverrat's avatar

I wanted to get one, but they wouldn’t work in my climate. They were only good to raise the water temp about 20 degrees. The plumber said the ideal use would be if I wanted to keep the water heater temp set relatively low for baths and showers, which makes sense if you have young kids, but could boost the temp if I needed to at the dishwasher or hot tub.

Since I don’t have kids, I just set the temp at the tank a little higher.

stanleybmanly's avatar

@ibsturbo. Yes that’s the idea. The on demand provides the initial heat until the arrival of hot water from the traditional tank. Lucky guy and snowberry sound like they’re talking about combat sized industrial devices. Of course in a big old rambling house, there’s always going to be the problem of cold water and cold pipes between the heater and the taps. Our house was re plumbed 6 years ago, and then I seriously considered an on demand heater at the junction where the water line sprouted into 2 lines. One to our newly tiled bathroom and the other to the laundry room. The run to the 2 taps would be less than 3 feet apiece and the heater would be mounted on the ceiling of the garage below. As it is now the house is plumbed such that there is a run of at least 80 feet from the heater to the 2 taps. I have a water pressure gauge attached to a tap in the garage which reads just under 90lb per square inch. Even so, it takes a full 30–40 seconds for the hot water to hit the shower. It’s money literally down the drain.

snowberry's avatar

@WestRiverrat Wow, the one we had in France, if you didn’t watch it, you could have raised blisters in the shower. That sucker was hot!. But like I said, it was a bare-bones affair.

LuckyGuy's avatar

I should add one other item. The unit was so loud everyone in the area.knew when you got up in the middle of the night and washed your hands.
I imagine you might have a similar situation indoors. But maybe they are much better now.

JLeslie's avatar

Why is it noisy? I had no idea it was noisy. I thought it is like running water through a coffee maker?

LuckyGuy's avatar

It was noisy because it was a gas fired system. You could hear the gas burner ignite and the blower fire up. Like on a gas clothes dryer.

JLeslie's avatar

Oh. I assume mine will be electric? Not sure. I do have gas lines to my house. I have a fantasy that some brilliant improvement will happen in solar and then having a primarily electric house will be a bonus.

SecondHandStoke's avatar

One should have an effective water softener anyway.

Nothing is a threat to one’s social life more than hard water:

Stiff rough clothes.

Brittle hair.

Ashen complexion.

Water spots and mineral scratches on one’s car finish…

SecondHandStoke's avatar

^^However life is too short for an electric range.

They call it “cooking with gas” for a reason.

ibstubro's avatar

I’m nearly as concerned about my water as you are, @SecondHandStoke, but an electric cooktop is nothing but a constant irritant. I have a dream that someone is going to give me a range with gas cooktop and dual electric convection ovens. Dream big: matching French door fridge with bottom freezer.

Judi's avatar

Love it. We have used Rinnai mostly.
I did see an awful review that someone had for an inexpensive point of use water heater that they thought should heat all the water in the house. I felt bad for the guy because he didn’t know what he was doing.
Just so you know you will need
¾ inch gas line
Isolation valves
Special duct work that both draws and vents (kind if expensive)
Even though hubby is a licensed contractor he leaves this specialized work to a licensed plumber. It’s not really a do it yourself project and the parts alone could be a few thousands.
Check out the Rinnai website to get an idea what parts you will need.

ibstubro's avatar

I think that might be a “Best Answer” for me and @JLeslie @Judi.

Judi's avatar

@ibsturbo, your askville” is showing. :-)

ibstubro's avatar

@Judi it’s been so long since Askville had a “Best Answer” option that I had forgotten it. I guess somewhere along the line I confused Family Fued and Askville, because I’ve clapped and exclaimed “Good Answer!” since Richard Dawson’s first season. lol

LuckyGuy's avatar

@ibstubro @JLeslie Here is a little physics for you that might help. All this will assume no efficiency loss. (Not possible but you can get close.)
Electricity:. 1000 Watts ~ 3400 BTU/hr
A “normal” 110 Volt outlet is 15 amps or 1650 watts ~ 5500 BTU/hour.
A big 220 volt service with 30 amps is 6600 watts ~ 22,000 BTU/hr

If you want to heat one gallon per minute that is 60 gallons per hour or ~500 pounds per hour. If your cold water is 50F and you want to get it up to 100F at that flow rate that means you need. 50×500 = 25000 BTU/hr

You can get a natural gas fired boiler for 75000 BTU/hr. It will heat all the water you need no matter how large the demand. That is why it sounds like a jet engine.

If you get a small electric unit and agree to have a reduced flow rate. You can get a perfectly quiet unit.

Try an experiment an see how much water you flow in a nhour. Also measure your cold temperature. It might be higher than 50F so you migh not need such a big unit. .

The laws of physics are hard to beat.

trailsillustrated's avatar

I have a relatively new rinnai one it’s gas, it works great I do lots of washing, it never runs out. I’ts not noisy.

ibstubro's avatar

Thanks, @trailsillustrated. Exactly what @JLeslie and I wanted to hear.

I was thinking that on-demand water availability would be reliable with a small holding tank. Reduce the efficiency a bit, but make it more reliable.

trailsillustrated's avatar

Mine is a small unit on the side of the house. There’s no tank that I can see. It’s economical and it’s never run out.

Judi's avatar

You don’t need a tank for a tankless water heater silly!

susanc's avatar

We put in one when we were sure we didn’t have kids any more and only two of us would be using the shower most of the time. That was about eleven years ago. We have two bathrooms back to back, a kitchen sink about ten feet away, a dishwasher right next to it, and a washing machine about ten feet in the other direction. So the runs aren’t that long. I have NEVER HAD A PROBLEM.
I’m running the demand heater on propane. Propane is a complete bitch, but I live in the sticks and can’t get on a natural gas line, so I heat with it when the blessed heat pump is overwhelmed by freezing air temps.
I do have to run the water for awhile before the hot water gets going. Yes, water costs money, but I fill up various pots with the pre-hot water and use it for cooking, pets’ water bowls, etc.
My greywater and excessive cold water flows from the bathtub/kitchen sink/toilets into a septic tank and from there into my drainfield. My grass is except in August.
Oh, another detail: my tankless heater is inside a sort of insulated closet that opens to the outdoors rather than the bathroom it’s next to – this saves on bathroom space. I keep a heat tape on the inflow pipes and turn it on around Hallowe’en, then off again around the Ides of March. I also filter all the well water that comes into the house through a whole-house filter but the water isn’t intrinsically very hard, it’s well water from a pretty friendly aquifer.

Yes, buying and installing the on-demand was expensive. Yes. But I had just gotten a big commission and this is how I celebrated.
Sexy, right?

susanc's avatar

… “my grass is” GREEN “except in August”. But you knew that.

JLeslie's avatar

I live in FL so the tap water temperature is fairly warm.

ibstubro's avatar

Great information/answer @susanc (you sexy thing)

susanc's avatar

Nothing like earning power to make a girl look hot. (This is for you, @ibstubro).

MadMadMax's avatar

My son put in a tank-less water heater and it’s fantastic. He restored a whole house and everything is uber “green” environmentally friendly. It’s gorgeous.

I wish I could afford the water heater. I wish I could own that house.

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