some plumber type questions about tankless water heaters

[Neal Clayton]

so my old water heater is on its last legs and i'm looking to get one of these. i'm familiar with how they work and all that but have a question on placement and which type to get.

a) prefer gas (going in a cold basement)
b) need a larger unit (old house, big pipes, high flow in the tubs/showers)
c) indoor or outdoor unit?

C is the sticking point. the existing unit is in an unfinished basement that vents to the outside via openings in the outside wall footings, as most old houses do. the indoor units seem to have a vent built in that must be directed somewhere. the outdoor units seem to have intake/exhaust right there on the unit itself with no way of running the exhaust through any sort of pipe.

so which should i want here? can the exhaust from an indoor unit be directed to share exhaust with my furnace or is that a no-no? can i vent an outdoor unit to the open air in the basement since there's a foundation vent right there or is that not enough? are indoor units going to perform well enough in a sub freezing basement if that's what i need, or is there no difference in them performance wise and indoor vs outdoor doesn't matter in that respect?

any advice appreciated

[Doug Herzberg]

This isn't something to mess with. The reason for the vent is to get the CO out of the house. I think most of these units, if not all, have powered vents and get their combustion air from outdoors. While you might get away with taking the combustion air from a well ventilated basement, you do not want the exhaust coming out under where you live. At least some of the vent kits are combination combustion air/exhaust vent piping.

You also do not want to share a vent with a furnace or other gas appliance. There may be condensates in the exhaust which could corrode the metal vent piping.

An outdoor unit is just that. No provision is made to protect the occupants of the house from the exhaust gasses, other than locating the unit away from windows and other openings. Don't try putting one indoors.

At the very least, please read and follow all the instructions and warnings about installation and venting. Better still, consult an experienced installer. And get a CO detector.

[Brian Elfert]

Can you use one of the units that vents via PVC pipe instead of sharing the furnace exhaust? You'll need to check the codes to make sure the exhaust will be far enough from windows and doors and all that.

[Mike Cutler]

Neal

Outdoor unit would be my choice in Little Rock Arkansas. The Delta T across the unit would never be very large for an extended length of time.
You'll also find that the Vent piping will be substantially less, almost non existent, in cost versus venting from the inside.
The Tankless water heaters cannot be vented indoors, at least those I know of. There is too much exhaust gas to vent in an enclosed space. They also hane minimum intake air requirement that need to be met, or they need air from outside.
These units run 200,000 BTU's for a stand alone unit. Depending on your gas type, piping size, and location, the gas piping can be costly to run into the house if it's not already in the house.
AO Smith, and Bradford White, both make high capacity , high efficiency, power vented storage tank units. These can supply over 200 gallons per hour at > 90 degree F rise. The Combi Core can supply upwards of 400 gallons per hour. They may be worth a look if the tankless becomes prohibitively expensive. They can also be looped to supply not only hot water, but act as a boiler for radiant heat.
I'm not clear if you currently have a Tankless Water heater, or just a gas water heater.

[Jim Matthews]

When we did our retrofit here in Coastal Massachusetts, we considered tankless boilers.

Instead, we went with one to "temper" the incoming well water, and the other to raise the temperature.
It's on a timer, so it only makes hot water four hours each day and that if the temperature falls in the boiler.
It has a powered blower to exhaust outside, through a wall perforation.

It's a natural gas burner, and we're down 7% since last year.

[Michael Weber]

+1 on the pvc vented units. They are most effecient. Otherwise you will likely go broke buying the dual concentric wall stainless steel pipe required. They have to have stainless steel because the gasses are much more corrosive that most gas fired units. You also cannot use a commen vent from another appliance. Outdoor unit would be ideal unless it requires long lengths of water line to your points of delivery. Check out the Rheem units at Rheem.com. They make two sizes of pvc vented heaters. An interesting note about tankless heaters is that the gas input is modulated to the demand. So if one has a little water flow or alot the btu input will vary in order to maintain the outlet temperature. So an oversized unit costs no more in operation than a smaller size because they would both modulate the gas input to the same level. That is, a large unit will use no more energy than a small unit for the same demand. Purchase price is a bit more but might be worth the piece of mind that you will always have enough capacity. I do believe that the general concensus is that payback would take a very long time indeed over the purchase costs of a tank type.

[Neal Clayton]

thanks guys, i should have clarified, the existing unit is a tank type, electric. it just can't keep up in the deep cold in that basement.

doug, not to worry, i'll have a plumber install it, i don't mess with gas ;). just doing my research on which type of unit i should look for. i was trying to keep the install simple since the existing lines and gas access are already in the basement, and the exterior wall is stucco from 102 years ago, mixed very hot and put on very thick, it's hard to get anchors in that stuff, nevermind drill through it.

[Greg Peterson]

Neal - I replaced my gas water heater with a tankless unit several years ago. Mine is an inside unit, with the configuration option to draw its intake air from outside.

The exhaust venting was pretty expensive as it is stainless steel. I kept the venting as short as possible. I used single walled SS vent tube through a wall thimble. The cavity where the vent tube exits the exterior wall is boxed in with cement board and the sealed off with high temp RTV silicone. Perhaps that is a little over engineered, but I feel better knowing the vent is not in contact with anything that is flammable.

One mistake I made on my installation was not installing a back flushing kit. These are isolation valves that go on the inlet and outlet that allow you to circulate a cleaning solution through the heater core without disconnecting the service lines. I installed the isolation valve kit last year. Back flushing removes any calcium build up and/or any thing that might accumulate in the core.

If the installer offers the isolation valves as an option, IMO, in the long run it will pay for itself as the cleaning task is quick and you can easily perform it yourself.

[Chris Kennedy]

Kind of a side issue -- does it have to go in the basement? My understanding was that one of the advantages of tankless is that you could place them more centrally in the house, provided you could still vent them.

Cheers,

Chris

[Pat Barry]

The nice thing about the electric heater is that you can insulate it. Also insulate the hot water pipe running from it as far as possible.

[Jim Becker]

Venting is a critical issue with these things and it was not until recently that one could get a tankless unit that could use PVC for venting. That kept me from installing one in the 205 year old portion of our home until this past summer after the hurricane flood out our tank-type (a very expensive version) and I had to put in a replacement. I installed the Paloma PHH-32RDV (http://www.tanklesswaterheaters.com/tankless-water-heaters/gas-tankless-water-heaters/paloma-phh-32rdv.html) which is a 94% condensing type and uses 3" PVC for both intake and exhaust. While this one isn't inexpensive, when you consider you don't need the very, very, VERY expensive stainless steel exhaust venting, it's not a bad price. And it was only about $250 more than a replacement would have been for the special configuration high-efficiency tank type we had previously.

http://www.tanklesswaterheaters.com/media/catalog/product/cache/1/image/300x300/87b089d16cf02c1971aa5732f367ae3f/t/a/tankless-installation-valves-600px.jpg (stock photo)

My installation:

http://img.photobucket.com/albums/v646/a-j-adopt/hot-water-system.jpg

You cannot use an "outdoor" type indoors. You must use an "indoor" type with proper venting for locating the unit in your home.