Tankless Water Heater

booshcat

Anyone have any experience with Takagi? I've installed a tk-3 in a new gas conversion and the HO loves it.

Double-A

You never want to mix natural draft appliances with forced draft appliances in a combined or common flue. The reason for this can be that the forced draft will cause an unnatural draft in the non-power vented appliance and cause combustion issues or even allow a flame out and reignition situation. It can also push its flue gases into the natural draft unit causing a back-drafting situation and a fire.

Some model codes allow this, but require automatic dampers to open and close and ignition lock-outs to be used to prevent simultaneous firing and also to prevent funky situations from arising with having a forced, high speed stream of flue gas being pushed into a cold stack of air (in the vertical part of the flue above the natural draft unit), and being forced into the building instead of being pushed outside.

Think of it this way... forced draft appliances pressurize their flues. When this happens, flue gases try to go anyplace they can to escape, and its not always up and out as intended. Don't mix natural draft appliances with forced draft appliances... ever, unless you're damn sure you understand exactly what happens at all times.

One other note is condensing appliances. They should never be put on a flue with a non-condensing unit unless there are ignition lockouts in place. I haven't looked at the latest model gas codes to see if this is addressed, but last time I looked several years ago, it wasn't.

Splinter

Wow, Im glad I stumbled on this thread.. I bought my house almost 10 years ago, and the previous owner had a new furnace installed a year before that. The forced air gas furnace has a power vent which was tapped into the same flue as the 40 gal water heater...

Now the pilot on the water heater has never gone out, and the CO detector has never registered any dangerous situation, but I want to fix it.

The chimney is in the center of the house with both appliances right next to it. Would I have to move the furnace to an outside wall and direct vent it? That would involve a lot of ductwork. Would I just have a new vent run to an exterior wall? It's at least a 20' run for that...

Double-A

A power vent or a draft inducer? Most furnaces have a draft inducer and it is not considered to be the same thing as a power vent.

I would a competent person (plumber perhaps, or HVAC folks) take a look and make recommendations. If it is truly a power vented appliance, it can have quite a long horizontal run with few problems if any.

Splinter

Thanks for the clarification... It was the cheapest model the previous HO could buy, so probably not a power vent unit, I suppose. I didnt realize there was such a thing as a draft inducer... I'm going to have the plumber stop by to verify though...

ExperiencedPro

RealPlumber is 100% correct
I've had to re-educate a lot of inspectors seldom with any thanks on tankless. At no time has Noritz or any Plumbing Code allowed a Type III/IV Power Vented appliance to share a flue with either a Natural Draft or Type I Fan Assisted Appliance (older 80% furnace).
It's dangerous, detrimental to the equipment and VOIDS any Warranty. Also "B" Vent is not allowed.
Only AL29C Stainless or on the new condensing (i.e. Bosch 800C, Noritz 841/842, Naviens and Takagi TH2, Eternals) units you can use Sch 40 PVC which also means solid core not cellular PVC.
We remediate 100's of units a year for mistakes like these and we're never really welcomed by Owners who already supposedly paid to have it done right.

ExperiencedPro

Navien is rolling out a new NR/NP series of models in the same 180/210/240 sizes but with a lot of the rubber they used first time around replaced by molded plastic and clips.
Not saying they got it perfect but at the efficiency they claim if they get press you'll be getting inquiries about them steadily.
Curious their original models were the only tankless units with a leak detector/shutdown system. Possibly a trouble hint?
Me, I'm not sold on their upside down flame. Seen it done on the eternal but in a more controlled flame drum.

JamesNLA

Almost all name brans tankless units have multiple fluid sensors on the bottom of the can. Why? Incase a leak devolops internally. This has been in existance for at least 3 or 4 years. I have done one of the new Navians,......not much has changed...still loud, still inefficient on the NR recirc design. Gas diaphram still uses substandars tubing. Flue collar is all plastic that will never last 20 years. It's a better idea than it is a product. They will still sell because of their price point but they are a middle of the line product with piss poor in house techs.

mrmike

The word Direct Vent is used because The heating appliance takes the air from the outside for combustion instead of using the air from the room that it is in.. Usually the pipe has two Chambers -one for exhaust-& one for fresh Air.