Installing a hot water radiant floor heating system. Thinking of using a tankless hot water heater as the heat source (like this one http://www.takagi.com/index.php?product_id=3&page_id=2)

It requires a Category 3 vent, which I intend to run through the roof. I understand that the vent needs to be 2' above the roof, but I can't figure out how to measure that vertical distance on a sloping roof.

Anyone know where I could find that answer?

Why not call your town's building inspector/construction official.
He would know.

If the roof pitch is not too great, just measure 2' from the highest point where the vent passes through the roof..........then, you can't go wrong.

However, if it's a steep pitch, 12 or greater, there will be issues with the distance between the top of the vent and the roof based upon the horizontal distance. In this case, you'll need to speak to the building inspector. You'll probably need to go taller.

http://www.saginawtx.net/bid-ho-waterheater.htm

Scroll down to a chart and diagram that might give you an idea.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Good rule of thumb:

http://www.selkirkcorp.com/installation-planners/PlannersSetup/PlannerFiles/support/chimneyh_en.html

I second the building inspectors word. Only costs a 5 minute visit and it's the 'right' answer.

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-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/showpost.php?p=831799&postcount=13
http://www.peachparts.com/shopforum/showpost.php?p=831807&postcount=14

Here in NY the stack needs to be "two feet higher than the highest point (roof surface) within ten feet"

Jim

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2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)

Thanks...........although it seems a bit onerous to me. It's going to be one ugly stack if he has to match that code.

He'd need a 5' stack on a 6" pitch roof.........and taller as the pitch climbs...........10' stack on a 12".

Not trying to give you a hard time but is a tankless the best way to go with radiant floor? I mean, response time is not a concern and isn't that the main advantage of the tankless heater?

In a big house in Berkeley where my buddy put in the heating system, they just got a huge water heater and it services everything, as the radiant lines are potable rated.

Downside of this, whenever the heater had a hiccoup, not only no heat but no hot water either.

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1986 300SDL, 362K
1984 300D, 138K

I have two tankless heaters in my house and both recommend that the heaters NOT be hooked up to a closed, recirculating system. Check with the manufacturer before you invest in this.

I like the idea of a tank-type hot water heater for this application. This particular manufacturer (Takagi) recommends this unit for radiant floor applications. One thing I like about it is that it is so compact. Space is a real concern in my application.

I did this about fifteen years ago. It was not recommended but I thought it would work anyway. The owners found it did not work correctly and had to install a small boiler to replace it.

You want to use either a small boiler or a tank type heater.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

FYI, solar hot water evacuated tubes work really well with radiant floor heat. A well designed system will provide most of your hot water (for showers, etc) and shorten your heating season. I helped install a system like that last week.

These comments are great.

I just had an interesting conversation with a plumber. He says that the reason Takagi recommends its tankless units for radiant floor applications is the amount of storage that is already in the piping system. I hadn't thought of that.

The supplier I intend to use for my tubing has 7/8" tube. Assuming that I have 750 feet of tubing, that works out to about 23 gallons in the tubes.

To put that in perspective, the smallest Takagi tankless heater goes down to an input 17,500 BTUH at 83% efficiency. At that level, it would take the unit between 4 and 5 minutes to raise the water temperature in whole system by 5 degrees.

That doesn't seem like such a hateful situation.

We have installed many, many systems in all of the above configurations, depending upon heat loss, client preference, and budget. Have used boilers, tankless water heaters, standard domestic water heaters and "Combi-Core" units (a tank style water heater with an internal coil/heat exchanger, specifically designed for this application). Haven't had any adverse feedback from any of them; some of the tankless installations were done 10+ years ago. I believe though that Kip Foss is correct; any of the tankless heaters have a prominent disclaimer about use for heating purposes. Find yourself a knowledgeable hydronics professional, buy him a beer and pick his brain.

Best of luck,
Jim

__________________
2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)

I think this book is the Bible on this subject, or maybe one of them:

http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?r=1&ISBN=9780766816374&ourl=Modern%2DHydronic%2DHeating%2Dfor%2DResidential%2Dand%2DLight%2DCommercial%2DBuildings%2D2E%2FS iegenthaler

I got one on Ebay a while ago.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra