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Old 01-15-2010, 10:25 PM   #16
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DIY gas tankless water heater


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Originally Posted by beenthere View Post
Lofar.
How did you determine that your pipe sizing to the area that your installing the tankless is large enough.

Did you measure the pipe length from the meter to the tankless area?
If so, how long is it? And did you include all the other appliances that will be tapping gas from that line.
No, i did not measure the pipes as that would be nearly impossible to do. I went back to the original building plans. Which basically state incoming gas is 1-1/4" off the meter at max length of 60ft, followed by a 1" line with max length of 25ft, the water heater and furnace then share a 3/4" line which I estimate to be no longer than 15 feet, the stove/oven, dryer and fireplace all have their own runs off various lengths of the 1" line. The current rating of all appliances is 160,000BTU, replacing the water heater with a 180,000 unit gives me a total load of 340,000BTU. My biggest worry at this point is actually the gas meter itself which is rated at 250CFM, which seems like would be unable to support such a load but I called the gas CO and they said I should be fine as long as the tankless water heater doesn't exceed 200,000 BTU. <shrug> Their problem if it doesn't work out I guess.

I also recently discovered the Hydronic air handlers offered by rinnai and am thinking that I will eventually replace my furnace with one of those and make use of the tankless water heater for air heating as well which will take 40-60kBTU off the load of the system. Of couse i'm also in southern california so I really question if I actually need a heater or even should count it in the loading equation.

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Last edited by lofar; 01-15-2010 at 10:27 PM.
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Old 01-15-2010, 10:49 PM   #17
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15' 3/4" line at 1/2 PSIG or less is good for about 234,000 BTUs. But doesn't allow for restrictions from tee's ell's or other fittings in the gas line. Allowing for a few fittings. Your 3/4" line is good for about 180,000 BTUs.

So if your tankless is 180,000, and your furnace is trying to run at the same time. The 3/4" line is too small.

Your 25' of 1" line would then also only be good for 315,000 BTu's not allowing for any of the above listed restrictions. Extending it the 15' that your 3/4" now runs. Would lower it capacity to 245,000 BTUs, not allowing for fittings. Allow for a few fittings, and its good for about 215,000 BTUs.

You'll need to repipe to use the tankless and have heat at the same time.
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Old 01-15-2010, 11:03 PM   #18
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Quote:
Originally Posted by beenthere View Post
15' 3/4" line at 1/2 PSIG or less is good for about 234,000 BTUs. But doesn't allow for restrictions from tee's ell's or other fittings in the gas line. Allowing for a few fittings. Your 3/4" line is good for about 180,000 BTUs.

So if your tankless is 180,000, and your furnace is trying to run at the same time. The 3/4" line is too small.

Your 25' of 1" line would then also only be good for 315,000 BTu's not allowing for any of the above listed restrictions. Extending it the 15' that your 3/4" now runs. Would lower it capacity to 245,000 BTUs, not allowing for fittings. Allow for a few fittings, and its good for about 215,000 BTUs.

You'll need to repipe to use the tankless and have heat at the same time.
I've been using the charts from my local gas company which is a bit higher than what you have here.

http://www.socalgas.com/construction...nts/tableb.pdf

This table is probably even lower than reality because in their other documentation the state that their standard delivery pressure is an 8" WC / 1/3lbPSIG, so there's probably another 12-15% in there to work with.

I am also confident for the 3/4" that there's not a lot of bends or Ts in the line. I can't see through the floors but the 3/4" most likely comes off a T on the 1" line just below floor level and goes straight up where it meets a T for the current water heater and then an elbow to the furnace. My plan (because i'm relocating the water heater a bit) is to replace the T to the water heater with a coupling and replace the elbow to the furnace with a T to split to the furnace and water heater.

Last edited by lofar; 01-15-2010 at 11:08 PM.
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Old 01-15-2010, 11:30 PM   #19
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Thats a chart for a .5" pressure loss. Used to cut cost by many. And often lets you end up with a whistling noise when the appliances are on.

I size for the gas so the equipment does not to whistle. And so that when a below normal temp period comes around. When the gas company can't maintain normal pressure in the lines. the system still works. And doesn't whistle.

Gas furnaces/appliances run into trouble when the gas pressure suddenly drops. Because another appliance has come on and the sudden pressure loss lowers the flame.

3/4" 90 ell is equivalent to 2' of pipe.
3/4" Thru tee is equivalent to 1.5' of pipe.
3/4" branch tee is equivalent to 4' of pipe.
1" 90 ell is equivalent to 3' of pipe.
1" Thru tee is equivalent to 2' of pipe.
1" branch tee is equivalent to 5' of pipe.


You may assume all you want. When you have troubles. if you call the gas company and complain. The first thing they'll bust on is the piping size. You'll know what you have to replace.
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Old 01-16-2010, 12:23 AM   #20
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Here is a link to find installers of Bosch tankless water heaters
http://www.boschhotwater.com/BoschHo...5/Default.aspx
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Old 01-16-2010, 01:23 AM   #21
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hmm.. Well. I guess I need to really look at the hydronic air handlers then, if I throw that in and then go with the 9.8GPM condensing rinnai heater that puts my total household load at 290,000 BTU and the load on the 3/4" to the water heater at 199,000 BTU which would just baerly fit under the numbers you gave above. Guess this might turn into a larger project than I hoped for... But I do need a new furnace, the piece of crap baerly works as it is and is probably running at about 20% efficency by now I'm sure.
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Old 01-16-2010, 05:12 AM   #22
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Be carefull of getting too large of a unit. Or it won't come on when you use some of your faucets.

It will take .X GPM to bring on the burner.

You might be surprised. How little water you draw at the vanity. And it may not be enough to activate larger tankless units.
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Old 01-16-2010, 10:56 AM   #23
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180,000 btu/hr with 50F incoming water going to 120F [70F temp. rise] equals 5 GPM continuously, forever.
That seems pretty high.
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Old 02-12-2010, 08:56 PM   #24
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Thanks for the help so far on this. I think i've finally figured out that I need to get a new line run, but i'm a little confused about exactly how to size it.

Including elbows and everything I have an equivalent of 136ft of distance I need to cover. The first 60 feet will be under the building, and due to space limitations I will need to use CSST as getting full length rigid pipe under the building just isn't going to happen. Looking at the tables from omegaflex in this document http://www.omegaflex.com/trac/techni...Omega_Flex.pdf 1 & 1/4" over 60 feet will carry 327CFH. From that I can use rigid black steel for the rest of the distance which is an equivalent of 76 feet with elbows etc. and at 76 feet a 1" rigid will carry 222CFH. So, can I assume that connecting the 1" to the 1-1/4" will give me the full 222CFH at the end of my line or am I totally missing something?

BTW, i'm having the gas-co come out next week and do some pressure tests and load test on the current line and I will probably discuss some of this with them so I want to make sure I have an idea of what i'm talking about.
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Old 02-12-2010, 11:07 PM   #25
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No.
You have to size the CSST to carry the full BTU of all appliances for the length of the longest run. not just that 60 foot.
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Old 02-12-2010, 11:51 PM   #26
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This will be the only thing on the new line, but if I understand you right i need to size the 60 foot csst section as if it was the full 136 feet even if I downsize the pipe further down the line? I guess that makes sense. Even so, that should probably still be ok, according to the chart I should get alittle more than 208 CFH out of 1-1/4" CSST. Of course wouldn't it be even higher than that since it appears CSST has a lower capacity of an equivalent sch-40 black steel and I won't be using CSST for the entire distance. But it's probably safer to just assume worst case scenario numbers.
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Old 02-12-2010, 11:57 PM   #27
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What the next section of gas pipe is(CSST, sch40, etc) never matters.

I have never replaced a gas line because it was too big.

But done lots and lots of replacements because it was too small.


When you say its the only thing on the line. What is the only thing on the line?

Are you running a separate line for the tankless now.
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Old 02-13-2010, 12:58 AM   #28
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Yes, that would be correct. Going to run a seperate line off a T right off the meter for the tankless. I did a lot more investigating and while the original building plans said 1-1/4" after more investigating (read lots of crawling in dirt) the vast majority of the line is only 1" and frankly it is amazing that I have working gas appliances at all after doing the math based on the 1" numbers.
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Old 02-13-2010, 07:16 AM   #29
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I think you'll find Ward is a bit generous with their CFH ratings.

At 136 foot, you'll barely have 180,000BTU's at a .5"PD.

You'll be closer to a 1" PD.
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Old 02-13-2010, 05:47 PM   #30
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Installed tankless DIY and had an HVAC guy install the hydronic air handler from the same manufacturer. I don't want to mention brand as they do state warranty is voided by DIY installation. Getting used to the "cold water sandwich" takes a little time but you change your habits accordingly.

Don't know anything about Grundfos circulation pump but the TACO D'MAND system eliminates the water waste while water is getting hot and can be used with tankless.

Getting gas line properly sized is a big issue, however with the units I installed they will restrict their output to match gas supply, but I am sure that is within reason. Home I purchased had no gas at all so it was a new LP install.

--Chris

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