Reducer Effect on BTUH

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  #1  
Old 07-25-09, 07:52 PM
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Reducer Effect on BTUH

Hi, my new tankless water heater requires 3/4 inch gas pipe for its 199,900 BTU burner. My current 75-gal tank runs a 1/2 inch T fitting down off the main 1 inch pipe.

How much would the flow be impeded if I were to replace the existing 1/2 inch pipe with 3/4 pipe using a reducer (reversed) off the 1/2 inch T? I'd rather not tap into the line if I can help it.

Thanks!
 
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  #2  
Old 07-26-09, 12:25 PM
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Gas Line

You have to know what the btu rating of the original heater was, could be that the 1/2 inch was barely enough to feed that. If you get this wrong your new heater could fail to stay lit or burn up the burner. Make sure you get this right.
 
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Old 07-26-09, 02:07 PM
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The current heater is a 75K btuh.

I've got a 5 psi, 275 CFH, 1 inch pipe going into the house.

I just wonder how much restriction the single 1/2 inch tee will have on a line that is otherwise a 1 inch to 3/4 inch reduction. Given that I don't have much elbow room.
 
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Old 07-26-09, 05:52 PM
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Here's in my house, aside from the water heater:

Dryer, 22Kbtuh
Furnace, 50Kbtuh
Gas Grill, 35Kbtuh
Stovetop, 20Kbtuh

Plus, the 199.9K tankless heater is actually going to run at 95% of capacity as I'll be using 3 inch exhaust instead of 4. So, talking through this, it seems as though I'll be fine, especially as

(1) I'll set the heater to 110, instead of 160, so it'll not be running a full load;
(2) I'll probably very rarely be running the furnace at the same time as the gas grill, due to weather conditions;
(3) It'll also be extremely rare that I'll be concurrently running the dryer, both stovetop burners, and pulling lots of hot water, and running the furnace. And if I do, it seems as though all of the appliances would take hits in that case, not just the tankless heater.

Someone punch holes in this. Am I looking at natural gas capacity / use the right way?

Steve
 
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Old 07-26-09, 06:18 PM
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No expert but a few minutes to kill...

If the burner pulls that much..not sure it will matter what you set the temp to. If it only has one burner, when it calls for heat...boom...full burner. If its some sort of staged thing..maybe..but who knows.

I don't think a 1/2" pipe will give the capacity you need.

Also, there are all sorts of things about temp rise and incoming water temp, etc etc.

If you don't plumb it the way the maker requires, I'd say you are setting yourself up for frustration and disappointment. Many tankless don't seem to work right even when they are installed correctly.
 
  #6  
Old 07-26-09, 06:29 PM
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Thanks, but I am not talking about plumbing with 1/2 inch - just running 3/4 inch coming out of the main 1 inch through a 1/2 inch tee...will just a little spot where its a half inch versus 3/4 make a difference?
 
  #7  
Old 07-26-09, 07:33 PM
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Yes, the restriction of the 1/2 inch branch WILL make a significant difference even if you immediately increase the pipe size to 3/4 inch.

Gas supply systems need to be engineered from the meter to the last appliance in the line. If you change any appliance significantly you must re-engineer the entire system. You may find that increasing the "draw" on your water heater from 75,000 BTU/hr. to 199,000 BTU/hr. (an increase of more than 160%) will require a new (larger) pipe from the meter all the way to where you would branch off for the water heater.

Furthermore, I don't know where you get the idea that lowering the set temperature of the output water will decrease the firing rate of the heater, because it won't. And unless the manufacturer has some special provisions for allowing a 3 inch exhaust you cannot arbitrarily use a 3 inch exhaust. It sounds to me as if you bought the water heater and now are trying to make it work with your existing piping and exhaust arrangements. If that is what you are attempting you are making a huge mistake.
 
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Old 07-26-09, 09:03 PM
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Wow, lots of assumptions built into your email there, and no specific info. Let me fill in the gray areas.

Spec says BTUH needs to range from 11k to 199.9K. That's why I assume a lower demand will correspond to a lower draw. If this is a mistaken impression, then why the range for btuh input?

Spec also makes provision for reducing 4 inch to 3 inch exhaust. That's where I got the 5% reduction in draw number.

I had a plumber out and his plan was to just tap a new 3/4 into the pipe. I am simply curious what effect trapping into the existing tee would have, with 3/4 pipe. Fluid dynamics would indicate not much; I am not as familiar with gas dynamics for this specific question. I wonder if someone on this board might be. Though I will probably have a new line put in further upstream, I like to explore all options.

I mentioned below that the pipe coming off the meter is 1 inch. Putting in a larger pipe than that one is a scenario similar to what I am describing herein, as the flow would still be restricted to the 1 inch pipe coming off the meter. Are you saying I might need a larger capacity meter? I am thinking of that as well...however, if that's not your point, then I am not sure why you think this might be effective, yet having a single 1/2 inch reduction inline between the 1 and the 3/4 would not be. You're still restricted to the meter's output capacity.

If anyone knows what effect the scenario I describe will have on flow, reducing a 1 inch pipe through a 1/2 inch tee then opening it immediately up to a 3/4 inch pipe for the rest of the 10 foot run, I'd be interested to hear it.
 
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Old 07-27-09, 12:01 PM
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Although this link is specifically for engine-driven electrical generators the principles are the same. I'll let you do the measuring and arithmetic.

Natural Gas Generator Pipe Size Chart
 
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Old 07-27-09, 12:10 PM
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Again..no expert..but these type of questions interest me...

What brand and model of WH are you installing....would prob help for those that have experience with them.
 
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Old 07-27-09, 12:24 PM
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furd,

That's a helpful link. I have already consumed that info, though; I know I need 3/4 inch pipe. Aside from whole house capacity issues, which I am drilling into separately with the utility company, my question is, what is the effect on the rated btuh flow if there is a single joint restriction of a 1/2 inch ID in what is otherwise a 1 inch to 3/4 inch line?

Is this something that is problematic?
 
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Old 07-27-09, 12:33 PM
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gunguy, it's the Noritz 842 indoor/outdoor single forced vent.

I'll probably be doing all the installation except perhaps the natural gas; I might have a contractor stub out the main with a 3/4, also depends on what the utility co. says re: capacity and flow rate.

Water and venting should be no problem; I'll be using pex for the water and as mentioned the company specs the appliance for a 4 inch to 3 inch reducer for the forced vent model.

Steve
 
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Old 07-27-09, 12:50 PM
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That chart that FURD gave you should answer your question.

......Any nipple or other piece that is smaller in the system and then goes larger again is to be rated from the reduced piece onward. For instance, if a 1 inch pipe runs 30' then drops to 1/2" for 6" and then someone installed 3/4" pipe and ran an additional 40 feet, the 40 feet of 3/4" pipe would all be considered 1/2". You will never get more gas through the 3/4" pipe than will pass thorough the 1/2" nipple........
 
  #14  
Old 07-27-09, 01:05 PM
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Ahh, I didnt catch that. Thanks for highlighting it!

OK, seems as though I definitely need to replace the T with a properly sized one. I feel good about this approach.

Now I just need to work with utility to ensure whole house capacity.
 
  #15  
Old 07-27-09, 01:11 PM
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I got news back from the utility company as well. I had queried them regarding flow rates, etc., specific to my gas line. My meter reads 275 CFH and 5psi.

Hopefully this helps other folks in similar positions:

The meter rating of 275 cfh means that there is a 1/2" water column (WC) pressure drop at the outlet side of the meter with a load of 275 cfh. The meter is capable of delivering much greater that 275 cfh except that, as the load increases the pressure drop will also increase.

At MGE, the regulator outlet pressure is set at 8" WC. With a 275 cfh load, the pressure at the outlet of the meter would then be 7.5 " WC

Most tankless water heaters we have seen require a minimum inlet pressure of 6" WC which leaves a fairly good margin for error. (The Noritz unit I specced requires minimum of 5 inches, so even better).

There can occasionally be gas pressure problems with tankless water heaters when the gas piping is too long or improperly sized which causes a pressure drop at the appliance, but we have found that homes with a typical heating load and a tankless water heater operate without any problems with a meter rated at 275 cfh.

:-)

So now all I have to do is have someone stub out the main line with a 3/4 inch pipe and I am good to go. Thanks everyone for helping me through this.
 
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Old 07-27-09, 01:49 PM
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I know this is late..and you already have your answer...but the way this was explained to me once back in the day was as follows.

Imagine a water tank with a 1" hole in the bottom..no restriction on output flow or makeup air/fluid. The column/stream of water out of that hole will be a constant 1" in diameter (we disregard and water column height numbers in this..assume the column above the hole stays constant, no friction or anything else either) and a constant pressure and volume.

Now, restrict that hole to 1/2" for 1' (2"or 2'...doesn't really matter)..the water flow will be 1/2" in diameter with constant volume and pressure.

Now upsize back to 1"...the 1/2" flow will just fall straight down the middle of the pipe...so no change.

As I said..it was just the way it was explained..not sure by who. Maybe one of my Navy instructors...or more likely a crusty old BTC on one of my ships.
 
  #17  
Old 07-27-09, 02:01 PM
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That's an illustrative example. I thought about that as well, as I do lots of plumbing for my brewery, but wasn't sure whether natural gas compressed more easily, etc. Plus an engineer friend of mine thought the level of loss might be related to length of restricted flow...Either way sounds like I am good to go with the new 3/4 fitting and available gas supply.

Thanks!
 
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