Gas lines

[Dentaltec]

1960's track home has a pool and upgraded gas meter to accommodate add on of pool heater and back yard hardline BBQ and fire pit. House currently has 199kbtu tankless water heater now that is attached to an existing 1/2" black pipe home run to the meter. No issues with existing tankless heater but we are upgrading to a more efficient recirculating unit. The installer that came to the house says new heater which has the same BTU rating will necessitate a 3/4" run. When I read the manual it says that if the volume of gas isn't sufficient the heater will protect itself and shut down. Its a significant expense to make a new home run in 3/4" so I am looking for suggestions. Is there a test I can run prior so I know if this is a non issue?

 

[LS6 Tommy]

Input is input. Efficiency ratings make no difference for pipe sizing. It all depends on length of run. Lookieheer:

https://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html


Tommy

 

[Dentaltec]

Input is input. Efficiency ratings make no difference for pipe sizing. It all depends on length of run. Lookieheer:

https://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html


Tommy

So if one heater with the same BTU is working the new one is bound to not have issues?

 

[rlitman]

So if one heater with the same BTU is working the new one is bound to not have issues?

Not necessarily.

 

[Dentaltec]

Not necessarily.

Here I thought I was in the clear, so what do I need to do?

 

[ljhhontx]

The installer is correct, 1/2" pipe will only carry 199 btu worth of gas for 1 foot, the old unit was probably starving and the new unit won't stay lit with that amount of flow. New gas appliances with electronics sense the flow problem and lock out. Check the tables at www.engineeringtoolbox.com/natural-gas-pipe-calculator for proper sizing with length of run. You can also get the amount of pressure drop allowed off the new heater and input that to correct the calculation but it won't make that much difference.

 

[MattT]

So if one heater with the same BTU is working the new one is bound to not have issues?

That depends on why the old one is working. First the older unit may not be sensitive to reduced gas pressure. If that's the case you'll have problems with a replacement which is pressure sensitive.

Second 199Kbtu is a lot of hot water in a warm climate so you may not be running it anywhere near full capacity. Without looking it up I'm guessing 6+ gallons per minute at 120*F. So get out the five gallon bucket and stopwatch and figure how much hot water you actually need. A smaller unit, which will work with your existing gas line, may be an option which would also save money on the unit itself.

Third you might have a 2psi feed and the installer didn't notice it. That would probably be enough for 199K.

 

[LS6 Tommy]

The installer is correct, 1/2" pipe will only carry 199 btu worth of gas for 1 foot, the old unit was probably starving and the new unit won't stay lit with that amount of flow. New gas appliances with electronics sense the flow problem and lock out. Check the tables at www.engineeringtoolbox.com/natural-gas-pipe-calculator for proper sizing with length of run. You can also get the amount of pressure drop allowed off the new heater and input that to correct the calculation but it won't make that much difference.

That depends on why the old one is working. First the older unit may not be sensitive to reduced gas pressure. If that's the case you'll have problems with a replacement which is pressure sensitive.

Second 199Kbtu is a lot of hot water in a warm climate so you may not be running it anywhere near full capacity. Without looking it up I'm guessing 6+ gallons per minute at 120*F. So get out the five gallon bucket and stopwatch and figure how much hot water you actually need. A smaller unit, which will work with your existing gas line, may be an option which would also save money on the unit itself.

Third you might have a 2psi feed and the installer didn't notice it. That would probably be enough for 199K.

X2. You guys beat me to it.

Tommy

 

[Dentaltec]

So thank you for all the great info, I enjoy these sites because I can learn a thing or two. The gas company is coming to check our service and do a test on the line. Its a 11gpm unit so its complete overkill we just want to take showers at the same time with out worrying if the dishwasher or the clothes washer are working at the same time.


https://www.rinnai.us/product/tankless-water-heater/rur199in

 

[MattT]

Its a 11gpm unit

Took a look at the specs and that's max flow, with a 35*F increase in water temperature which isn't near hot enough to suit most folks. I'm guessing you'll need a 60*F temp rise to get the 120*F hot water normal people expect. Which downrates that units flowrate to 6.5gpm. So bear that in mind if you do decide to consider a smaller unit.

 

[pbon]

Has the OP removed the flow restrictors from the 2.5 gpm shower heads? Is he running 2 showers on full hot?

 

[Dentaltec]

Took a look at the specs and that's max flow, with a 35*F increase in water temperature which isn't near hot enough to suit most folks. I'm guessing you'll need a 60*F temp rise to get the 120*F hot water normal people expect. Which downrates that units flowrate to 6.5gpm. So bear that in mind if you do decide to consider a smaller unit.

This is the reason I opted for this unit, and we will hack this unit to get the top temp to 140 as well ( no kids or elderly in the house to scald)

As a betting man I am really hoping not to have to replace the line but I am preparing for the worst.

On the bright side I get $600 tax credit

 

[Dentaltec]

Has the OP removed the flow restrictors from the 2.5 gpm shower heads? Is he running 2 showers on full hot?

We have one shower that has been drilled out and the the other than has multiple heads. We have not been able to run 2 concurrently with the old system which has specs of 7.4gpm with the 45* rise and considered "mid efficiency"

This new unit has a built in recirc pump which will give us an instant on and can be set with wifi to cycle at the times where we will use it most. It also does not require a return line and allows for a simple modification to the system with a cross valve or what ever its called on the farthest faucet in the house. So instead of having a pump running 100% of the time this will save some money but can be programed to fell like it does at peak times.

The great deal I got on this heater on amazon with a special coupon will most likely cost me more in the long run to get it working effectively so not free lunches ever when you so call "Upgrade"

 

[brewchief]

How long is the gas line? What type of new gas line would be used? Depending on the length and type of pipe to be used 3/4" might not be large enough either.

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[Jackfre]

You are describing a Rinnai RUR unit, I think. Rinnai is the only unit that authorizes the use of the cross over valve. That is due to the way they handle an internal by-pass. I am of the opinion that a dedicated recirc line is best. I recently installed a Rinnai RUR with a dedicated recirc line at my daughters home and reduced the wait time from 2.5 min to 10 seconds. Pretty cool. Here in CA energy efficiency is necessary, but since that 5 yr drought, water conversation is more important. The by-pass valve has a smaller flow rate but does work. Do not control your recirc with a Rinnai’s timer control. Use the WiFi accessory with the wireless push button in the kitchen and the wireless motion sensor in the bathroom. It will be better for you and the system.
If your unit has a negative pressure gas valve it will not fault out on low gas pressure unless it is extreme. You just don’t get what you paid for. You don’t list any specifics on the gas line orientation or total btu load. With your shower load I will say that you should increase the gas line size. You are running on vapors with 1/2”.

 

[HoosierBuddy]

I agree with what others have said. Your current line is undersized and it's somewhat amazing it works. It would be really interesting to put an inches water column pressure gauge at your water heater and watch your pressure drop when it kicks on.

My guess is the only thing saving you is it can modulate and never really tries to pull the 199,000 BTU it's rated for.

I have such a gauge on my 150,000 BTU modulating wall boiler and it drops almost a full inch w/c when it comes on....hooked up to 3/4" pipe. For reference...3/4" Pipe will haul abut 2.25 times as much gas as 1/2" pipe at the same pressure drop.

Phil

 

[BD1]

Did you read the manual ? It should have a chart related to gas line size and length
IF it doesn't, contact manufacture or local utility company.


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[Dentaltec]

Gas co is coming on Wednesday to test the line and evaluate before I make final decision.

Paying out the *** right now to repipe the copper based on slab leaks so if I can get away from doing the gas line I will be happy.

 

[Dentaltec]

Update....

Gas company came out and did the test, put in a new meter in to handle up to 630k btu to handle all of our upgrades. Line tested out within spec at the 1/2 as the run was much shorter than I thought only 40 feet.

Thank you to all that shared their wisdom in this thread.

 

[HoosierBuddy]

Update....

Gas company came out and did the test, put in a new meter in to handle up to 630k btu to handle all of our upgrades. Line tested out within spec at the 1/2 as the run was much shorter than I thought only 40 feet.

Thank you to all that shared their wisdom in this thread.

40 feet of 1/2" pipe with 7 inches on the inlet will drop to 3.8 inches at the outlet at 199,000 BTU/Hour.

Something does not compute. What do you mean "Line tested out within spec?"

Phil

 

[MattT]

40 feet of 1/2" pipe with 7 inches on the inlet will drop to 3.8 inches at the outlet at 199,000 BTU/Hour.

Something does not compute. What do you mean "Line tested out within spec?"

Specs for that unit call for 3.5" minimum so it is just barely "within spec" if it's a 7" feed. Still sounds sketchy to me but maybe it'll work??

 

[Dentaltec]

So installer did a pretty clean instal as far as I can tell, hard plumbed everything, one thing I noticed is the T & P and condensation line are tied together and done in coper with no way to remove with out cutting "no unions" Not the end of the world just surprised me he didn't use PVC here.

The new 3/4" supply just ties in to the 1/2" up in the attic with a straight run back to the meter and seems to be working quite well thus far, almost instant on of hot water and so much quieter than the Rhem.

View media item 97754

 

[brewchief]

Manual specifically says not to connect drain and t&p together and to use plastic pipe for the condensate drain.

Hard tell if vent is proper, always see pvc vents on condensing units here.

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[Dentaltec]

Thats frustrating but not sure why it matters. The vent was the old Stainless Steel Concentric Venting from the Rheem that was installed prior. Hope I will not have an issue, thanks for sharing

 

[LS6 Tommy]

So installer did a pretty clean instal as far as I can tell, hard plumbed everything, one thing I noticed is the T & P and condensation line are tied together and done in coper with no way to remove with out cutting "no unions" Not the end of the world just surprised me he didn't use PVC here.


View media item 97754

Was that even inspected? They violated a bunch of codes, get the installer to come back. The T&P cannot be installed into an elbow like that (I'm not even sure how they did that without breaking off the element), it must have it's own drain and the condensate line must have a neutralizer.

Tommy

 

[Jim greengo]

So installer did a pretty clean instal as far as I can tell, hard plumbed everything, one thing I noticed is the T & P and condensation line are tied together and done in coper with no way to remove with out cutting "no unions" Not the end of the world just surprised me he didn't use PVC here.

The new 3/4" supply just ties in to the 1/2" up in the attic with a straight run back to the meter and seems to be working quite well thus far, almost instant on of hot water and so much quieter than the Rhem.

View media item 97754

Did he own a level?

 

[BD1]

Did he own a level?

Not only that, if he would have placed valve in horizontal he would have saved coupling.
AND he could have fitted the riser into unit , plumb level, measure, and add exact size ******. Typical non union installer. I see that all the time . Many installations are have issues and then they call in union contractors to fix their poor workmanship.



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[Jim greengo]

Not only that, if he would have placed valve in horizontal he would have saved coupling.
AND he could have fitted the riser into unit , plumb level, measure, and add exact size ******. Typical non union installer. I see that all the time . Many installations are have issues and then they call in union contractors to fix their poor workmanship.



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Not all non union contractors are hacks.