Heater supply gas pipe confusion

Bradenwb

Member
Jul 13, 2020
22
Minnesota
I have a Jandy Laars LX 400k BTU heater that just died. It was installed in 2004. I was looking to replace it with the Raypak 406A equivalent but my pool guy is insisting that I go with the 336k BTU version because the supply gas line is a 5/8" diameter.

Reading through the Jandy and Raypak manuals, they both say 1 1/4" pipes. Searching on TFP, reading a lot of the same. This heater lasted 18 years and cranked out the heat. I am confused how this is even possible if the lines were too small? Is it possible to have the gas company increase pressure or something so a 400k works on 5/8 pipe? Is it possible with a high pressure meter?
 
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JamesW

TFP Expert
Mar 2, 2011
28,291
Do you have a regulator near the heater?

5/8" might be acceptable if the gas pressure is 2 psi to the heater and stepped down to inlet pressure with a regulator.

How far is the heater from the meter?

What is the meter rating for cubic feet per hour?
 
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Bradenwb

Member
Jul 13, 2020
22
Minnesota
Do you have a regulator near the heater?

5/8" might be acceptable if the gas pressure is 2 psi to the heater and stepped down to inlet pressure with a regulator.

How far is the heater from the meter?

What is the meter rating for cubic feet per hour?
I believe there is a regulator, see picture. The line on the left is from the meter which is maybe 25 feet away. It is the 5/8 line. The line on the right goes to the fireplace in the pool house - it is 1/2 diameter. I believe the thing in between is a regulator?

How can I tell meter rating? Is it on the meter somewhere or do I need to call the gas company?
 

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JamesW

TFP Expert
Mar 2, 2011
28,291
Assuming the pressure is 2 psi, the charts indicate that a 1/2" line is ok up to 50 feet total equivalent length for 400,000 btu/hr.

What is the btu/hr rating for the fireplace?

The meter should have a model number and a rating of CFH.
 
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JamesW

TFP Expert
Mar 2, 2011
28,291
Assuming a 2 psi pressure and a 50 ft total equivalent length and a 1/2" pipe, the line can carry 673 cubic feet per hour or 673,000 btu/hr.

If the heater is 400,000 btu/hr, the fireplace can be up to 273,000 btu/hr.
 

JamesW

TFP Expert
Mar 2, 2011
28,291
Almost definitely the pressure is 2 psi to the regulator and then it steps down to heater inlet pressure.

So, I think that the supply is probably fine for the heater assuming the fireplace will not run at the same time or if it will use less than 273,000 btu/hr.

You should have a qualified licensed gas contractor or HVAC service technician check the supply and verify the capacity before choosing the heater size and then have the same technician connect the gas and test run the heater to verify that it works correctly.
 

Bradenwb

Member
Jul 13, 2020
22
Minnesota

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Bradenwb

Member
Jul 13, 2020
22
Minnesota
So if I am reading this right, my meter is capable of 425 CFH with 1/2 pipe? So sounds like I have the pressure and meter to use a 400k BTU heater with 5/8? The fireplace is only ran in winter when the pool is closed.

Next step would be to get someone to come test pressure to verify before I order heater? I am assuming that if I have the gas pressure for 400k BTU's, I want to do that over the next step down - correct?
 

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JamesW

TFP Expert
Mar 2, 2011
28,291
The meter is good for 425 cubic feet per hour or 425,000 btu/hr.

If you have other appliances that will run at the same time, you have to add that to the total load.

The tubing should be fine for 400,000 btu/hr at 40 feet of tubing length assuming a 2 psi line pressure.

Have a qualified licensed gas contractor or HVAC service technician check the supply and verify the capacity before choosing the heater size and then have the same technician connect the gas and test run the heater to verify that it works correctly.

Table 5 - Maximum Capacity* for Copper Tube in an Elevated Pressure System of 2 psig and Pressure Drop of 1.0 psig, CFH.**
1633023031529.png
https://www.copper.org/applications/fuelgas/specs/lng/table05.html
 

Bradenwb

Member
Jul 13, 2020
22
Minnesota
James, thank you for taking the time to respond.

Gas company just confirmed they are delivering 2 PSI to the house and with my meter I can go up to 950 BTU at one time. The only other thing on that line for the pool heater is a fireplace that is not ran during pool season so I should be good to go and order the 400k.

One question for you - Any other reason I would want the 366k BTU over the 400k? I believe I read, buy as big as you can afford?
 

JamesW

TFP Expert
Mar 2, 2011
28,291
Any other reason I would want the 366k BTU over the 400k?
For a hot tub, the exit temperature can sometimes be excessive with a 400,000 btu/hr heater.

For example, at the minimum of 40 gpm, the temperature rise is 16.8 degrees.

If the heater input temperature is 103 degrees, the heater exit temperature would be about 120 degrees, which can be too high for someone directly in front of the jets/returns.

For a pool, the input might be 86 degrees and the exit would be 103 degrees, which is not dangerous and no one is sitting in front of the return anyway.

If you increased the flow to 80 gpm, the exit temperature would be (103 + 8.4) = 111.4 degrees.

A 400,000 btu/hr heater can heat faster, which can matter if you only heat occasionally.

If you keep the pool heated, then the speed is less important.
Looking at the picture above, do you know why I have a regulator right before the pool heater?
The heater needs a gas supply pressure of about 7" of water column.

So, you need a regulator to step down the pressure from 2 psi to 7" w.c.

2 psi = 55.4" w.c.

You have a 2-stage supply with a high and low stage.

1633028993662.png
In a single stage supply, the pressure is 0.5 psi (about 13.85" w.c) (Typical pressure is between 7" and 14" w.c).

Due to the lower pressure, the pipe has to be bigger to supply the same amount of gas.

For a low pressure single stage supply, there is no regulator at the heater.
1633028946754.png
1 psi is 27.7076” w.c or 2.30897 feet of water.
 
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JamesW

TFP Expert
Mar 2, 2011
28,291
The manual will specify the pipe size and the heater gas valve inlet minimum and maximum gas pressure.


A minimum of 6 in. WC and a maximum of 10.5 in. WC upstream pressure under load and no-load conditions must be provided for natural gas.

The Raypak manual shows the low pressure gas pipe sizing chart, but it does not show a 2 psi chart.

In any case, a service tech who is qualified in gas should be able to verify the supply being adequate for the heater.

1633031381507.png
 
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