Real world gas line size for 400k btu heater

[Sws]

Hi Everyone,

I was planning on doing a 250k or 300k pool heater that needs roughly 90 ft of mdpe plus risers. I got 1 1/4 mdpe BUT I'm now considering a 400k heater. Anyone here have a similar set up? I'm curious what kind of pressure drop you're seeing from the meter to the heater with 100ft of 1 1/4 and 400k.

FWIW, we have 50psi in the street and 7"wc running after the meter. It's a 1000 meter and 2 inch header. Total load with the 300k is 1mil. If I go to the 400k then load is 1.1 mil. I cant see ever using more than 700k at once.

It's a small 11k gallon semi inground pool that we generally use in the AM so my goal is to occasionally get it up 4-8 degrees before I go in.

 

[JamesW]

I got 1 1/4 mdpe BUT I'm now considering a 400k heater.

I'm curious what kind of pressure drop you're seeing from the meter to the heater with 100ft of 1 1/4 and 400k.

409 at 80 feet is 0.5" drop using Polyethylene Plastic Pipe.

So, at 100 feet, using Polyethylene Plastic Pipe, the drop will be 0.625" w.c.

Max load at 100 feet and a 0.5" w.c drop is 362 cubic feet per hour or 362,000 btu/hr.

Most design is specified at a max pressure drop of 0.5".

So, for a 409,000 btu/hr heater, you can go up to 80 feet using Polyethylene Plastic Pipe.

The Mastertemp Manual allows 100 feet of SCH 40 black iron pipe at 400,000 btu/hr.

The Raypak allows 95 feet of schedule 40 black iron pipe at 400,000 btu/hr.


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https://www.pentair.com/content/dam/extranet/nam/pentair-pool/residential/heaters/mastertemp/manual/mastertemp-heater-manual-english.pdf

https://s3.amazonaws.com/AWSProd/sites/raypakcom/documents/241236.pdf

Pipe Sizing for Gas Connection.

These capacities shown below based on using SCH 40 black iron pipe.

For capacities using other materials, consult local code.

 

[JamesW]

Poly pipe has an ID of 1.328" and iron pipe has an ID of 1.380", which is why the iron pipe is rated higher than the poly pipe.

If you can increase the pressure after the main regulator to 8" w.c, then it might be fine.

There are 3 important questions for the design:

1) Will it work? Probably.
2) Does it meet code? Maybe, depending on the local code and if the inspector approves.
3) Does it meet manufacturer instructions? Since they do not specify the length of poly pipe, you would have to get their approval.
4) Will the manufacturer raise an objection if a warranty claim is submitted?

 

[JamesW]

In some code, the pressure drop is not specified to a specific number.

G2413.6 (402.6) Allowable Pressure Drop.
The design pressure loss in any piping system under maximum probable flow conditions, from the point of delivery to the inlet connection of the appliance, shall be such that the supply pressure at the appliance is greater than or equal to the minimum pressure required by the appliance.

 

[JamesW]

For pressure at 8" to 11" w.c, you can have up to a 3" w.c pressure drop.

At 11" to 13.8" (inches of water column, you can have up to 6" w.c loss for metallic pipe.

Did you check with your licensed gas contractor?

 

[Sws]

In some code, the pressure drop is not specified to a specific number.


G2413.6 (402.6) Allowable Pressure Drop
The design pressure loss in any piping system under maximum probable flow conditions, from the point of delivery to the inlet connection of the appliance, shall be such that the supply pressure at the appliance is greater than or equal to the minimum pressure required by the appliance.

Thanks,

I can't see anyone besides me taking pressures at the meter and heater so I'll be the only one who knows. I can get the reg bumped to 9" max if I need to. .625 drop doesn't seem too far off of the .5 so I would imagine that it would be fine in the real world.

If I even have 8 running at the meter, it's still over 7 at the heater when it operates as low as 4.

I'm not sure if the extra 100k BTUs is worth figuring it out though. I'm probably best off playing it safe with the 300k.

 

[Sws]

The utility here won't go to 14 unless the minimum input of an appliance is over 7" I'd also have to put a step down reg on the house if I managed to pull it off.

 

[JamesW]

If I even have 8 running at the meter, it's still over 7 at the heater when it operates as low as 4.

It's a 1000 meter and 2 inch header.

Meters are sized based on allowable pressure drop through the meter.

Some gas companies use the 0.5" w.c pressure drop rating, which is conservative and some use the 1" pressure drop rating.

If your pressure before the meter is 7" and you have a 1" drop, then your pressure out of the meter is 6" and then you have a 0.625" w.c pressure drop through the gas line, which leaves you 5.375" w.c.

Assuming a 0.5" pressure drop through the meter, you will have 6.5" w.c out of the meter and then a 0.625" w.c loss through the line, which is 5.875" w.c at the heater inlet.

 

[Sws]

Meters are sized based on allowable pressure drop through the meter.

Some gas companies use the 0.5" w.c pressure drop rating, which is conservative and some use the 1" pressure drop rating.

If your pressure before the meter is 7" and you have a 1" drop, then your pressure out of the meter is 6" and then you have a 0.625" w.c pressure drop through the gas line, which leaves you 5.375" w.c.

Assuming a 0.5" pressure drop through the meter, you will have 6.5" w.c out of the meter and then a 0.625" w.c loss through the line, which is 5.876" w.c at the heater inlet.

View attachment 611424


View attachment 611425

Thanks,

I have an Al1000 with roughly 60psi in the street, and 7in WC running after the meter. If I have 7 after the meter and a .625 loss from the length of mdpe, then I assume I'd be at 6.375 at the inlet of a 400k btu heater. Does that sound right?

 

[JamesW]

You probably have 7" w.c after the main regulator.

The pressure after the meter depends on how much gas is flowing.

At 0 flow, there is no pressure drop and you will have 7" after the meter.

At 1,000 Cubic feet per hour, you have a pressure drop of about 0.5" w.c, which leaves 6.5" after the meter.

The formula is y=0.0005X^2, where Y is the loss in milliinches and x is the flow in cubic feet of gas per hour.

At 500 cubic feet per hour, the loss is 0.125" w.c and the pressure after the meter is 6.875" w.c.

y=0.0005X^2, y from 0 to 500 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

I have an Al1000 with roughly 60psi in the street

60 psi is high for street pressure.

Did you get this directly from company information?

If I have 7 after the meter and a .625 loss from the length of mdpe, then I assume I'd be at 6.375 at the inlet of a 400k btu heater.

At full meter capacity, the loss is 0.5" w.c.

7" = 0.5" = 6.5" w.c. after the meter.

6.5 - 0.625 = 5.875" w.c at the heater inlet.

Note: Get all plans approved by a licensed gas contractor before deciding what to do.

 

[Sws]

You probably have 7" w.c after the main regulator.

The pressure after the meter depends on how much gas is flowing.

At 0 flow, there is no pressure drop and you will have 7" after the meter.

At 1,000 Cubic feet per hour, you have a pressure drop of about 0.5" w.c, which leaves 6.5" after the meter.

The formula is y=0.0005X^2, where Y is the loss in milliinches and x is the flow in cubic feet of gas per hour.

At 500 cubic feet per hour, the loss is 0.125" w.c and the pressure after the meter is 6.875" w.c.

View attachment 611491

y=0.0005X^2, y from 0 to 500 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

60 psi is high for street pressure.

Did you get this directly from company information?

At full meter capacity, the loss is 0.5" w.c.

7" = 0.5" = 6.5" w.c. after the meter.

6.5 - 0.625 = 5.875" w.c at the heater inlet.

Note: Get all plans approved by a licensed gas contractor before deciding what to do.

Thanks,

It's definitely 60 psi in the street and I currently have 7" downstream of the meter under a full load (about 700k btu) so the meter loss and regulator droop are already accounted for.

I'm really just trying to see the estimated pressure drop from the outlet of the meter to the inlet of a 400k heater. Still seems like I'll be 6.375ish at the heater if I'm starting at 7" after the meter.