Plumbing - NPT vs straight thread

[Donald]

New water heater has NPT pipe nipples to attach hot & cold. All the flex and corrugated lines at HD and Lowes ars straight thread female (with gasket) at each end.

Looked for some type of adapter female NPT to make straight thread. Could not find one. Asked. No luck.

What am I missing.

 

[CDX825]

They just thread on and the gasket seals them. I'm not crazy about them but they work and I have used them many times in the past.

 

[Donald]

They just thread on and the gasket seals them. I'm not crazy about them but they work and I have used them many times in the past.

Are you saying you can screw a straight thread female (with gasket) onto a NPT male?

 

[John Galt]

NPT is a tapered threaded joint used for hard walled pipe - usually “iron” and sometimes brass. The seal is caused by the threads (with an appropriate sealing compound in this instance) filling in the space between the two threaded surfaces as it is tightened.

The flex pipe uses a tubing flare type straight thread and as you mentioned the seal is sometimes done by a gasket. They are not compatible or interchangeable in any way. Many will want to put Teflon tape on a flare tubing connection thinking it makes it leakproof and that is almost a sure way to create a leak and if an inspection is done, Teflon tape on a tubing flare connection will almost assuredly cause a problem.

The NPT pipe to flare fitting adapter will usually be made of brass. The pipe end will usually be male NPT and the tubing side will be called a flare fitting. Here is an example, make sure to get one allowed for potable water service:

Amazon.com

 

[Rogueman2016s]

New water heater has NPT pipe nipples to attach hot & cold. All the flex and corrugated lines at HD and Lowes ars straight thread female (with gasket) at each end.

Looked for some type of adapter female NPT to make straight thread. Could not find one. Asked. No luck.

What am I missing.

just like a garden hose the water tight connection is done at the washer and mating of the two surfaces

 

[CDX825]

Are you saying you can screw a straight thread female (with gasket) onto a NPT male?

Those ones for water heaters you can. Just screw them on until they are snug.

 

[Donald]

NPT is a tapered threaded joint used for hard walled pipe - usually “iron” and sometimes brass. The seal is caused by the threads (with an appropriate sealing compound in this instance) filling in the space between the two threaded surfaces as it is tightened.

The flex pipe uses a tubing flare type straight thread and as you mentioned the seal is sometimes done by a gasket. They are not compatible or interchangeable in any way. Many will want to put Teflon tape on a flare tubing connection thinking it makes it leakproof and that is almost a sure way to create a leak and if an inspection is done, Teflon tape on a tubing flare connection will almost assuredly cause a problem.

The NPT pipe to flare fitting adapter will usually be made of brass. The pipe end will usually be male NPT and the tubing side will be called a flare fitting. Here is an example, make sure to get one allowed for potable water service:

I don't think it's flare. This is the end of the hot water heater SS flex line. How do they expect this to connect to a 3/4" NPT.

 

[oilcandave]

Just like one has said it is like a hose connection. Make sure the nipple it screws on to is flat and square.

 

[burbguy82]

The seal is caused by the threads (with an appropriate sealing compound in this instance) filling in the space between the two threaded surfaces as it is tightened.

No, the seal is caused by the wearing of the threads together, the "sealant" is not a sealant, but a lubricant. Yes, some pipe dop says "sealant" but it is not. Teflon is not a sealer, it is a lubricant. Common misconception.

teflon tape on a tubing flare connection will almost assuredly cause a problem.

Correct, but teflon paste or "pipe dope" will not. I use that or oil of whatever kind.

I don't think it's flare.

it is not, proceed with the connectors to the heater, do not muscle on, tight, but not gorilla tight

 

[y_p_w]

Just as an aside (since some are looking here who might know), how big a deal is it that brass fittings be rated for use with natural gas. I'm kind of wondering about the brass hex nipple I got that's 1/2" MIP on both ends and went into a dryer gas line. I didn't see anything on the package that indicated that it was rated for natural gas.

If it's really that important, I may need to re-do it with a gas rated one. I suppose this one would work.

1/2 in. MIP, Both Ends - BrassCraft

From the industry’s most common brass fittings like compression, flare, pipe and hose barb to customer specialty engineered products, Brasscraft

www.brasscraft.com

https://d2cl5to9q9oami.cloudfront.net/doc/300.224_PHN_Series_Hex_Nipple__MIP_x_MIP_SUB.pdf

Use: For use with water, grease, fuels, LP and natural gas, refrigeration, instrumentation, and hydraulic systems in low and medium pressure pipe line connections. Use in combination with brass, copper, and iron pipe.​

There's also the issue of these tapered connections. I suppose the idea it to do it by feel where it needs to be tight enough to properly seal, but not so tight that the threads are damaged.

 

[Rogueman2016s]

To save yourself headaches since there’s such lousy plumbing fittings coming from overseas out of spec dies cheap metals etc. get yourself a good pipe dope safe on plastics and brush a thin… thin coating on the lip/face of the mating surfaces. Something like the pipe dope pictured below. It will help in the future with disassembly and allow your connection to go together with less torque and effort. It’s ok if some dope gets into the threads but not going to aid much especially in a garden hose style connection where the seal is created at the washers mating area unless there’s some slop in the threads then the dope can help a bit there. I always use pipe dope anywhere there’s a threaded or compression connection even around ferrules.

 

[burbguy82]

wondering about the brass hex nipple I got that's 1/2" MIP on both ends and went into a dryer gas line

the nipple you have listed is not for flare.....just pointing that out. It is nothing more than a brass nipple. Some call them apliance nipples, or service nipples, as opposed to just a nipple.........

Brass can be used for gas. Brass, stainless or black iron, but not galvanized

 

[y_p_w]

the nipple you have listed is not for flare.....just pointing that out. It is nothing more than a brass nipple. Some call them apliance nipples, or service nipples, as opposed to just a nipple.........

Brass can be used for gas. Brass, stainless or black iron, but not galvanized

I get it. It's 1/2" MIP on both sides, but after I installed it I saw something claiming that it wasn't for use with natural gas. I don't understand enough to know the practical difference between what Brasscraft sells as suitable for natural gas and what HD sells as a suitable for water, oil, or air. I didn't really look over the package when I got it.

This is what I installed:

Everbilt 1/2 in. x Close MIP Brass Hex Nipple Fitting 802279 - The Home Depot

Brass pipe nipples are made from solid, lead free brass stock and have male tapered pipe threads on both ends. Brass is very durable and resists corrosion. The fitting diameter is measured using the outer

www.homedepot.com

 

[Rogueman2016s]

BrassCraft 1/2 in. MIP x 1/2 in. MIP x 36 in. Gas Connector (1/2 in. OD) w/Safety+Plus2 Thermal Excess Flow Valve (71,100 BTU) CSSD64TE-36 X - The Home Depot

BrassCrafts Safety+PLUS2 gas connector combines patented excess flow technology and thermal heat activation with the advanced corrosion resistance, to provide a safer gas connector for your home and family.

www.homedepot.com

Swipe through the photos it will show you what’s needed for a gas connection. It’s a cone/bevel shaped fitting at one end

 

[Donald]

To save yourself headaches since there’s such lousy plumbing fittings coming from overseas out of spec dies cheap metals etc. get yourself a good pipe dope safe on plastics and brush a thin… thin coating on the lip/face of the mating surfaces. Something like the pipe dope pictured below. It will help in the future with disassembly and allow your connection to go together with less torque and effort. It’s ok if some dope gets into the threads but not going to aid much especially in a garden hose style connection where the seal is created at the washers mating area unless there’s some slop in the threads then the dope can help a bit there. I always use pipe dope anywhere there’s a threaded or compression connection even around ferrules.

View attachment 212956

View attachment 212957

The thread was about NPT pipe nipple on water heater and the SS braided supply lines that are FIP.

 

[Trav]

Go to a real plumbing supply shop and get one of these or something similar.

https://www.amazon.com/Brass-Adapter-Female-Thread-Fitting/dp/B07SKTYMTX

 

[SHOZ]

I've had that gasketed type connection for my water heater for decades. Easy peasy.

I actually have a corrugated copper tube.

 

[Donald]

I've had that gasketed type connection for my water heater for decades. Easy peasy.

I actually have a corrugated copper tube.

My guess is the nipple on top of your water heater was straight thread. So the gasketed supply line works. The nipple on my water heater is NPT. Tapered.

 

[mk378]

As others said, straight pipe thread has the same thread pitch and nominal diameter as tapered. It will engage with NPT and tighten mechanically, though the threads will not be watertight, even if sealant is used (so it is pointless to apply sealant).

That is not a problem since the gasket makes the seal.

 

[SHOZ]

My guess is the nipple on top of your water heater was straight thread. So the gasketed supply line works. The nipple on my water heater is NPT. Tapered.

Actually I have this going to a 3/4" NPT nipple on a tee on top the out and in and to 3/4" PVC male connector. They fit NPT or straight threads because the gasket does the sealing.

Just like your garden hose does though it has garden hose threads.