I'm considering purchasing a Cummins Onan RS 20A standby generaor to be used on natural gas. The gas pressure requirement per the manual is 5-11 inches w.c. 240 mbtus per hour. I'm in a low pressure natural gas system where the pressure varies between 4 and 7 inches w.c. Seems like it should work most of the time.

However, will the generator work properly if the pressure drops to 4 inches w.c.? Can i just increase the size of the gas pipe to compensate for the lower pressure.

Incresing the pipe size does nothing to change the pressure, however, the requirements are not always "set in stone", However you do need to size the gas ine according to established rules to ensure that it is adequate.

Welcome to the world of genset. The unit you are considering is an excellent natural gas generator. The unit must be mounted on a concrete pad. Startup on the 20KW could easily draw as much natural gas as all of your gas appliances in the house (dryer, stove, water heater). Its more then the pressure, its the size of your delivery pipe and Meter/Regulator. Your local gas supplier is where I would begin. I believe, the Onan-Cummins 20A has to be installed by a qualified company representative and start-up with the same rep to obtain the 5 year warranty. Once a year the oil and filter is changed under the program.

Also consider the automatic switch unit and indoor operating meter. The indoor meter allows you to see what your genset is doing and if there are any faults.

Best Wishes

Thanks for your input. I have checked with the gas company and all they need to do is change the meter for the required volume of MBTUs coming into the house. They also said the gas pressure normally runs higher than 4 w.c. inches, however they can not guarentee more than 4 w.c. inches. So what happens to the generator when it's in the 4 to 5 w.c. inch zone? Will it not start or stall?

i would never advise someone about whether a gas appliance would function properly or not without enough gas pressure.
contact the manufacturer for that information. Cummins Onan...

And under piped system will draw all the natural gas line, depriving the other appliances, until the start-up is complete. Only then can the line recover to provide natural gas to the original appliances. Rather then tapping into an existing line in the house, we added a "T" at the meter and added a seperate line directly to the Onan-Cummings Generator. The meter/regulator was increased to handle the extra demand. A 20 kw generator will easily handle 100 Amp service. It will handle an air conditioning system and all of your electric appliances without a problem.

I'm hoping that you are working with an Onan-Cummings Certified Installer and installing a automatic cut in/ cut out switch.

Best Wishes

The plan is to use a professional Cummins installer and master plumber. The pipe size requirement for my situation is 1 1/4 a separate run from the meter to gen, approximately 70 feet to get the required MBTUs even at a lower w.c. pressure. I just don't know if the gen would function at 4 w.c inches even if it has sufficient supply of gas. Maybe I need a low volume gas booster to raise the pressure. Not sure but would like to know before committing.

A booster, if there even is such a thing, could not "create" more gas than is available.

Most gas-run appliances, if the pressure was a little low, just won't give the full output, because the combustion will be weak. At some point, it would probably fail to fire the pistons.

We also used a licensed electrican to install the Automatic Switch Unit and connect the wire from the breaker box to the Break a Way (auto). The Onan-Cummins Installer made the conntections from the AutoSwitch to the panel on the generator. They filled the oil and did the first start-up. The same unit does a test run every Sunday about 3:15 to lube the o-rings and keep the motor lube'd. The inside meter, lets you know what is happening on the outside.

Best Wishes

quote; I just don't know if the gen would function at 4 w.c inches even if it has sufficient supply of gas. Maybe I need a low volume gas booster to raise the pressure

1. If the pressure is too low, there will NOT be a sufficient supply of gas because the pressure will drop to zero and the unit would shut down momentarily, until the pressure was restored.
2. What is a "low volume gas booster"? The only way to raise pressure is with a pump and I do not know of any pump that will work with gas.
3. The reality is that the gas company's mains run at many POUNDS of pressure, except is some cities such as Chicago. The regulator at your houss is adjusted to give you the low pressure your appliances need. In certain situations it is adjusted to give 2 psi into the house, then the individual applances have ""high to low pressure" regulators.

"And under piped system will draw all the natural gas line, depriving the other appliances, until the start-up is complete. Only then can the line recover to provide natural gas to the original appliances. Rather then tapping into an existing line in the house, we added a "T" at the meter and added a seperate line directly to the Onan-Cummings Generator. The meter/regulator was increased to handle the extra demand"

Wheelchair......I disagree with this statement. An improperly piped gas system would never recover to allow the other appliance to operate correctly. If the unit is rated for 240000 btus, that's the most it will ever use, whether on start-up or during operation.

There is such a thing as a low volume gas booster. See the following link [www.etterengineering.com]

Also, many generators in my area have been installed in the same type of natural gas system without problem. I think the gas company is being conservative and causious with only guaranteeing 4 w.c. inches. The gas pressure typically runs higher than 5, so i should be good unless there is an extremely high demand in my area. And thinking about it, generators will kick on when the power goes out and that's when natural gas demand will be low because many heating systems in my area are forced air/ electric blower driven.

Paul48. That is exactly why we had the gas engineers out to inspect and evaluate the gas line from the street (600 ft) to the meter. It took 3 guys with their college books to determine exactly what we need to meet a demand of 220,000 btu from the generator and the other gas applience in the house. I believe 400,000 btu's was the magic number for us. We had around 100 ft between the gas meter in the back left corner of the house to the front right corner where the generator pad sat. After the meter/regulator was upgraded an 1-1/4 black pipe was added. It too entered the house basement and made 2 bends before it was reduced to 1 inch for the last 30 ft, out of the other side of the basement, underground where the pipe exited the ground and another angle to the flexline (24 inch) to the generator.
The entire installation was engineered after the 1st Generator failed on the first try, as the installer tried to circumvent the entire installation by not installing the proper KW sized generator (Onan-Cummins 12kw). The Onan-Cummins dealer would not take back the generator and it was finally traded for a Generac 20kw generator. It was a bid job and the electric contractor thought he had enough training to do the piping, the installation and the startup. Boy was he wrong.
The town inspector came down on him for not pulling a permit or inspection until we complained. This ended up bringing out the city inspector, the utility engineers and the bid contractor. The electrican had to return a 100 ft of 1 inch yellow flex stainless steel pipe to where he purchased it. The job was bidded in December. The 1st Generator was delivered on Jan 8th in snowy freezing weather. The sub-contractor dug the foundation for a 3 X 6 concrete pad right over the main electric and tv cable drop service line. He cut the TV Cable before he made it to the electric service. Never took the time to have the ground utility sevice guys mark over the property before he dug, 3 ft from the house and pour'd his pad from several bags of concrete. After the pad cured he transferred the Generator and began the process of having it wired and the gas service installed. The General Contractor claimed that he was not aware of what was going on with the subcontractor and that's when the dung hit the fan. The subcontrator had to return and remove and repackage the old generator. Utility company had the subcontractor dig up the concrete pad in early March clay and mud and move the concrete pad off their electric service line. The pad was restablized before the new Generac 20kw was installed and wired, at no additional costs to us. The sub-contractor contacted a licensed plumber bid (site unseen) to extend a gas line in the basement, under the kitchen store to the generator. When the owner of the plumbing company came out to see the job, he pulled his guy and walked off the job. The sub-contractor then had to rebid the job (properly) before the 100 ft of gas pipe was installed to the flexline. The job was finally completed, tested, inspected by early June of that year. Were there headaches?? yes. But in the end, it was done right and we are the pride of the neighorhood each time the power goes out for more then 12 hours. Doing the job right might cost more but its worth it when you really need 100 amps.
Best Wishes

I just can't seem to get gas pipe sizing right. No matter which method I use....Longest Length or Branch, I come up with 1.25" feeding your generator. What am I doing wrong?

quote; An improperly piped gas system would never recover to allow the other appliance to operate correctly.

Correct, The laws for a "gas" state that the pressure will be the same throughout the system, so, assuming the pipe sizes are correct, one appliance cannot "draw" all the gas, all it can do is reduce the pressure "everywhere". If the "culprit" shuts down because of insufficient gas, THEN the other appliances can function normally again.

quote; There is such a thing as a low volume gas booster

And do you have any idea what that thing would cost. Besides, it is powered with an electric pump. Therefore, you don't have electricity which is why you need the generator, but the generator needs the "gas pump", which requires the generator to provide electicity. Sort of a "Catch 22" isn't it? If you install it and have a problem, have the gas utility convert it to a 2psi system with a "regulator" for the existing piping near the meter, and another one AT the generator.

quote; It took 3 guys with their college books to determine exactly what we need to meet a demand of 220,000 btu from the generator and the other gas applience in the house.

And people wonder why we deprecate "engineers". It should have been a simple 30 minute job to make the computation. ALl the other "gyrations", were caused by incompetence on several different levels, but WHY incompetent people were involved is subject to interpretation. One part of the installation is "questionable". Once you determine the pipe size, that size is for the entire lenght, NOT just 1 1/4" SO FAR, and then convert to 1" the rest of the way.



Edited 2 times.

quote; No matter which method I use....Longest Length or Branch, I come up with 1.25" feeding your generator. What am I doing wrong?

What makes you thing YOU are wrong? The sizing tables use the "total developed length" from the meter to the appliance, or the furthest fixture on THAT run, whichever is greater. That same "distance column" is used for ALL branch sizing calculations also, regardless of their individual lengths.

I was wrong before about sizing, and just assumed I was still doing something wrong.I even went back and watched an online tutorial about pipe sizing. But assumed that with all the folks involved with wheelchairs install, they would have gotten it right.

Paul, it took 6 months from the beginning of the project until the end with all the fuss n errors. For our purposes, there was not much for us to say our insurance company paid for the generator, and its installation. The contractor, tried to cheat the insurance company by undersizing the original specification and the quasi sub-contractors went along with the plan, until the orignal job was completed and we requested the city to inspect the job, before the subcontractors left the job. Once we were able to have the code inspector formally evaluate and issue his report, were in the position to contact the insurance company and present them with the written report.
The contractor and subcontractor all lost big money on this venture as they had to remove everything and start from the beginning... this time doing everything right and according to code that was reinspected and passed.

Best Wishes

I don't know what the repercussions of down-sizing that last 30 ft to one inch would be, but I would find out.The gas charts for 220000 btus at 100 ft call for 1.25".

Wheelchair......I consulted the most knowledgable person I know re: gas, and he says it is fine. He did say it is always a good idea to have a fairly large manifold in the last 6-8 ft to help with the surge in demand when the unit first fires.

I own a Cummins-Onan 30kw full house generator with an ATS and it works excellent...luckily, I only have the generator using natural gas and I have the meter 6 feet from the unit...I recommend you get a surge switch (not sure about the name) installed on your air conditioner to prevent the surge...My a.c. repairman installed it for $150.00..Now there is little to none when the a.c. kicks on..Also, I highly recommend synthetic lube oil for the oil changes since the unit only self tests every 1 or 2 weeks...This will insure proper lubrication of the engine parts..In closing, dont forget to run the unit under load for at least 30 minutes once a month so the generator will get used ( This process is easy...locate your house Main Electrical panel box where the ATS is installed and turn off the main breaker to the house....the ATS will then sense there is no electricity and take over from there...after 30 minutes, turn the Main breaker back on and everything else will be automatic)...I love my unit..I'm from Louisiana and several times we have gone weeks without electricity due to a hurricane and old Genny does the trick...good luck

Nothing will happen if you increase the pipe size as the gas pressure will be coming low from the source. I was also facing low gas pressure problem at my home than my friend suggest me to use IDI Gas Amplifier the Amplifiers can produce up to 250 psi with flows up to 28 SCFM (6881 Liters/M). It fixed my problem. Have you tried such amplifier? have a check on it.

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