Consequence of undersized heater gas line

[walbur]

We recently build a spa with a Pentair MasterTemp 400k BTU natural gas heater. To save costs we used an existing gas line that's a little small (1 inch instead of 1.25 to 1.5 in the manual, for a run of about 60-70 ft). Builder said it's fine, because the heater can adapt to the lower gas flow (?) and the only consequence is that the water would not be heated as fast to the heater's potential, which is totally fine for us.

However today our pool guy came and said that because of the undersized pipe, the heater is gonna "make whistling noise" in maybe a year and when that happens the heater would be damaged and need to be replaced, if we don't resize the pipe.

So which of these two is correct?

 

[HermanTX]

Welcome to TFP.
What is the rating of your Gas Meter? There should be a tag on it. See example

 

[walbur]

It reads "MAOP 5 PSI 250 CFH@ 1/2" DIFF."

 

[JoyfulNoise]

If you gas flow runs too lean, then you will get very inefficient heating. Your master temp is a 400kBTU unit and so the air flow into the combustion chamber is expecting a certain amount of gas flow. You are choking that flow off by quite a lot as both your plumbing is inadequate and your gas meter only allows for 250CFH of flow which is roughly 250,000BTUs total heat load.

Do you have the gas appliances in the the home? If so, running the heater with those appliances can put excessive demand on that residential meter which could lead to system pressure drops and appliances shutting off. At the very least, you won’t have a sufficient amount of gas to the heater which can cause the flame to shut off and cause the heater to intermittently fire and shutdown.

I don’t know if whistling will be the problem for the heater but the low gas flow is not good either way. By trying to “save money” your builder will cause you headaches down the road when they are long gone and not around to fix the issue. The gas meter and gas line should have been upgraded to reflect the greater demand of the pool heater as well as any appliances in your home. Not that the pool is installed and the trenches are all covered over, the problem is exponentially harder to fix.

 

[ajw22]

Your meter can only provide 250 CFH while your heater alone needs 400.

Your gas service is undersized.

The gas to air mixture will be off with too little gas for the air setup. The flame will burn hotter then designed and your heater will have problems in a few years. That is how your heater will adapt.

In addition to your heater not producing the heat it was designed for.

If you intended to keep this gas service the builder should have installed a MasterTemp 200 heater sized properly for your gas service.

 

[walbur]

Thanks for the replies. These make total sense to me. To clarify, the "cost saved" is for replacing the (small) gas line we already had in ground before building the spa.

I'll definitely call my gas company to see if they can replace the meter with a bigger one. But can this gas line still be used? For example, with a higher gas pressure to that line? I may eventually replace that line when I get around to redo the pavers on top of it. But that may happen next year. Before then, can I add a new regulator for this line to increase the pressure?

 

[ajw22]

You need to talk to a gas technician for what your options are in your area.

 

[1poolman1]

Thanks for the replies. These make total sense to me. To clarify, the "cost saved" is for replacing the (small) gas line we already had in ground before building the spa.

I'll definitely call my gas company to see if they can replace the meter with a bigger one. But can this gas line still be used? For example, with a higher gas pressure to that line? I may eventually replace that line when I get around to redo the pavers on top of it. But that may happen next year. Before then, can I add a new regulator for this line to increase the pressure?

PG&E will replace the meter for free (which is very unusual). You will not increase the pressure, which is not the issue. It is the amount of gas available to the unit. You heater wants 400BTU (400 cubic feet, 4 therms) per hour and your meter will not allow that through it.

As with most installers, yours never told you to get a larger meter. Usually that is because you immediately think the unit will use more gas, but not so, it will use what it needs, or try to get it (whistle) and sound like it has asthma. So does your meter. Get the larger meter and see what happens. If you need a larger gas line, you will have to get that, too, (or a smaller heater) but even at the distances you mention I've seen them work OK with your size pipe, its just not ideal. Try to not have anything else that uses gas on at the same time.

Heaters never "adapt" to the low flow, they just have performance issues that can lead to damage (sooted heat exchanger, poor heating, burned-out burners, noise, etc.).

The builder/installer can't get the meter for you as the owner of the utility account is the only one that can request the change. Lost a few installs because of that as I would not install a heater to a system with inadequate gas supply. Haven't seen it with your brand heater, but have seen the damage it can do to a normal atmospheric one. Have seen those heaters installed with flex gas line, hopefully yours isn't, that barely worked. Replaced that with the proper hard pipe and problems were gone, including the whistling.

 

[Scout101]

You’ll likely find that your gas line from the street to the meter will be too small too. They won’t put a big line in if math says that 250 meter was sufficient. I had a similar problem at my house, the meter said 650 so everyone thought it was ok. Added a generator too. Meter was bigger than the line could support (say, 450). Ended up having gas company install new line to street (supports 1.5M), and have a 1M meter now. Technically 895k connected load, and gave myself room to get a 400k heater instead of 300k later.

Gas line replacement was cheap since I was technically adding appliances that they’d be able to charge consumption on (but appliances were already there). Cost me like $500, no brainer.