Takagi error 991

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Joseph Skoler

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I installed a Takagi 110u about 3 months ago. It is the only LP device in the house and has a 3/4" iron pipe feeding it.

Whenever anyone takes a shower for more than 10-15 minutes, the unit fails with error 991.

Researching this it looked like a venting problem.

The entire length of exhaust pipe is less than 10'

I removed the back draft prevented.

I had a PVC elbow just sitting on the air intake port (there should be plenty of fresh air in the room in which the heater is installed) and took it off (so there is nothing connected to the air intake on the Takagi).

I tried the temp setting at 110 as well as 120.

I checked the dip switches (they are correctly set for LP).

I tried changing the dip switches to indicate a longer exhaust pipe

None of this fixed the problem.

I notice that with the shower running, the unit cycles every 2-3 minutes (i.e., the flame goes on and then off). The flame also seems to modulate (gets stronger and weaker).

Unit says flow is 1gpm with shower running.

What could be going on?

Thanks!
 

Bannerman

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That error code relates to imperfect/abnormal combustion which is often caused by insufficient combustion air or an obstructed exhaust vent, but there can be other reasons.

Although you said the gas pipe is 3/4", you didn't state the piping distance which will include allowance for elbows and other fittings.

As the unit seems to be operating correctly for the initial 10-15 minutes, the issue maybe insufficient gas pressure after that time. If so, suspect the issue may have become worse over the 3-months since installation, and particularly after sundown now that nightime conditions are becoming cooler.

LP stands for Liquified Petroleum gas. As such, the fuel is in a liquid state within the tank, but the exterior air temperature will cause some of the gas to boil, vaporizing the liquid into a gaseous state which is then supplied to the WH as fuel.

What size of LP tank is supplying the WH?

When the LP storage tank is too small to provide sufficient surface area for the surrounding air to heat the liquid fuel, then when gaseous fuel is rapidly removed to supply a high consumption appliance such as your 140,000 BTU WH, the rate of vaporization will be insufficient to continue to supply gaseous fuel to keep up with ongoing demand.

Through use, the amount of liquid fuel within the tank will continue to be reduced, which will provide increasingly less exterior tank surface area in contact with the liquid fuel to absorb sufficient heat to satisfy the rate of vaporization needed. As the exterior air temperature will be further decreasing as we approach winter, the rate of vaporization will further decline.

Page 3 of the document at the link below, specifies a 250 gallon LP tank when filled to 60% capacity at 0°F exterior temperature, will vaporize 331,200 BTU/hr, but only 149,040 when there is 10% liquid fuel remaining.

Although your WH consumption rate maybe less than 140,000 BTU/hr once running and supplying a relatively low water flow rate, we will assume the full burner capacity for this example. While a 250 gallon tank containing 60% liquid fuel can vaporize 2.3 times the fuel your WH will consume while 0°F outside, that same tank will just barely satisfy the vaporization rate required when there is 10% liquid fuel remaining. When the temperature is colder than 0°F, those vaporization rates will be even less.

Suggest consulting with your LP supplier to determine the size of tank recommended to satisfy the vaporization rate needed for your WH at your location.


https://www.gasequipment.com/catalo...UQFnoECB8QAQ&usg=AOvVaw2xZGl2JUJyUiWB2nAIQGcF
 
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Joseph Skoler

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Embarrassing, but I think my problem was that I didn't convert to LP.

I thought the guy who installed it did it and he didn't know it needed to be done.

I just did it today and it seems to be fine.

Thank you so much for the detailed info and education -- it will indeed come in handy.
 

Fitter30

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After running three months with the orifices and pressure. Can u see the flame is it blue (good) ye!low, yellow tips, lazy flame burner could be sooted up need cleaning and proper setup.
 

Joseph Skoler

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After running three months with the orifices and pressure. Can u see the flame is it blue (good) ye!low, yellow tips, lazy flame burner could be sooted up need cleaning and proper setup.

The flame looks quite nice -- smallish blue consistent across the jets. It wasn't used much at all.

Thanks!
 
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