Author Topic: Low water cut off for condensing boiler  (Read 174 times)

Offline Xiaopig9

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Low water cut off for condensing boiler
« on: November 30, 2017, 09:50:37 PM »
Do we need to install an external LWCO on a wall mounted condensing boiler by Ontario building code or plumbing code? If do, where is the best location? Thanks.

Offline Admin

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Re: Low water cut off for condensing boiler
« Reply #1 on: December 01, 2017, 09:11:35 AM »
I would refer to the boiler installation manual regarding the suggested LWCO location.  Here are some Codes from the B149.1-15 Amendment FS-225-17.

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Clause 7.1.4 - A boiler not covered under 7.1.1 shall conform to the requirements of clauses 7.1.5, 7.1.6, 7.1.7, 7.1.8, 7.1.9 & 7.1.10 as applicable.

Clause 7.1.5 - Every steam boiler not under continuous attendance by a certified operator shall be equipped with a low-water fuel cut-off device that serves no other purpose, that cannot be rendered inoperative and can be tested under operational conditions.

Clause 7.1.6 - Except as permitted under clauses 7.1.7, 7.1.8 and 7.1.9 every automatically fired hot-water heating boiler shall be equipped with a low-water cut-off device to shut off the fuel supply in the event of low water when:
(a) the input to the boiler is in excess of 120 kW (400,000 Btuh); or
(b) portions of the circulating system are located below the boiler’s lowest safe permissible water level regardless of the input, and the sensing element of the device shall be located above the lowest safe permissible water level established by the boiler manufacturer.

Clause 7.1.7 - The circulation system indicated in 7.1.6 (b) does not include:
(a) Piping, headers and components required for the bottom connections of the boiler and piping within 6 feet (2 m) of the boiler, or
(b) Residential Combo Fan Coil units or in-floor heating applications and all connecting piping and required components.
Boilers not requiring Low Water Cut Off devices by meeting the exemption requirements of 7.1.7 (a) or (b) shall be equipped with a flow-sensing device installed integral to the boiler. The function of the device shall be to shut off the fuel supply when the circulating flow is interrupted.

Clause 7.1.8 - A coil-type boiler or a water tube boiler having an input in excess of 120 kW (400,000 Btuh) requiring forced circulation to prevent overheating of the coils or tubes, shall be equipped with a flow-sensing device installed integral to the unit or within the outlet piping in place of the low water fuel cut-off device required in sub clause 7.1.6 (a), and the sole function of the device shall be to shut off the fuel supply when the circulating flow is interrupted.

Clause 7.1.9 - When two or more hot water boilers of the coil or fin-tube type are installed in one system, a low water fuel cut off device shall not be required on each boiler, provided that a low water cut-off device is installed on the main water outlet header and a flow switch is installed integral to the unit or within the output piping of each boiler that will cut off the fuel supply to the burner on the boiler. These devices shall be installed so that they cannot be rendered inoperative. The installation of low-water cut-offs shall be such that they can be tested under operating conditions.
Note: The term "tested under operating conditions" is a procedure that ensures closure of the fuel supply valves in response to a simulated low water condition.

7.1.13 - A pressure type low-water fuel cut-off device is not acceptable for compliance with clauses 7.1.5 & 7.1.6

Most new boilers come equipped with a factory installed pressure switch type Low Water Cut Off (LWCO).  Clause 7.1.13 does not accept this type of LWCO if there are components installed as per Clause 7.1.6(b).  If any of your components (like the rads) were installed lower than the boiler you would need to install another LWCO.  Most TACO LWCO instructions will say,

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1. Install the probe above the minimum safe water level, as determined from the boiler manufacturer's literature. (See Fig. 1)
NOTE: This may be in a tapping on the boiler or in the boiler supply or return piping.
2. Install the probe to extend into the boiler cavity or piping to make contact with the water.
3. Install the probe so that the exposed portion of the stainless steel is a minimum of 1/4" from any grounding surface inside the boiler (to prevent the probe from shorting). (See Fig. 2)
4. Tighten probe securely with a wrench, using bushing flats provided. Use pipe thread sealant to seal the probe threads.

Offline Xiaopig9

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Re: Low water cut off for condensing boiler
« Reply #2 on: December 01, 2017, 04:57:01 PM »
Thanks a lot, I would to read the boiler manual. We usually install Navien boiler or combi, for navien boiler manifold there is a port to install LWCO, but it's underneath of boiler, how does it protect the boiler, and for navien combi boiler with the service valve kits, it comes with shutoff valve, could I install LWCO after shutoff valve?

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Re: Low water cut off for condensing boiler
« Reply #3 on: January 02, 2018, 05:07:13 PM »
The Navien manuals I have read say to install external LWCO above the boiler.  You need to pipe the bottom port up at least 6" above the boiler.  See the attached picture of the Navien NHB-55 boiler manual.  This applies to both the Navien NHB-55 and NCB-E Combi models.

Offline Xiaopig9

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Re: Low water cut off for condensing boiler
« Reply #4 on: January 10, 2018, 08:06:06 PM »
Thanks again Admin, I had read the navien manual. Thank you clarify the code amendment.