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electric tankless water heater UL 499

Discussion in 'Plumbing Codes' started by Flexo, Jul 10, 2018.

  1. Flexo

    Flexo Registered User

    Jan 28, 2018
    Likes Received:
    Are you or your jurisdiction requiring P&T valves on electric tankless water heaters? We use the 2012 codes,
    P2803.1 in the IRC requires all residential water heaters to have P&T relief valves. UL 499 allows for no P&T relief for electric tankless units meeting the UL 499 standard. I know that the IRC would preempt the UL and that the most restrictive regulation shall govern. Does your jurisdiction require a P&T relief valve in this instance?
    Thank you.
  2. Pcinspector1

    Pcinspector1 Platinum Member

    Oct 28, 2009
    Likes Received:
    The ones I've inspected so far have been installed, did not know that there was a UL 499 exemption.
    Francis Vineyard likes this.
  3. ICE

    ICE Sawhorse

    Jun 23, 2011
    Likes Received:
    I've never inspected one. It may come down to the volume that the heater can contain. For example, pool heaters are seldom equipped with a TPR valve but the volume of the coil qualifies the heater as a storage capacity heater and as such requires a TPR valve.

    Gas fired tankless water heaters always have one. A reasonable assumption is that the coil holds enough water to be classified as a storage capacity water heater.
  4. Rick18071

    Rick18071 Sawhorse

    Nov 28, 2009
    Likes Received:
    I saw a few the said right on the unit that a P&T is required, that is the only time I required it.
  5. Francis Vineyard

    Francis Vineyard Sawhorse

    Jan 1, 2010
    Likes Received:
    Don't recall all the complexities of how the fuel-fired tankless system works but my understanding is the fuel burns at one temperature and therefore has the potential to exceed the temperature for the flow of water to be heated and require a PTR valve. Whereas electric has a duty cycle (on/off) and better control the heat the water to the set temperature.

    An example of a manufacturer's installation guideline of an whole house electric tankless unit invites


    A code change proposal:
    RP54-13 P2803.1, P2803.2 (New)
    Revise as follows:
    P2803.1 Relief valves required.
    Storage water heaters and hot water storage tanks shall be protected against over-pressure and over-temperature conditions by one of the following methods:

    1. A separate pressure-relief valve and a separate temperature-relief valve

    2. A combination pressure- and temperature-relief valve.

    P2803.2 Instantaneous fuel-gas fired water heater relief valve.
    A fuel-gas fired instantaneous water heater shall be protected against over-pressure conditions by a pressure relief valve. The valve shall be located on the cold water inlet piping to the heater at a point that is downstream of all external valves except where the heater manufacturer’s instructions require the valve be located elsewhere.

    Committee Action: Disapproved Committee Reason: This proposal doesn’t address small tankless water heaters. The manufacturer’s instructions take precedence. https://www.iccsafe.org/wp-content/uploads/10-IRC-P.pdf

    R102.4 Referenced codes and Standards.
    The codes and standards reference in the code shall be considered part of the requirements of this code to prescribed extent of each such reference and as further regulated in Section R102.4 and R102.4.2

    Exception : where enforcement of a code provision would violate the conditions of the listing of the equipment or appliance, the conditions of the listing and manufacturer’s instructions shall apply.

    R102.4.1 Conflicts. Where conflicts occur between provisions of this code and referenced codes and standards, the provisions of the code shall apply.

    R102.4.2 Provisions in referenced codes and standards.
    Where the extent of the reference to a referenced code or standard includes subject matter that is within the scope of this code, the provisions of this code, as applicable, shall take precedence of the provisions in the reference code or standard.
    #5 Francis Vineyard, Jul 10, 2018
    Last edited: Jul 10, 2018
    ICE and Pcinspector1 like this.
  6. HForester

    HForester Member

    Sep 21, 2015
    Likes Received:
    The proposal's reason statement:

    "A question that is often asked is whether existing Section P2803.1 applied to instantaneous (“tankless”) water heaters as it is impossible to install a temperature relief valve (in accordance with Section P2803.4) as there is no tank! A new section is added to cover gas instantaneous (“tankless”) water heaters. The fuel gas-fired instantaneous water heater industry is waffling about whether a pressure relief valve is required and most have in their instructions “ it’s up the local code official or jurisdiction”. Many questions come up about this and code officials are not sure what to do. We need to settle the debate by simply requiring the pressure relief valve at least for the gas-fired tankless water heaters. Note that electric instantaneous water heaters are exempt from having a PRV by the UL listing for those products. While the code could be written to require PRVs for electric tankless water heaters, it is impractical to install PRVs for the small electric units (think of the one fixture, under the cabinet type) and then the issue is where to route the discharge pipe. Generally, the gas instantaneous (“tankless”) water heaters are for the whole building and are installed in a basement, garage or other location (outside) where routing of the PRV pipe is no more difficult than it is for a storage tank water heater."

    It seems to me that if UL doesn't care about a pressure relief valve for electric tankless water heaters (of any size) why should the code be worried? An overabundance of caution will only lead to complications (where to route the discharge?) that will virtually eliminate anyone choosing to use small [electric] tankless units.

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