I had a tankless water heater installed with isolation valves. The setup that I got is a little different from what I normally see, and I'd like to make sure that this is a good solution and I'd like to ask a few questions about descaling with this setup. There are two shutoff valves, one each on hot and cold lines. However, there are no drain valves on these valves, all they do is shutoff water with no way to attach lines for descaling. Instead, flexible hose is used to attach from these lines to the tankless heater. Here is an image of the attachments from the lines to the hose:enter image description here

Then, the hose attaches to the heater as shown:enter image description here

The hose is marked as stainless steel water heater hose, 3/4" FIP.

My questions are:

  1. Is this setup an ok installation?

  2. To descale with this setup, I assume that I just close the valves, unscrew the hose, and re-attach it to a pump to run descaling solution through. Is this so? Is it better to unscrew the hose at its attachment to the lines or at its attachment to the heater? When I re-attach the hose, should I put anything on the threads such as pipe dope? Will there be any problem long term with repeatedly unscrewing and rescrewing the hose every time I descale it?

1 Answer 1


This kind of connection does not need pipe dope because a plastic washer or seal is compressed between two hard metal rims. The threads are not exposed to water pressure and are not part of the sealing so pipe dope would not be useful.

Twelve years ago I bought and supplied to the plumber who installed my tankless heater a set (hot and cold) of Webstone 50443 service and isolation valves. I had bought two of these Bosch heaters as discontinued models on sale at Home Depot, intending to put one in my house and the other in a rental house I owned, but I chickened out on doing it myself and hired a plumber. He acted like he had never seen valves of this type. These are specifically for tankless heaters and allow cutting off the cold water supply and the hot water piping of the house from the water heater and then opening up the heater for draining and for descaling with vinegar.

I had it descaled once in 12 years of service. The plumber who did the descaling was not familiar with these valves and wrenched one in the wrong direction over the stop. Luckily this didn't break it and I was able to remove the handle and restore normal operation. He used a pump in a bucket to circulate a couple of gallons of white vinegar but decided to turn the heater on with the vinegar circulating. I don't think that is a good idea.

I wonder whether descaling is necessary for most installations. If yours is new just use it for 10 years and then think about descaling.

What model is your tankless water heater? Mine is a Bosch Aquastar 125 (?), a discontinued model with a standing pilot light that does not require electric power.

Noritz seems to think that there is no one universal descaling time interval. http://www.noritz.com/blog/biggest-myth-tankless-water-heater/

  • The Webstone valve set I got had a pressure relief valve (or temperature/pressure relief valve) included. This attaches to a port on the hot valve. Does your installation have a relief valve and where is it connected? Commented Jan 16, 2017 at 13:22
  • Thanks for the info. Can I re-use this kind of connection multiple times? That is, can I detach and re-attach it each time I descale without problem? There is no relief valve on the installation and no drain valve. The valves are just levers to shut off the flow of water through the line, with no other connection on them.
    – user65138
    Commented Jan 16, 2017 at 15:06
  • I think you could disconnect and reconnect the supply and output lines multiple times for descaling. However, it will not be convenient. As to the absence of a pressure relief valve in your installation, this is a major deficiency and I must think is a code violation. Tank water heaters have a temperature/pressure relief valve, in which a temp sensor is in contact with the water in the tank. Tankless water heaters are, I think, pressure relief only because a heat sensor would not be effective. Commented Jan 16, 2017 at 15:15
  • What is the make and model of your water heater? Commented Jan 16, 2017 at 15:22
  • You must have a pressure relief valve on the hot output side of the tankless water heater and it must be one which is specified for tankless heaters. You will not be able to use the TPV for a tank heater. You might be able to rig up a way to connect the pressure relief valve using standard copper or brass fittings, but the normal way would be to use the specific valve for that purpose. Where in the US or elsewhere is the structure with this tankless heater? Commented Jan 16, 2017 at 15:34

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