continuous hot water with shower mixer

Hi BBJ, If you want to run two showers at the same time, it more depends on how your hot water pipes are setup. Best setup I am told is to run a larger diameter 'manifold' from the HWS for a short distance, then run smaller individual pipes to each outlet...or at least to each area. But this is really only possible normally in a new build.

Also with instant gas system of that size you will need at least a 3/4 inch gas pipe to the unit as they consume a high level of gas during operation (~200 mg/h I think).

In terms of temps, if you install a 60 degree model legally the plumber has to also install tempering valves to the bathrooms to limit the temp to 50deg. With some models of the 50deg (eg Rinnai) you can bump up the unit temps to around 55 legally to allow for losses in the pipework so that technically it is still around 50 at the tap.

Hope this helps. BTW I'm not a plumber, but have been building so researching this stuff a bit of late.

Thanks for your replies everyone! Sounds like its more confusing than I thought LOL

Hi BBJ, It is a bit more technical I guess than just picking a 'big' enough HWS unit.

But from what I have been told and researched, your 26-27L rated instant gas units will be more than enough in terms of their ability to produce enough hot water for two 7.5l/m shower heads.

The main issues might be:

• Does your gas line carry enough gas to feed the units when running at full capacity? I am told you need 3/4 inch gas lines to the unit
• Also does your hot water setup currently resemble a typical 'trunk and branch' setup? Where you have one decent sized pipe running a loop around the whole house, and then other pipes (typically smaller diameter) branching off to the outlets or main areas. With this setup, when you turn on another outlet or two because they all run off the main branch you will notice at least some pressure loss. Whereas, if you had smaller diameter pipes coming off near the HWS and each went separately/directly to the main areas/outlets, not only is there less cold water to 'clear' before the hot arrives, you don't suffer noticeable pressure loss when running multiple outlets.

Hope this helps? So basically installing just a large enough unit doesn't guarantee you can run both your showers with no noticeable pressure or hot water loss unfortunately.

This plumber actually has some videos on the topic, so this might explain it better than a non-plumber like myself:

https://www.youtube.com/watch?v=XpMjqAV-rIQ