One of my main jobs in plumbing is hot water upgrades, mainly from gas storage to instantaneous. I get a lot of enquiries for this type of work but the one thing that is always said to me is “I want a 26L/min cont flow to replace my gas storage unit”.
26L/min is one of the largest domestic units on the market. But the biggest fundamental problem with an upgrade from storage is the pipe sizing and im not talking about the gas. Storage units are piped up with 15mm cold inlet and 15mm hot outlet to feed the whole house, to get the full benefit out of a 26L/min cont flow you need a min 20mm cold feed and min 20mm hot outlet to the first bathroom. One of the main reasons people want a 26L/min unit is to run two showers at the same time but I can tell you no matter how large the unit is weather its 26, 27 or 32L/min you will not run 2 showers at the same time if you do not upgrade the pipework correctly. 15mm pipe will only ever deliver a certain L/min down the pipe no matter how big the hot water is. Not to mention frictional pressure loss the longer the length of pipe from the unit to the outlet. Now you can imagine upgrading hot and cold pipework can add a substantial cost to the job but if you do not want to upgrade the pipework due to cost then do not go any higher than a 20L/min unit you are simply wasting your money if you do not do it.
15mm pipes max flow rate is 0.3L/sec = 18L/min
A 20mm pipe holds almost double the volume of water than a 15mm pipe and the frictional loss of pressure is far less in a 20mm pipe so you will get a greater flow rate through a 20mm pipe than a 15mm pipe.
Another thing to consider when deciding on the size of hot water unit, the water authority prescribed pressure and flow to your house is 20L/min to the nearest tap to the meter, so if your house has say 22L/min recorded at the front garden tap what is the point of installing a 26L/min unit. A flow rate check should be done at the tap to determine if a 26L/min is suitable for the property.
Hopefully, you now understand a little bit better when I say biggest is not always best when it comes to cont flow water heaters, if you require a hot water upgrade please feel free to call me.
Jason your local Clarkson plumber