sizing hot water pipework for HWS

[emptybucketman]

I'm thinking of getting an instantaneous HWS and wondering whether or not I should also replace the main 1/2 inch hot water cu pipe from the existing water heater with say 3/4 inch lagged cu pipe. The system will be located at the rear and adjacent to the kitchen and I need to supply the bathroom nearer to the front of the house. What are the pros and cons to increasing the main feed to the bathroom to 3/4 inch lagged pipe? What are the regulations?

[wonderplumb]

Depends on how far the run is and how accessible it is. Most instantaneous heater manufacturers like to see you run 3/4 at least to the first branch so the thing works properly,this is assuming its a gas heater your thinking of installing. Also if your replacing a storage heater with instantaneous you must install a tempering valve, which governs the water temp to 50degrees to prevent scalding unless the heater you buy is factory preset at 50.

[emptybucketman]

thanks wp.

The first branch is less than a metre from the hot water outlet with one feed going straight thru to the kitchen, another up the cavity to the bathroom (via the roof space) and another to an external (free standing) laundry in the opposite direction to the bathroom. The plan now is to have 3/4 for the main pipe to the first branch (split off to the laundry) and each branch using 1/2 inch lagged. Is the short 3/4 main pipe ok?

[bricks]

Just a note: Instantaneous hws use the flow of water to lift the gas valve and let gas to the burrners- more water = more gas.

That is why hws manufactures want 3/4 to the first branch, it encorages the unit to work more efficiently. The way you want it should be ok. Keeping in mind that the more pipe you have in a larger size the more cold "ded leg" water you'll draw off before you get hot water.

As a general rule I put in 3 meters of 3/4 at the start of the run if possible. I have put less in under otherwise adverse conditions but not less than 1.5- this isnt law ,. its just i figure ive come to useing - because 3meters of pipe is 1/2 a length- therfore i get 2 hws out of one length.