I've just been asked whether it's worth lagging hot water pipes. The questioner said: "We have quite long hot-water pipe runs in our home. We've fitted the system boiler and thermal store nearer the bathroom, but we still have to run a lot of cold water to get hot water in the kitchen 2 floors below. Is there any merit in insulating the hot water pipes so that the water in the pipes does not cool so quickly? Our plumber thinks we're eccentric to ask for this."

Well, plumbers will be plumbers.

Think about it. Let’s assume the “quite long hot-water pipe” is 20m long, and15mm pipe. That will hold 3.53 litres of water. Let’s also assume that the water enters the pipe at 65^oC and cools to 15^oC. It will lose 204 watts of heat. Say the hot water tap in the kitchen is used 4 times each day, that is 819W of energy lost each day, or 299kWh each year at a cost (using a condensing gas boiler) of £19 and 88kg CO² each year. The insulation would cost £10.

I am guessing that the plumbers argument is that the heat is lost to the house and is therefore not “lost”. But what that means is that the central heating design and the control system are being ignored, as is the convenience of having hot water come out of the hot water tap.

There are two lessons to be learnt: 1) avoid long pipe runs and 2) insulation is king, the plumber is not.

Photo by John Carl Johnson

The author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks. 

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

Click here to ask an Energy Expert a question, and sign up to RSS feeds, to make sure you don't miss the answer.