Lag your pipes, Missus!

Reader Question

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.

Answer

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 65⁰C and cools to 15⁰C. It will lose 204W 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. What would the insulation 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.

Two lessons to be learnt; 1) avoid long pipe runs and 2) insulation is king, the plumber is not.