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Lag your pipes

Posted by Tim Pullen on 16 July 2009 at 8:18 pm

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 65oC and cools to 15oC. 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.

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one


BethCMahoneyComment left on: 14 October 2020 at 4:50 pm

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Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 16 November 2011 at 10:10 am

Yes, insulation is king.  You can also save on your bills by insulating what's called the Primary Pipework - that's the pipes that carry the very hot water from your boiler to your hot water tank, and back. Because the water inside these pipes is much hotter than in the hot water pipes to your taps, more heat is lost from them per metre of length - it's the temperature difference that determines how much heat is lost.

You can see the primary pipework entering and leaving your hot water tank and it's usually quite obvious which ones they are, as they are the hotest of all the pipes in your airing cupboard.  You can get pre-formed and split cylindrical lagging very easily now in the DIY sheds and it's not too difficult to fit - it's easy to cut to length and you just snap it on.  If you can, you should insulate the whole length of these very hot pipes inside your airing cupboard.

While on the subject of pipe lagging, don't forget that you also need to insulate cold water pipes to prevent them freezing in the winter.  Many older homes have pipe runs on outside walls where they are exposed to the cold outside. 

Often, these runs are on the corner of two outside walls, where they will become extremely cold at night when your central heating is off.  Combine this with the temperatures we achieved last winter, down to 14 degrees below zero on some nights, and you have a recipe for a frozen pipe - and when this thaws, what you get is a leak that may cause damage, and is almost always costly to fix.

It's worth remembering that any insurance against leaks may include a clause that says it won't pay up for bursts that occur to unlagged pipes.  So check your insurance before the winter comes, and get the insulation done if you need to, especially if you are going to be away from home for a longish period during the cold weather. 

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Advanced Insulation Manufacturing

Advanced Insulation ManufacturingComment left on: 14 August 2009 at 4:26 pm

It always amazes me that people don't lag their pipes properly; an investments of less than £10 saves about 20% to 30% of the energy used by the system

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