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Combi Boiler – Pre Installation Water Checks

Here’s a true story. An installer successfully quoted for an entire new heating system installation, including a combi boiler, for a rural farmhouse.

On the first day of the installation, immediately after all the materials arrived, he used the property’s upstairs bathroom. When he opened the cold tap to wash his hands, he was horrified to find that no water came out. The reason? Another tap was running downstairs and the dynamic water pressure of the cold mains was so low that there was no flow – in fact, air was being sucked into the pipework through the tap!

That was an expensive mistake as there was no way a combi boiler was going to be suitable for that property and the whole system had to be redesigned.

This account illustrates well that spending a few minutes undertaking some basic checks before quoting for the installation of a combi boiler can save a lot of pain later. Let’s consider four tests that are essential.

Static water pressure test

This identifies the maximum pressure in the cold mains pipework when all taps and outlets are closed and no water flowing.

Why undertake this test?

Too great a pressure can damage boiler components and/or cause leaks. Boiler manufacturers will specify a maximum permitted water pressure in their installation instructions, and this should be checked – it could be lower than you think. If the static water pressure exceeds the maximum permitted by the boiler manufacturer, a pressure reducing valve will be required.

How to perform this test

Attach a pressure gauge to either an outside tap or other outlet on the cold mains pipework and slowly open the tap/outlet and take the reading. Bear in mind that readings can vary at different times of the day, with greater static pressure likely at night when no-one in the neighbourhood is drawing water. Keep the pressure gauge connected in this way for the next test.

Dynamic water pressure test

This identifies the minimum pressure in the cold mains pipework when taps/outlets are open and water is flowing. This will always be lower than the static pressure.

Why undertake this test?

If dynamic water pressure is too low, the boiler manufacturer’s published hot water flow rates will not be achieved. Boiler manufacturers will specify a minimum permitted dynamic water pressure in their installation instructions, and this should be checked – it is usually somewhere between 1.0 and 1.8 bar.

How to perform this test

With the gauge connected as shown above, run one or two cold water outlets and take the reading.

Flow rate test

This identifies how much water will flow from the taps when one or more taps/outlets are open and water flowing.

Why undertake this test?

It cannot be presumed that there will be an adequate flow rate of hot water just because the minimum dynamic water pressure is acceptable. There is not a direct link between pressure and flow. It is possible to have high pressure with low flow and vice versa. Therefore, the flow rate at the property’s taps/outlets needs to be checked. Otherwise, the boiler manufacturer’s published hot water flow rates may not be achieved. This is especially likely when other taps/outlets within the home are used at the same time.

How to perform this test

Using a weir gauge/flow cup at a mains water tap, measure the flow rate. Next, open another cold mains water tap/outlet or two. Compare these readings to the water demand for the property (the 2021 Domestic Heating Design Guide details minimum flow rates for simultaneous demand.

Water hardness check

This identifies the ‘temporary hardness’ of the incoming cold mains water.

Why undertake this test?

Regional building regulations guidance documents state that water with a temporary hardness in excess of 200 ppm should be treated to prevent limescale build-up. Some boiler manufacturers set a much lower maximum hardness in their installation instructions, and this should be checked as warranties will not likely be honoured otherwise.

How to perform this test

Simple, inexpensive test kits are available. For example, test strips are available that only need to be dipped in a sample of the water for a few seconds before having their colour change compared to a chart that indicates the temporary hardness.

Turbidity Test

This identifies the cleanliness of the existing heating water.

Why undertake this test?

The manufacturer will most likely void the warranty if you have connected a new boiler to a dirty system. Would you put old dirty oil in a new car?

How to perform this test

There are test kits readily available.

Summary

In total, the checks above will only take a few minutes to undertake but will prevent you the pain associated with specifying combi boilers that will not perform as anticipated.

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