Its All About Flow Rate

I get a lot of customers with open vented central heating systems assisted by heat only boilers, requiring updates.

What I have noticed is that a lot of people get advised by friends and family that installing a combination boiler is the best way to go. I personally have to inform them that although Combi boilers are great, they do not suit everyone.

There are a few things us as engineers need to cover upon viewing the clients property and before providing them with a quotation for a combi update.

Take in to consideration the size of the property. Ideally the properties that are best suited to combis are smaller properties i.e  flats, apartments, semi detached house or terraced houses with one bathroom.

Although the are some quite large combis for example , a Worcester Greenstar High Flo 440 that are purposely designed for the larger properties.

This however is dependent on the incoming water mains flow rate. The customer needs to realize we are not talking about pressure, we are talking about the speed of which water travels at. This is measured in litres per minute.

Pressure is a measurement we give to the force of water when contained within the pipe work, this is measured in ‘bar’.

A quick way to explain it would be , we could have 10 bar of pressure however if we do not have any flow rate, as soon as you open the tap you will a burst of water followed by a small trickle of water.

We can compare this with electrics, you could have a 1000 volts however if you didn’t have any current ,  you wouldn’t get that 1000 volts to travel through any conductor sufficiently enough to operate anything.

Therefore in accordance with most manufactures guidelines, we require between 20 to 16 liters per minute incoming water main at the boiler to be able to provide approximately 12 to 8 liters per minute of hot water at each hot water outlet.

Anything below 16 liters a minute, provide a written disclaimer or a warning notice on your quotation that the water main is insufficient to supply a combi boiler so it will affect the performance of the boiler. If the client is adamant they want the combi, this explains that they were warned prior to the installation.

THE TEST:

1. Obviously using your flow cup, measure the water at the kitchen tap.
2. Record this measurement.
3. Determine whether the bathroom is an indirect or direct cold water supply.
4. If a direct cold water supply, ask the customer to turn on the bath cold tap whilst you stay recording the second measure at the kitchen tap. check to see if the flow rate is affected. Record the second measurement. Should you be locating the boiler above the kitchen , either on the first, second floor or loft, leave the kitchen tap on and take a third measurement at high level, this may involve taking a reading at the cold water storage cistern in the loft. For an indirect cold water supply, get your customer to turn the hot and cold taps on to allow the cold water storage cistern in the loft to fill as you take your second reading at the kitchen sink.
5. Take into consideration the mains water supply at peak times. Ideally carry out these tests during peak time or alternatively ask questions about the performance of the cold water main in the morning.

HOW WE GET AROUND THE ISSUE OF INSUFFICIENT FLOW RATE.

Keep the cold water storage tank in the loft and fit a single impeller pump to feed the boiler with indirect cold water rather than it being mains fed.

However check with the manufacturer of the boiler you are supplying first.