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Demand Gas Hot Water Heaters

Gas Water Heaters

Probably the cheapest way to get hot water is to install a demand gas water heater. They are low cost, effective and never run out of water.

What is a 'demand' gas water heater how do they work, what problems do they have and what are their advantages and disadvantages?

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Storage and Demand Water Heaters

Multipoint or demand gas water heater

Gas water heaters are very popular in many countries of the world notably Britain and Australia where they make up 50% of all new water heater sales.

They come in two basic types.

1. Storage water heaters which, like solar, air conditioner and most electric water heaters, heat a tank of water which is stored ready for use.

2. Demand or instantaneous water heaters (also known as multipoint heaters, tankless heaters, geysers or ascots). These heaters heat the water "on demand" as it flows through the pipe to your hot water tap.

Demand Gas Water Heaters

Geysers have been around for many years. Early models In Britain were often mounted on a wall in a confined space such as over the kitchen sink and were known for their idiosynchratic behaviour and had rather a habit of scaring people.

There have been a lot of technological advancements and these days gas water heaters are a lot safer than they were in the early days although the same safety considerations still apply but we'll come to that later.

Advantages and Disadvantages

Demand gas water heaters are very popular and understandably so.

Advantages are:

  1. They are cheap to buy and operate.
  2. They are quick and easy to install.
  3. They are very compact and don't take up much space
  4. They heat the water as it is needed, you never run out of hot water
  5. From an environmental point of view, are efficient when compared with electric heaters.

Disadvantages are:

  1. You have to keep changing the gas cylinder if you are not on reticulated gas supply.
  2. You may have to change batteries for the ignition system.
  3. You may have to get the heater serviced from time to time particularly if the gas is dirty and the jets become blocked.
  4. Controlling the water temperature can be tricky at times.
  5. They do not operate well with low water pressure.

How do they work?

Demand gas water heaters vary in size but all follow the same basic design. Inside the outer casing is a heat exchanger, a vertical flue copper cylinder with a heating coil, a spiral of pipe, wrapped around it. The flue rises to the top, and sometimes above the top, of the heater. At the bottom of the flue is a circular gas ring with a series of gas jets forming a ring of flame.

The cold water enters at the bottom of the heating coil and is heated by the heat from the gas ring as it rises up the cylinder and is hot by the time that it leaves the top.

Water flow turns the heater on and off

When you turn on a hot tap, water starts to flow through the heater which senses the flow of water, turns on the gas and electrically ignites it. The heater roars into action. As soon as you turn the water tap off, the water flow stops and the gas flow is cut off.

Heater Controls

Controls on the front of the heater allow you to adjust the gas flow, the water flow and the size of the flame and thereby the output temperature of the water.


Older models had a pilot light which burned permanently ready to ignite the gas jets when the heater switched itself on. Pilot lights had a habit of blowing out in a strong draught. These days the pilot light is gone. The heater is connected to a battery or an electrical supply and the gas is ignited by an electrically operated 'piezo' ignition. This is the main reason why modern heaters are far safer than the geysers of old.

Problems with gas water heaters

Adjusting the water temperature

Now we come to a critical element in the gas water heater's design. If the water flow is too low to carry away the heat the heat exchanging coil will overheat and be damaged and so, to safeguard the heater, the water flow is monitored and if it falls too low the gas supply is automatically turned off.

Gas water heaters are very efficient and can produce very hot water so when you are in the shower you might only need a little hot water to mix with the cold. When you turn the hot tap down the flow of water through the heater may not be enough and the gas heater switches itself off.

When people have problems with their gas heaters they often make the mistake of turning the gas up to maximum to try and solve the problem. The hotter the gas setting the more water you need to flow through the heater to stop it turning itself off.

So, if your heater keeps switching itself off while you are trying to shower the starting point is to turn the heater down.

Low water pressure

There are other common problems with gas demand water heaters. Many people have low flow rates through their gas water heaters due to the small diameter of pipe in the heat exchanger coil.

Hard water can block the heating coil with scale

If you have hard water the inside of the heating coil can very easily scale up with calcium which, due to the heating effect, is deposited inside the pipe. This reduces the diameter of the pipe and reduces the flow of water through the heater - this is a very common problem. Heaters can be repaired but it is important you take it to someone who knows what they are doing.

Difference in pressure in the hot and cold water pipes

Another more complex problem relates to the difference in the water pressure in the hot and cold water pipes to the shower. Very often the water pressure is lower in the hot water system due to the restrictionmto the water passing through the heat exchanger. Unequal pressure in the shower mixer tap may result in higher pressure water in the cold water pipe pushing back up the lower pressure hot water pipe which can slow down the flow and switch the heater off.

Intelligent plumbing design can solve this problem.


Finally some words of warning. Gas water heaters have naked flames inside them care must be taken in their installation and operation.

Gas heaters should be mounted outdoors if possible and always in a well ventilated space.

They should not have anything flammable above the flue and you certainly should never put anything on the top of them.

Gas cylinders must always be stored ina well ventilated space and away from the ignition flame of the heater

Regulators and gas hoses should be checked regularly to make sure they ae in good condition and working correctlt.

Gas heaters are also something that you should never attempt to repair yourself. These heaters can hang on your wall and quietly and efficiently run for years with little attention but, if you have a problem, call a technician who knows what he/she is doing.

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Copyright © Phil Wilson May 2010
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