Hot Water Cylinder with Wetback Coil

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What a wetback to your heat pump hot water system adds:

A wetback is a coil connected to your fireplace that can top up the cylinder heat. When you have the fire going, the wetback transfers some of the heat from the fireplace into your hot water cylinder, reducing the amount of energy your heat pump needs to provide.

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​Before deciding whether the cylinder with wetback is the right option for you, please consider:

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Economic reality-check up front

You are already saving up to 75% of your hot water heating cost with the heat pump. The wetback can only touch the remaining 25%.

 

Only if you have free access to firewood, or are fit enough to cut, chop and stack your own trees, then a wetback could be beneficial for you. If you, however, have to buy your firewood, it is simply not economical, let alone the extra purchase and installation costs.

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There are also other important things to consider, as a wetback is considered an uncontrolled heating source:

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Capacity of your wetback element (the part that is in the fireplace)

Wetback elements are available in various capacities, ranging from 1.5 up to 5 kW. Realise that these capacities are measured in test labs, and the real output varies with the type and condition of the firewood used. Nevertheless, it will give a good indication of the amount of energy you can add to the hot water cylinder with a wetback.

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How many hours do you light the fire daily in winter?

When you know your wetback element capacity, you can multiply this by your estimated average running time of your fireplace. When you light your fire from 5 PM until 10 PM, with a wetback element of 2 kW, you are adding 5 × 2 kW = 10 kW of energy to the hot water cylinder. It will take about 4 kW to increase the temperature of a 300L hot water cylinder by 10 degrees. 10 kW would raise it by about 25 degrees.

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However, when you light the fireplace 24/7 in winter, with a 2 kW element, this would add 24 × 2 kW = 48 kW, and raise the water temperature by 120 degrees per 24 hours. If this was possible.

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How many showers / baths do you take daily in winter?​

Of course, when you use hot water, cold water is introduced, so more energy could be added.

A 10-minute shower, using 100L of water at 40 degrees (10 degrees inlet temperature), contains about 4 kW of heat. A 200L bath contains about 8 kW.

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Overheating of the hot water cylinder

When you are adding more energy to the hot water cylinder than you are taking out, it is a matter of time before the hot water cylinder – or the water in the wetback circuit between the fireplace and hot water cylinder – will reach boiling point. This is something you want to avoid. Our hot water cylinders are rated for temperatures up to 90°C.

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Location of the cylinder vs fireplace

Wetback heating normally works on thermosiphon. Colder water is heavier than warmer (expanded) water, so cold water ‘sinks’ down and warmer water rises up.

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When the wetback element of your fireplace is lower than the wetback coil in the cylinder, and there are no airlocks in the pipes, the coldest water will be in the wetback element.

 

When the fire is lit, that water is heated and rises to the wetback coil. When the water in the cylinder is cold, the heat from the wetback coil will be transferred to the water. The water in the wetback coil cools down, becomes heavier and ‘sinks’ to the wetback element again.

 

The thermosiphon flow will be restricted if the height difference is low, the pipe diameter is small, there are lots of bends in the circuit, or there are airlocks.

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Double coil cylinder option

If you want to use the wetback heat to heat some radiators, we have a 400L cylinder with two internal coils.​

 

The lower coil will be used for the wetback element to heat up the cylinder. The top coil can be used to branch off heat for a couple of small radiators to heat up bedrooms or bathrooms which cannot be heated by the fireplace.

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The advantage of the 400L is a greater heat storage capacity.

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Please realise that the heat pump cannot be used as a full-time heater, as the heat pump capacity is ‘only’ 4 kW. For larger-capacity radiator heating, please visit our hydronic heating section.  

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Regardless of the heat taken by the top coil, there will always be about 150L of hot water available for showers or baths.