Hot Water Cylinder with Wetback Coil

​

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.

​

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

​​​​

Economic reality-check upfront

As 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, a wetback could be beneficial for you. If you, however, have to buy your firewood commercially it is simply not economical, let alone the extra purchase and installation costs.

​​

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

​​​

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

Wetback elements are available in various capacities, typically ranging from 1.5 to 5 kW. Please note that these capacities are based on test conditions, and the actual heat output can vary depending on the type and condition of the firewood, the fireplace and how the fire is operated.

​

Nevertheless, the rated capacity gives a useful indication of how much heat the wetback can potentially transfer to the hot water cylinder.

​

How much heat can your wetback provide?

When you know the capacity of your wetback element, you can multiply this by the estimated number of hours you run your fireplace to calculate the amount of energy it could add to the hot water cylinder.

​

For example, if you light your fire from 5 PM until 10 PM, with a wetback element rated at 2 kW, the theoretical energy added to the hot water cylinder would be: 5 hours × 2 kW = 10 kWh of energy.

It takes approximately 3.5 kWh of heat energy to increase the temperature of a 300L hot water cylinder by 10°C, assuming there are no heat losses. Therefore, 10 kWh could theoretically raise the temperature of a 300L cylinder by approximately 25°C. In reality, the temperature increase will be lower because of heat losses and hot water being used during this time.

 

However, if you were to run the fireplace 24/7 throughout winter, with a 2 kW wetback element, the theoretical heat input would be: 24 hours × 2 kW = 48 kWh of energy per day

 

For a 300L cylinder, this represents enough energy to theoretically raise the water temperature by approximately 137°C in 24 hours. Of course, this cannot happen in practice, as the cylinder temperature is limited and the system needs to safely manage excess heat.

​

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

Of course, when you use hot water, cold water enters the cylinder, and more energy is required to heat it back up. For example, a 10-minute shower using 100L of water at 40°C, with an inlet water temperature of 10°C, requires approximately 4 kWh of heat energy. A 200L bath, heated from 10°C to 40°C, requires approximately 8 kWh of heat energy.

 

These examples give you an indication of how the heat supplied by the wetback compares with the amount of hot water energy your household uses.

​​

Overheating of the hot water cylinder

If you have the fire running continuously, day and night throughout winter, there is a risk of overheating 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 – can reach boiling point. This is something you want to avoid. Our hot water cylinders are rated for temperatures up to 90°C. With normal use, such as having the fire on for a few hours in the evening, the risk of overheating is neglectable.

​

Installation: 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.

​

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, when there are lots of bends in the circuit, or when there is an airlock.

​

​

​

 

 

Double coil cylinder option

If you want to use the wetback heat to heat some radiators, we have a 400L Wetback & Hydronic 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.

​​

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

​​

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.  

​​​

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