Specialists in Low Energy and Heat Pump Heating Technology

Domestic Hot Water

A heat pump system is not like a combination boiler and is not able to provide instantaneous hot water. It will therefore need a hot water cylinder.

If you think that this is inefficient, don't! Modern heat pump cylinders are ultra efficient and losses through the well insulated cylinder are minimal. Even a large 305 litre cylinder loses less than 2KW per day. Compare this with the losses through the flue from an oversized gas combi boiler which will typically put 10% of its energy out through the flue. To get anything approaching the hot water availability of a tank system you will need a 30KW combi boiler so 3KW of heat will go out of the flue every hour this boiler is producing hot water.

We have various options of hot water cylinders suitable for a heat pump. We don't recommend that you re-utilise your existing cylinder as the heat exchanger and insulation are very unlikely to be sufficient.

What size cylinder do I need?

We quote the MCS guidance as below

Fabdec make the following recommendations

As for ourselves, we would normally use 150 litres for a small house or apartment, 200 litres for a small house and 250 to 300 litres for a very large house.

The largest cylinder we have ever supplied was 500 litres for a country mansion with multiple bathrooms connected to a 16KW heat pump which was only for hot water production. The owner was concerned about running out of hot water and urged us to be ultra cautious in our selection. We understand that he has never come close to running short and is utterly delighted with the results.

Panasonic branded hot water cylinder

Panasonic have two rages of hot water cylinders available

Panasonic standard cylinders with G3 kit, immersion heater, 3 way valve and control sensor

 Cylinder CapacityHeight Diameter Weight Kg Heater KW Coil m2  Heat loss
 198 litres 1150mm580mm 49kg 3KW 1.4m2 1.9KW/day 
 287 litres1600mm 580mm 65kg 3KW 1.8m2 2.3KW/day 

Panasonic High efficiency twin coil cylinders with G3 kit, immersion heater, 3 way valve and control sensor

 Cylinder CapacityHeight Diameter Weight Kg Heater KW Coils m2  Heat loss
180 litres - Slim line1,790mm475mm 33kg 3KW 3.01m2 2.01KW/day 
180 litres 1,295mm 550mm 33kg 3KW 1.36 + 0.68m2 1.40KW/day 
300 litres2,020mm550mm49kg3KW 2.04 + 0.97m21.93KW/day
400 litres2,040mm630mm61kg3KW 2.91 + 1.27m23.33KW/day

If you don't use the second coil in the standard cylinder for a second heat source then you can pipe it in parallel for a larger heat pump coil.

Fabdec Excelsior range high efficiency water cylinders

Fabdec Excelsior stock HP range with G3 kit - requires tank kit (see below)

 Cylinder CapacityHeight Diameter Weight Kg Heater KW Coil m2  Heat loss
130 litres 958mm578mm 40kg 3KW 1.29m2 1.0KW/day 
150 litres1086mm 578mm 45kg 3KW 1.29m2 1.2KW/day 
175 litres 1243mm 578mm 50kg 3KW 1.85m2 1.3KW/day 
 215 litres1485mm 578mm 55kg 3KW 1.85m2 1.7KW/day
 255 litres1753mm 578mm 60kg 3KW 1.85m2 1.8KW/day 
 305 litres2029mm 578mm 65kg 3KW 1.85m2 1.9KW/day 

Fabdec Excelsior Large Coil special build range with G3 kit - requires tank kit (see below)

 Cylinder CapacityHeight Diameter Weight Kg Heater KW Coil m2  Heat loss
150 litres1086mm578mm 45kg 3KW 2.00m2 1.2KW/day 
215 litres1485mm 578mm 50kg 3KW 3.00m2 1.7KW/day 
255 litres 1753mm 578mm 55kg 3KW 3.00m2 1.8KW/day 
305 litres2029mm 578mm 60kg 3KW 3.00m2 1.9KW/day
500 litres1780mm 750mm 70kg 3KW 4.00m2 4.4KW/day 

The above large coil models can also have a solar coil fitted on request (the solar coil is 1.1m2 up to 305 litres or 2m2 for the 500l), although the heat pump coil drops in size from 3m2 to 2m2 on models 215 to 305.

Fabdec Slimline and Horizontal Cylinders

We have a range of Slim 450 and 500mm diameter cylinders up to 244 litres plus a range of horizontal cylinders that are only 715mm high and may be more suitable for lofts or other areas with limited height.

Third Party Cylinder kits

In order to use a third party cylinder, or the Fabdec models above, you will need a control sensor and valve kit.

We offer two types of sensor. These are the CZ-TK1 with a brass pocket or either the PAW-TS1 (6m lead) or PAW-TS2 (20m lead) which are the sensor only for insertion into a pre fitted cylinder pocket, such as are supplied with the Fabdec cylinders.

We offer two types of valve each in two sizes. You can either use a Y plan with a single 3 way valve in 22mm or 28mm or an S Plan using a pair of 2 way valves in 22mm or 28mm.

We suggest that models of 7KW and below can be used with 22mm valves but 9KW and above models should use 28mm valves.

If you are using your own third party cylinder then it should meet the following standards as a minimum -

  • Heat loss of less than 2.5KW per day
  • Minimum coil surface area of 2 m2
  • Maximum pressure drop of 30KPa at 50 litres per minute
  • Empty sensor pocket between 1/2 and 2/3 of the way up the tank
  • 3KW immersion heater in the bottom half of the cylinder
  • Overheat cut out internal to the immersion heater

If it does not meet these criteria then you should not use it, unless it has already been tested and approved by ourselves or Panasonic.

Remember - if a cylinder is physically smaller than the one that we offer it is probably for a very good reason whether it is a smaller coil or less insulation.

Managing Legionella and Scald Risks

We are aware of discussion within the Renewables and Heat Pump industries about legionella risks and associated sterilisation cycles. We are very concerned that there is less discussion about the risk of scalding. The time taken for a serious burn with water at 50C is around a minute while at 65C it is around a second.

There is a standard function which can be enabled or disabled in an Aquarea to produce a weekly sterilisation cycle. The problem is that with this function enabled then the water will be boosted up to 65C once a week. So while for most of the week the water coming out of the tap or shower is at 40 to 45C which is not a particular risk. But the householder has to remember that once a week it will come out scalding hot. We are not happy with that risk but believe that there is an alternative - that of holding the cylinder at 50C without a sterilisation cycle.

MCS requires that the guidance of the HSE is followed as far as the prevention of growth of bacteria, including legionella, is concerned. The HSE requires a risk assessment to be carried out and part of that risk assessment is the temperature the water is being held at. There is considered to be a risk if the water is stored at a temperature of between 20C and 45C.

The two charts above show that while legionella can multiply at up to 43C and survive up to 47C, at 50C they are dying. However at 48C a scald will take three minutes, by 50C it takes one minute and at 52C only thirty seconds.

We believe, as stated, that the best compromise between scald and legionella risk is to disable the sterilisation function but hold the water temperature in the DHW cylinder at 50C. This is safely outside the HSE risk zone and minimises the potential for serious burns that come with a sterilisation function. We would be delighted to discuss our reasoning for this and please contact us for further details.

Now you have selected your hot water cylinder click here to continue