A hot issue

Guy Cashmore, Kensa looks at performance issues with domestic hot water and heat pumps

Performance issues with domestic hot water probably cause more technical calls than any other subject. This is especially true where ‘off the shelf’ indirect cylinders have been combined with larger size heat pumps.

Many of the issues relate to size of the coil within the cylinder, not only do these coils need to have sufficient surface area for heat transfer, but also be large enough in internal diameter to allow sufficient flow rate to meet the flow requirements of the heat pump – these are two distinct and separate requirements.

Surface area

When choosing a cylinder to go with a heat pump, obviously the manufacturers’ recommendations should be followed. However, a good check or ready reckoner is to allow a minimum 0.2 m² surface area for each kW of heat pump output. So a typical 10kW heat pump needs a minimum 2m² coil. When you realise that a typical standard unvented cylinder from a plumbing merchant will only have a 1m² coil, you can see how things can go wrong.

Another common issue is with finned or ‘integron’ coils in cylinders. These usually only have the claimed surface area on the outside of them, rather different from a true coil and they simply don’t work as well.

Coil diameter 

Heat pumps work best with high flow rates going through them. Again, follow the manufacturers’ recommendations, but as a quick check aim for three litres of flow per minute, per kW of heat pump output. The installer needs to check that the pressure drop caused by the coil is not too great to get this flow rate and using a larger diameter coil is always preferable to fitting a bigger pump!

Stratification and pumped loop systems

Larger houses often use pumped loop DHW systems to provide instant hot water at the taps, typically the loop return goes to a mid point connection on the cylinder. These systems work fine with boilers but can cause a lot of issues where heat pumps are used. The through pumping stirs up the natural stratification within the cylinder, causing the incoming cold water to mix with the hot, so the delivered temperature drops off rapidly and because the water wasn’t quite so hot to start with (compared with a boiler), it rapidly becomes too cold to use.