Expert Tips for Troubleshooting Heat Pump Issues

Paul Spence is an expert heat pump installer, Technical Manager for heatly and curator of Heat pumps U.K., the Facebook group with nearly 15,000 followers that provides help and advice for heat pump owners and installers:

Over the coming months, Paul will be answering some of the common queries posed on the group.

At the time of writing normal rainy, above zero UK winter has resumed, but roll back a few weeks and much of the country was experiencing a deep freeze. With many people spending their first cold season with a heat pump, panic ensued on Heat pumps U.K. as poorly installed and/or poorly run heat pumps failed to keep their owners warm.

Let’s be clear, heat pumps work fine in very cold conditions. They’re a well-established technology in the coldest parts of Europe where below zero is an all-winter state, not just the occasional day. So what’s going on here, and how do installers and their customers ensure a heat pump meets the challenge of freezing temperatures?

Typically, homeowners complain that their systems are not warming the property as desired or as alluded to in the design documents. The reasons for this are many and varied.

Heat loss

Some are caused by the homeowner, leaving windows open, letting the house get too cool etc. Preventing draughts is key – I am a staunch believer in ‘fabric first’ and in an ideal world, adequate draught proofing and insulation would’ve taken place before the heat pump was installed. This can of course take place retrospectively, however significant energy efficiency upgrades will affect the heat loss and the size/performance of the heat pump – not necessarily in a good way (see the point about oversized heat pumps, below).

Be careful of projects where the customer says they plan to add insulation in the future, and you base your calculations on these assertions. If they don’t make the upgrades, your heat pump scheme may not work as it was intended, leaving you vulnerable to complaints. For more of my views on why fabric first is always best, click here.

Controls

The bigger issues are usually caused by problems at the initial system design, the stated flow temperature not being achieved via the controller settings, or the weather compensation controls incorrectly adjusted. It might seem obvious, but reading the manuals and familiarising yourself with the functions is important to your understanding of how the heat pump system differs from the traditional boiler arrangement.

If the issues cannot be resolved by basic heating control adjustment a closer look at the system design may be required:

Radiators

Has the radiator circuit been adjusted to the correct differential temperature (dt) (has the circuit been balanced)?

Are all the radiators open allowing all rooms to heat evenly? Turning off a radiator in one room could cause the adjacent room to lose heat faster and not achieve target temperature. This is probably something your customers don’t know.

If all the radiator valves are open and rooms still remain too cold, then perhaps the flow temperature needs increasing. When using weather compensation controls this could mean increasing the upper curve parameters by a couple of degrees at your design outdoor temperature, which will essentially make your radiators warmer.

If the rooms still do not reach target temperature, then check the following:

Are the radiators correctly sized for the mean water temperature available and room temperature required? Do not assume the heat pump stated outputs are all based upon dt5, the dt* is determined via the maximum permitted flow rate through the appliance in litres per second or typically litres per hour. The specific heat capacity of the water changes with increases to flow temperature, as does the density.

It is essential that installers obtain information from the heat pump manufacturer regarding the bandwidth of dt the heat pump will operate under. A 7kw heat pump with a maximum flow rate of 1205L/h cannot deliver 8.4KW unless the dt changes across the heat pump, in this example the dt must increase and small margins make a big difference. The devil is in the detail.

Heat pump size

Don’t size the heat pump at its absolute maximum capacity, the unit will defrost more frequently and the heat output will actually reduce so the house will never reach temperature at peak load. Allow a margin – the manufacturer will provide guidance on the oversize factor, i.e. some manufacturers say 105% whereas for others it’s 115%.

Digital design tools

The latest digital design tools are a really useful way to help ensure accuracy from the start. Whether on paper or on a screen, seeing the impact of design tweaks and making mistakes before the system is installed, will save expensive headaches and dissatisfied customers further down the line. Heatly will digitise and simplify heat loss calculations and room surveying, while also allowing installers and end users to see the impact of different levels of insulation, for example.

We are currently looking for installers interested in trialling heatly, you can register your interest here.

Ask questions!

Don’t be afraid to ask questions. The manufacturers are there to help you and so are we! Heat pumps U.K. is full of friendly installers, some of whom are very experienced heat pump engineers. Whether you’re a heat pump installer or owner (or both!) it’s likely any problems you’ve been facing have been experienced by someone else.

*A quick reminder about differential temperature (Delta T, dt, T, Δt or ΔT)

Differential temperature is a term that refers to a difference in temperature through a section at any moment in time. Or more simply expressed, the difference between two temperatures.

What does Delta T mean in heating?

The applied meaning of dt is different depending on what you are talking about, for example, air source heat pump (ASHP) or radiator, and the context.

Heating appliance DT = the difference between primary flow and return heat transfer fluid (water) temperatures.

• Standard efficiency gas or oil boiler: DT10, which means a 10°C difference between flow and return, for example 80°C flow temperature and 70°C return temperature, referred to as 80/70.
• High efficiency gas or oil boiler: DT20, a 20°C difference between flow and return temperatures, for example, 50/30.
• ASHP: usually DT5, representing a 5°c difference between flow and return temperatures, for example, 45/40. ASHPs can also be DT7, for example, 45/38, although this is less common.

The heating appliance primary flow and return DT is required to calculate the mass flow rate: kW output ÷ (DT x 4.2) = kg/s mass flow of heat transfer fluid water or use the number 3.8 (DT x 3.8) for water with glycol added. These are generic figures that are used for quick calculations, in the future, heatly will be able to adjust these numbers for the specific application, therefore providing more accurate results.

Heat emitter DT = difference between inlet flow temperature and outlet return temperature

Heat emitters require differing DTs depending on type (radiator, underfloor heating, ceiling panels, fan convectors, etc) and application, for example, the heat source, type of appliance they connected to, whether the heating circuit is connected directly to the appliance or whether hydraulic separation has been utilised (a buffer, low loss header, close coupled tees).

Radiators connected to a standard efficiency boiler (SE) would be DT10, representing the temperature loss across the inlet and outlet flow and return of the radiator. Radiators connected to a high efficiency boiler (HE) would be DT20.

Underfloor heating (UFH) would have a required DT determined by the manufacturer’s specification or designer’s stipulated calculations.