How does a heat pump differ from a gas or oil boiler? How do you operate a heat pump?
In this blog I explain the differences between operating a gas or oil central heating system versus an air sourced heat pump.
There is a lot of detail to take in, so I have split it into two parts.
Part A: The differences between Gas or Oil heating and a Heat Pump
Part B: A Guide for Householders – heat pump controls
Many of my readers will be content to just read Part A; those with a real interest in understanding how to operate a heat pump can also read Part B.
Please note that your heat pump installer will set up the controls to suit your house and your lifestyle needs, so in theory you won’t need to change them. However, in practice it is likely that you will want to adjust the controls at some point. Understanding how your heat pump works could help to avoid excessive electricity bills and will enable you to have constructive conversations with your installer if there are any issues with the operation of your heat pump.
An alternative, and one I would recommend, is to sign up to a remote monitoring and control service where a company can remotely adjust your controls and fix minor faults.
Part A: THE DIFFERENCES BETWEEN GAS OR OIL HEATING AND A HEAT PUMP
In some respects, heating your home by gas, oil or by a heat pump are similar. They can all work with wet based central heating systems that circulate warm water around your house. Many of the basic controls are the same, such as a house thermostat and a programmable timer. However, there are some important differences to understand.
Heat Output
A heat pump has a smaller heat output than a gas or oil boiler; typically 4-15kW versus 10-40kW. Given the lower heat output, a heat pump:
- to work well it must be designed and installed correctly and have its controls set to suit your needs;
- will work more efficiently if you heat all rooms to a similar temperature - a heat pump and its radiators are sized to heat the whole house;
- provides a steadier air temperature, reducing the likelihood of cold spots, condensation and damp;
- needs to have a cylinder to store domestic hot water. Gas or oil boilers can also work with a hot water cylinder, or combi boilers can heat up hot water instantly;
- may require more space in your house to host the hot water tank and buffer tank. The heat pump must be placed outside near an external wall.
Flow Temperature
A heat pump operates at a lower flow temperature (the temperature of the water circulating to the radiators) than a gas or oil boiler. Typically, a heat pump operates at 35-55°C compared with 70-80°C. Whilst gas and oil radiators feel hot to touch, heat pump radiators are luke warm or even cool to touch. Given the lower flow temperature, a heat pump:
- heats your rooms more slowly. It should be switched on at least 2-3 hours before you need the room to be warm (some people keep them on 24 hours a day);
- requires a ‘setback’ temperature to be set (say 17°C), which is the minimum temperature that the house can fall to overnight if the heating is switched off;
- will cause less corrosion inside your water pipes and radiators;
- is less likely to overshoot the thermostat temperature and waste energy by overheating your house;
- will stir up less dust given the reduced convection;
- will struggle to cope with narrow radiator pipes (15mm is a good size);
- needs to have underfloor heating or radiators with a larger surface area.
Efficiency
A heat pump is far more efficient than a gas or oil boiler:
- a heat pump extracts heat from outside your house and as a consequence can operate at 250-400% efficiency;
- a modern gas or oil boiler only operates at 90% efficiency, but often even lower if the flow temperature is set too high (then the condensing feature doesn’t work properly);
- a heat pump will work much more efficiently with in-built weather compensation, whereas this is an optional feature for gas or oil boilers. Weather compensation automatically adjusts the flow temperature dependent on the outside temperature.
Fuel and Carbon Emissions
A heat pump is powered by electricity. Unlike oil and gas prices, domestic electricity can be charged at different rates at different times of day in order to benefit both the supplier and the consumer. A heat pump:
- works well in conjunction with smart meters which enable access to cheaper tariffs;
- can be cheaper to run than gas, especially if combined with solar pv and/ or a storage battery;
- can provide nearly free hot water all summer if combined with solar pv;
- has far lower carbon emissions. With its high operating efficiency, a heat pump emits 65% fewer emissions than a gas boiler in the UK, and this is improving every year as our electricity grid continues to decarbonise. Meanwhile, a boiler burning gas or oil will continue to pollute for ever.
Safety
A heat pump is considerably safer:
- a badly maintained natural gas boiler risks death by carbon monoxide poisoning;
- natural gas can leak, and occasionally causes catastrophic explosions.
Part B: HEAT PUMP CONTROLS - A GUIDE FOR HOUSEHOLDERS
This guide is for an air sourced heat pump used to heat water filled radiators and provide hot water needs. It is based on a 7kW Vaillant Arotherm plus model, but most modern heat pumps will share similar features.
Index
- The Basics
- What does 7kW output mean
- Controller
- Temperature Settings and Timer (basic level)
- Temperature Settings and Timer (advanced level)
- Flow Temperature and Weather Compensation
- Heat Curve
- Hot Water
- Coefficient of Performance (COP)
- Calculating COP
- Summer/ winter
The Basics
The flow temperature refers to the temperature of the water that is circulated from the heat pump to the radiators or underfloor heating.
The coefficient of performance (or COP) is a measure of efficiency. It is the ratio of the electricity required to operate the heat pump versus the total useful heat being put into your house.
A heat pump circulates warm water round your radiators in a similar way to gas central heating but at a much lower flow temperature. The lower the flow temperature the greater the efficiency.
A heat pump is powered by electricity. Maximising the system’s efficiency is important to reduce your consumption of electricity.
A heat pump works more efficiently the higher the source temperature (for example, if the outside air temperature is warmer) and the lower the required flow temperature.
What does 7kW output mean?
Domestic heat pumps typically vary in maximum heat output from 5kW to 15kW dependent on the size of the house and how well insulated it is. Fortunately, my 7kW heat pump does not use 7kW (7,000 watts) of electricity. A heat pump extracts heat from the outside air and boosts it to the temperature required. If my heat pump is operating at a COP of 3.0, then it will use 2,333 watts when operating, although like a refrigerator, a thermostat will cycle it on and off.
A small ‘buffer tank’ helps to reduce the frequency of this on/off cycle.
Controller
There are four ways to control my heating system:
- Thermostats on individual radiators
- A controller (Vaillant sensoCOMFORT) with a built-in thermostat, fixed on a wall near the centre of my house
- A thermostat within the heat pump to measure the outside temperature
- An optional app to enable remote monitoring and control (which I didn’t buy)
Temperature Settings and Timer (basic level)
There are four temperatures to be aware of:
- The temperature you want to heat your house to
- Individual room temperatures
- The ‘setback’ temperature
- Frost control temperature (set by the system at say 6°C)
I heat my house to 19°C which I feel is adequate if you wear a jumper in winter. Unlike heating your house with natural gas, with a heat pump it is best to heat the whole house to a similar temperature. There is therefore little need for thermostats on individual radiators.
If you use a room thermostat to reduce the temperature in one room this will reduce the surface area that emits heat. The remaining radiators in your house will have to run hotter to compensate. It can also create bottlenecks in the water flow around your radiators and cycle the system on and off more frequently – all reducing operating efficiency.
In a well-insulated home, there is an argument to maintain the same temperature in your house 24 hours a day. This will maximise the efficiency because running it all the time means that you are never trying to raise the room temperature by more than a fraction of a degree. A low flow temperature is therefore sufficient to keep your house warm.
However, warm air leaks out faster if your house is warmer. So, as a compromise between heating 24 hours a day and having the heating on when you really need it; you should switch heating off overnight, and perhaps for a few hours during the day if you are usually out.
However, if your house cools down too much, then your heat pump will need a much higher flow temperature to heat the house back up. To prevent this from occurring the system has a setback temperature. If the house falls below the setback temperature when the heat pump is switched off (eg overnight) the heat pump will override this and automatically cycle back on. I set mine to 17°C. If your house takes too long to heat back up, then raise this setback temperature.
The frost control feature is purely designed to protect your water pipes from freezing during periods when you switch your heating off completely, for example, if you are away on holiday during the winter.
Example (4-time bands)
4am-8am 19°C
8am-3pm, switched off, but setback of 17°C
3pm-10pm 19°C
10pm-4am, switched off, but setback of 17°C
Temperature Settings and Timer (advanced level)
My setup is more complicated as I have solar pv and a battery and I am on the flexible Octopus ‘Cosy’ electricity tariff. This provides cheap electricity between 4 and 7am and 1 to 4pm; with an expensive rate in the early evening 4 to 7pm. To minimise my use of electricity during peak rate I try to slightly overheat the house during the cheap time periods.
With the Vaillant controller you can split a day into up to seven different time bands, but this is probably overly complicated for most people.
My example (5-time bands)
4am-7am 19°C
7am-1pm 18°C
1pm-4pm 19.5°C (overheat)
4pm-10pm 19°C
10pm-4am, switched off, but setback of 17°C
Flow Temperature and Weather Compensation
Surprisingly, you don’t set the flow temperature. Instead, you set the ‘heat curve’ (see below).
You may hear people saying that their “heat pump operates at 40°C”. This isn’t quite right. Your heat pump should have ‘weather compensation’ which automatically operates the heat pump at the lowest suitable flow temperature dependent on the outside temperature. This greatly helps to improve efficiency.
For example, if the outside temperature is -5°C then you need a flow temperature of say 40°C to maintain the required temperature in your house. If the outside temperature is +10°C then a lower flow temperature of 30°C may suffice. The weather compensation feature adjusts this flow temperature automatically [incidentally if the flow temperature is less than 37°C body temperature the radiators won’t feel warm – this is normal and doesn’t mean the heat pump is broken].
If the temperature is very cold, say -15°C then a high flow temperature of 50°C may be required at which point the heat pump does become expensive to operate (but hopefully this is only for a few days per year). To avoid excessive bills, I have capped my maximum flow temperature at 45°C (which means my house may feel a bit cold in very cold weather).
Heat Curve
This is the complicated section, but I do feel it is important to understand.
A heat curve is the relationship between the outside temperature and the flow temperature of the water that the heat pump delivers to your radiators. Heat curves are a way to optimise the efficiency of a heat pump by adjusting how it operates based on the outside temperature. The goal is to provide just the right amount of heat to maintain indoor comfort without wasting energy. A heat curve of 1.0 means that when the outside air temperature is one degree colder, the flow temperature will be one degree higher.
A heat curve is measured by a decimal fraction. Mine is set to 0.6. Vaillant provide a graph in their manual which shows the relationship between outside temperature and flow temperature at different heat curve settings.
A high heat curve means that the system will provide a higher flow temperature at any given outside temperature. For example, at -5°C outside, a high heat curve of 1.0 may provide a flow temperature of around 50°C, whilst a low heat curve of 0.6 may provide only 40°C.
The difficulty is that the optimal heat curve varies between every house dependent on the desired room temperature, the building’s insulation properties, and the size of your radiators. Even the installer won’t know the optimal heat curve and will may therefore ‘play safe’ and set it too high. Try turning it down and see what happens.
The key point is that to maximise efficiency you want to set the heat curve as low as possible. But, if your house isn’t heating up quickly enough, then you need to increase the heat curve. Normally once the heat curve is set for your house you shouldn’t need to change it again. However, in extreme winter conditions you may need to temporarily increase your heat curve (or increase your setback temperature).
Hot Water
We have a 190-litre insulated hot water tank. We set it to 48°C and time it to heat up between 1 and 2.30pm each day. This coincides with the highest likelihood of warmer air outside and, for us, free solar pv and our cheap electricity tariff. If you have no solar then you might want to heat your hot water up overnight on a cheap tariff.
Once a week our heat pump automatically heats the water to 60°C to prevent any risk of legionella bacteria from growing.
Note that the heat pump can heat hot water or provide central heating, but not both simultaneously. This means that in very cold weather our house can noticeably cool down between 1 and 2.30pm.
Coefficient of Performance (COP)
The COP is a measure of the efficiency of the heat pump. It is the ratio of the electricity required to operate the heat pump versus the total useful heat being put into your house. A higher COP is desirable.
In the UK, electricity costs around four times as much as gas per unit. Given that a gas boiler has an efficiency of 85-90%, you need a heat pump to have a COP of 3.4 to break even against the cost of natural gas. Of course, heat pumps have additional advantages over gas. They can be much cheaper to run if you have solar pv, a battery or can access variable rate electricity tariffs. They also massively reduce your carbon emissions (see my previous blog).
The highest efficiencies can be achieved by accessing a higher source temperature and/or operating at a lower flow temperature:
- Turn down your house thermostat to the lowest comfortable level
- Install large radiators, or underfloor heating
- Install a ground source heat pump - their source temperature tends to be higher (particularly in mid-winter) than for an air source heat pump
Be aware of the following:
- The COP is higher in the spring and autumn - the outside air temperature is higher than winter so the heat pump can use a lower flow temperature
- However, the COP in summer is lower - when your central heating is switched off the heat pump is only being used to heat your hot water (to 48°C) which is higher than the flow temperature required for winter heating
- Following on from the above point, a badly insulated home may have a higher COP than a well-insulated home. A higher proportion of your total heat is used to heat your rooms, with a lower proportion for hot water. Conversely in a well-insulated home, a larger proportion of your total heating is used for hot water.
So, achieving a high efficiency (COP) is important but is not everything. A well-insulated home will need much less heat and therefore will use less electricity and will cost less to heat.
Example
A well-insulated house may have an annual heat demand of 10,000 kWh and a COP of 3.4 = 2,941 units of electricity used
A badly insulated house may have an annual heat demand of 12,000 kWh and a COP of 3.6 = 3,333 units of electricity used
Calculating COP
The Vaillant controller provides information on the ‘environmental yield,’ being the amount of energy sourced from the outside air; and ‘power consumption’ being the amount of electricity used. ‘Total power consumption’ is the sum of the electricity used for heating plus hot water.
The COP is calculated by dividing the total energy input to the house (which is the sum of environmental yield + total power consumption) divided by the total power consumption.
Our COP averages 3.5 over the year. Around 2.2 in the summer months, 4.1 in the spring and autumn and 3.3 in cold winter months. The COP for the summer may seem poor but is mostly powered by ‘free’ solar pv.
Summer/ winter
You could leave your heating on all year; the thermostat will prevent it from switching on most of the time in the summer. However, you will waste electricity through stand-by losses and defrost cycles. I switch the heating off completely between May and October, leaving the hot water on only. I use the ‘holiday mode’ if I am away on holiday to temporarily switch off both the heating and hot water.
Conclusion
Manufacturers and installers should take note. You must write better manuals, in plain English, to help your customers. This will reduce the number of ‘faults,’ complaints, and remedial call-outs required.
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