Plants on the hearth as stove no longer being used.It’s February 2025, and we’ve been living with our Air Source Heat Pump (ASHP) for over a year, so this is the second winter. I thought it was time I did an update on comfort and running costs, and customisations to the system.

If you’re wondering how we made the decision to move to a heat pump, and how we found our installer, see my earlier post Buying a Heat pump – Explorations. This page is just about our experience this past year.

For context, our home is a 3 bed timber frame bungalow with suspended floor, self-build from 1972 to the building regs of the time. The outer stud walls have 2″ fibreglass in them, and 2″ gap to the external single skin brick wall. The uPVC double glazing and the 10-12″ of loft insulation are around 20+ years old. The area is 114m². We live in the Midlands of England.

Comfort

The first lesson was understanding that one runs this kind of a system ‘low and slow’. So instead of hot water to the rads now and then, followed by them going cold, you have warm water that circulates all the time. It’s just warm enough to maintain the temperature you’ve selected, in relation to the temperature outdoors. It is possible to run some gas heating systems this way, and they should be cheaper to run if you do, but that’s not something I’ll cover here.

Do you have arguments in your house, about whether it’s too warm, or warm enough? What temperature to set the thermostat? That basic difference of opinion isn’t solved by heat pumps! You will need to come to  an agreement about what the temperature settings will be. A higher temperature will always cost more than a lower temperature – but perhaps not as much more as you might think. As a personal choice, we choose not to heat our home to summer temperatures in the winter. We still wear sweaters, and I (being one who feels the cold) have my extra thermal layers still, as I need them everywhere I go.

Even so, when we get up on a morning when there has been a hard frost outside, and see the white coating on everything, it’s a surprise. The steady indoor temperature makes one less aware of what is going on outside.

The profile we’ve settled on, and which seems to be working for us through the winter, is 18° from 22:00 for the night, which steps up to 19° at 04:00 while we still have the cheap overnight rate for the electricity. By the time we get up at 07:30 it’s up to temperature. Then at 14:00 it steps up again to 20.5° (which it takes a few hours to reach) and we’ve found that to be a comfortable level for when we’re sitting about in the evening.

However, very many people run theirs at 21° or 22° all the time, or more. The heat pump can do that without trouble. You choose the temperature you want. I think we’ve noticed that the re-radiating effect of warmer surfaces means we don’t need as high a temperature setting, because we don’t have to make up for the cold surfaces.

The hot water is set to heat to 50° during the night time cheap rate. We do our washing up by hand, plus there are showers, and even so we only need to heat water once per night (sometimes every other night) which normally takes about 3/4 hour, or 1-2 kWh. An important point for saving: you probably don’t want to have the hot water on ‘constant’ because 1) it’s hot enough just heating it once or twice a day, and 2) if it’s always shifting to water heating periodically during the day it will cost you more with no extra benefit. You may even be colder, because when the hot water is heating your radiators won’t be. We do an occasional “Legionella cycle” (60°) via the immersion heater when it’s a particularly sunny day, or Octopus are offering free power.

Customising

Our system (Vaillant) has three options for how it’s controlled –

  1. Weather compensation
  2. Active
  3. Enhanced

It is generally recognised that running like a normal gas system on a standard thermostat, for a few hours a day, is the worst choice – the most inefficient, therefore most expensive way to run a heat pump. It turns on and off more frequently, and is less steady.

Very many people find that Weather Compensation is the most efficient control setting for them. This monitors the external temperature and adjusts how warm to make the radiators accordingly without reference to a thermostat. Set and forget. It’s controlled by setting up the weather curve, which adjusts the water temperature flowing through the radiators in relation to the external temperature, and the temperature the water is when it returns to the pump.

Active will actively adjust the weather curve if the temperature on the room monitor deviates from your selected set point. This works for us when it’s cold and gloomy, Nov-Jan. But when there’s extra temperature input it’s not as responsive and may keep going when you don’t need the heat.

We have south facing windows that pick up a lot of solar gain when the sun is shining, so we’ve chosen Enhanced. This is basically the same as weather comp, with a little added adjustment according to the room temperature. In addition, when it goes above set point the pump turns off. That means it will run less when the house up to temperature because of the sunshine coming through the windows, even if outside is cold. This probably is helpful if you’re running a stove.

There’s little to choose between Active and Enhanced, for most people. For new systems, start with full Weather comp and adjust the curve up or down until it’s right for you. If you then involve the internal monitor, set it to a moderate temperature, say 20°, and experiment from there. Give each setting two or three days to see how it is for you. You may find the depths of winter (around freezing) need a different setting from when it’s always above 10° in Spring and Autumn.

Costs

The numbers below are simply what the heat pump data tells us. Our final data for 2024, rounded for simplicity, is –

  • The heat pump used 1,500 kWh in total, mostly at the 7p overnight rate.
  • For that input it produced 7,500 kWh of heat
  • If we had that much warmth from gas, assuming 90% efficient boiler, it will need 8,300 kWh of gas at about £520 plus £115 standing charge.
  • At the price cap for Oct-Dec 2024 (0.245), that would cost £367 with no battery
  • On our tariff with Octopus, we actually paid £180 for the whole year’s heating.

The solar panels are a 14 year old 4kWp array, small by today’s standards but still a useful size. The 8.2kWh GivEnergy LFP battery we bought 5 years ago – again, technology has moved on, but it is doing very nicely just the same.

Having a battery enables us to use twice as much of our free solar generation as we would be able to use with just the panels. It also allows us to run the heat pump through the day using mostly the stored overnight rate electricity, topped up with solar when the sun shines. The battery isn’t quite big enough to cover the heating for the whole day in the coldest gloomiest months, but it is the majority. We just pay the day rate for the rest.

For the bigger picture –

  • Total electricity use for the year was 4,500 kWh (inc EV, about 500 kWh)
  • We imported 2,500kWh from the Grid, mostly at the 7p overnight rate
  • We generated 3000 kWh from the solar panels (4kWp array)
  • Of that we exported 1100 kWh surplus generation to the Grid

The strange thing with a water based heat pump system, it is cheaper to run all the time at a steady temperature, than it is to just have it on for two hours in the morning and six in the evening. You will also be a lot more comfortable, because at such low radiator temperatures it won’t be able to heat the space from cold in a time to fit such short running. Also, by maintaining the temperature nice and steady, the walls and furnishings themselves become, in some sense, part of the heating system. They are radiating back to you the ambient temperature they are kept at, rather than the colder temperatures from being switched off.

There is a lot of discussion about whether one should get a system like ours, based on radiators (Air2Water, or A2W), or the sort which is the air conditioning type of system (Air2Air, or A2A). Both are Air Source Heat Pumps (ASHP). Both will keep you warm, both will save a lot of CO² and be similar or less than gas in running cost (provided they’re correctly sized/designed and installed/set up). A2A is potentially cheaper to run, if you want to have on-demand heating rather than the steady warmth approach.

If you already have radiators and space for a water tank, a radiator system seems like a simple option, but you need a good installer for an efficient system. If you are moving from some form of electric heating or want to remove your radiators, air conditioning will be simpler and cheaper to install. Keep in mind your water heating needs to be considered separately. Air movement and fan noise is more noticeable with the air conditioning approach. You’ll have to make your own choice regarding which look and feel you prefer (radiators or air units).

At the time of writing, a grant of £7,500 is available towards Air2Water systems (if carbon fuels are fully replaced), but Air2Air does not get the grant, nor do Hybrid systems which keep gas or oil as a backup. This may or may not change.

Resources

A great resource for our learning has been the Facebook group Heat Pumps UK and Ireland. It has been a helpful source of information and discussions, but we’re always aware that a confident or strongly expressed opinion doesn’t mean they’re right. Another helpful resource has been the Heat Geek channel on YouTube, which explains a lot of the technical decisions in consumer-friendly language, and does experiments exploring different aspects of installation and controlling. There have also been a few accounts of personal heat pump experiences such as from the Electric Vehicle Man channel, or Tom Bray.

References