There has long been an idea that you should improve your home's insulation before you install a heat pump. That idea is changing as gas prices increase relative to electricity prices, and as the urgency to address climate change increases. Here at Sero we think the best path for your home depends on your objectives, and on the condition of your existing heating system.
Installing a heating pump is the biggest thing you can do to reduce your home's carbon footprint. If that is your top priority then you should do easy insulation measures such as loft insulation, and install a heat pump straight away. There is a myth that heat pumps can't heat poorly insulated buildings. This isn't true. You just need to have a heat pump that's powerful enough that it can provide heat as fast as your home looses it. You will also need to ensure that your radiators (or other emitters) are sufficiently sized so that the heat pump can distribute heat at that rate. This will often mean that you need to increase the thickness of your existing radiators and/or add more emitter area (in the form of more radiators, underfloor heating or fan coils). This makes your heat pump install a bit more disruptive, but ensures that your heat pump can run at a low flow temperature and therefore a high efficiency (see Will installing an Air Source Heat pump reduce my energy bills? for more info on this).
If you later improve your home's insulation then your heat pump and radiators will be oversized. Having oversized radiators isn't an issue - it just means your heat pump can run at an even lower temperature, and so even more efficiently in your newly insulated home. Having an oversized heat pump can be problem though. You will have spent (slightly) more money on the heat pump than if you insulated your home first, although heat pump prices don't actually vary that much with size. More importantly, if the heat pump cannot 'turn down' enough to match your home's new lower heat loss, then it will turn on and off more. This frequent turning on and off, referred to as cycling, can reduce the efficiency and the lifespan of your heat pump. Some heat pumps have a much greater 'turn down' ability than others though, so a good option is to select a heat pump which covers your heat load now, but which has a really good ability to turn-down so that it doesn't cycle too much when you do insulate your home. Ask for the manufacture's minimum kW capacity for the unit recommended. If you go down this route, remember to think about how your heat pump install will fit physically with measures you plan in the future - such as external wall insulation.
If you want to avoid installing a larger heat pump and upgrading your radiators, then you should do all of your insulation measures first, and only install a heat pump at the end once your space heating demand has been minimised. The downside of this approach is that you continue burning fossil fuels in your home for longer, and so your carbon emissions will be much higher than if you install a heat pump straight away.
Your existing heating system
Deciding to take out a functional boiler is rather different to making the choice between a new heat pump or a new fossil fuel boiler. We think you should avoid installing a new boiler. If your existing boiler is struggling but you don't feel ready to install a heat pump, see if you can find a great heating engineer to repair your existing boiler and keep it going until you are ready to switch to a heat pump. That way you won't waste money on a new boiler that you will only use for a few years.
If your current boiler is working fine you can still install a heat pump right away if you want to - there is just less urgency in making that decision. If you do want to keep your current system running for the moment, it's worth getting a heating engineer to check that it's running as efficiently as possible. A lot of gas boilers are running at unnecessarily high flow temperatures resulting in low efficiencies and unnecessarily high bills