IT WILL soon become clear that converting existing UK housing stock to low-output heat pumps is not only difficult, but close to impossible.
Those selling the idea of transforming our homes to eco-friendly heating systems seem to accept that the bill will be substantial, but also think that the process involved is as simple as replacing a gas boiler with a new unit. It isn’t. Experience tells me that the people selling the green dream could not hang a shelf or wire a plug.
Take a hypothetical Mr Green, who lives in a standard three-bedroom semi where he enjoys the benefits of gas central heating via a combi boiler. It fits neatly into a small cupboard, provides heating and hot water on demand, is cheap to run and needs a basic service once a year.
To replace that boiler with an air-source heat pump (ASHP), Mr Green will need more than a decent plumber. In particular, he will need sufficient space to install a 300-litre hot-water cylinder and possibly expansion tanks too (if the system is unvented). Ask yourself if you have a cupboard big enough to accommodate such a cylinder. They stand two metres (6ft 6in) high and half a metre (20in) wide!
Oh, you have only showers? Fine, install a 200-litre tank. But you’ll still need the same footprint and it had better be downstairs, because a filled tank weighs nearly a third of a ton.
Assume Mr Green finds the space for the cylinder. He’ll need to get to know a few tradesmen to help him with the rest of the installation.
Before we get to this tedious task, where does the heat pump go? Well, typically it will fit next to the house, at the rear or side, but location is an important part of achieving good performance. There needs to be at least 1.5 metres (5ft) of clear air in front of the fan, with as few reflective surfaces round about it as possible. Unless the ASHP is located correctly, it will never achieve the promised 1:3 coefficient of performance (in other words, you get three units of heat for every joule of electricity you put in).
Mr Green will need to get used to some noise too. Heat pumps whirr constantly, and reflective surfaces amplify the sound. It’s a shame that he has only an area next to the patio where he and his wife like to spend an hour or two reading the Guardian and fretting about climate change. Never mind.
OK, instead of £1,500 for a new super-efficient gas boiler with weather compensator, Mr Green says he’s ready to part with some hard cash for the ASHP. Cool. Let’s get it installed!
The thing is, an ASHP heats water to only 55°C instead of the 75°C from his gas-fired boiler, so he’ll need to do a few upgrades. He’ll need to change the radiators for a start. They have to be aluminium and oversized given the insulation levels in his house. Temperature is not the same as heat. A 1500W steam iron gets very hot, but it cannot heat a room, whereas a radiator with a large surface area can heat a room at a lower temperature. This is why underfloor heating works best for ASHPs.
Mr Green has already lost space to the water cylinder, so he’s reluctant to install bigger radiators everywhere. In any case his central heating system is fed by micro-bore pipework, which won’t carry enough lukewarm water to heat his house. If he was going to use radiators, he’d have to rip it all out and install new pipework throughout. Mrs Green would be fuming!
Right, so underfloor heating (UFH) it is. There are some ace systems out there to consider for refurbishments, but there are no quick fixes when you’re living in the home. It’s a big job.
Mr Green’s ground floor is an uninsulated concrete slab covered with carpets and some lovely engineered oak flooring through the hall to the kitchen and conservatory. Nice, but it will have to go. Unless he gets some hairy builders in to break the floor up, insulate it, lay a new damp-proof course, fit the UFH pipework, screed to existing levels and replace the carpets and wood.
That sounds expensive. And it is, but we can install a retrofit track system to the existing floor. It’s only about 30mm (2in) higher. Woohoo, that’s great! All we need to do is remove all the doors, skirting and architrave, trim them down and put them back again. It’s OK, it’s just a chippy and decorator. The low-threshold front door and UPVC patio doors might look odd when it’s done, but we’ll tell Mr Green that later.
Upstairs, the floors are timber and there are fitted wardrobes, carpets, and the balustrade runners are tight to floor, so underfloor heating is a no-go up there. That’s OK, we’ll put some oversized radiators in. C’mon Mr Green, it’s only replacing half the micro-bore and it’s for the planet!
Right, we’re set. The plumbing and gas engineer that the Greens have used for years had to be sacked because he’s not F-Gas registered, so we found one who is. The carpenters have quoted for the refit. Mrs Green is bloody annoyed and off to Carpetright, and the decorator is drawing up his estimate.
How long this will all take? About a week (probably). We’ll have to move all the furniture upstairs, so it’s probably best that Mr and Mrs Green visit relatives during the works.
It’s a deal. The total cost will be about £20,000 and the Greens will have their dream heating system. Hang on, did anyone calculate the building envelope for thermal heat-loss and airtightness? They forgot?
Ah, it’ll be OK though. I saw a portable heater in the conservatory. They can use that when it gets cold.