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Hi Romaron
I don't have access to any useful data sheets giving the radiator size reqs. But its a rather simple equation sq.m x heat loss assuming you are running off thermostats.
For 500sqm house built to modern building regs you have an estimated heat loss of ca. 20 -25kW. You cant start a 25kW system on single phase electricity, so you would need 2 x12kW units with staggering, so they don't start at the same time. Back in the day that would have cost ca. 25k. You would need a ca. 1m3 heat sink at least for a dx system. Good luck.
Hi Romaron andTigar
I don't disagree with any of your comments re builders cutting corners, nor that older houses are leaky. Firstly i will level with you, I am Swedish but live in the UK. I have a dx system and my family used to sell HPs 15-20 years ago. To really answer your questions would take an age, but there are a couple of things to understand about heat pumps. There are your traditional ground source HPs (pipes in the ground or borehole). If installed correctly (i.e no shortcuts on laying the pipes) they are very effective in colder climates, but usually only heats water to ca. 35'C. They last forever and the only part you need to replace is the compressor when it goes after ca. 40k hours. You then have the dual air to air HP's that can be used as both AC in summer and Heaters in the winter. Very efficient provided your outside air temperature does not go below zero for more than a few days a year . You then have the more modern Airsource HPs that heat water. These come in two formats dx (direct exchange) which operate best if you have a proper heat sink /store (usually ca. .5m3) or normal system that have a heat exchanger and no heat sink. In the UK you don't get many dx systems, as they require more investment (heat storage tank) and also requires not only a plumber but also a refrigeration engineer to install. Normal non DX systems come pre-gassed and dont need to be installed by a refrigeration engineer. The big difference is that a dx system also provides you with your domestic hot water (at 85'C which can only be accumulated in the heat sink). Essentially water forthe heating comes out of the bottom half of the tank (stratification) and the domstic hot water comes from the super heated top half of the tank. Before someone jumps on me and says that is not possible, it is, but the domestic water is obviously not mixed with the central heating water, but is heated from the superheated water through a normal coil. There is obviously a limit to how much hot water comes out as you will drain the heatsink if you are using a lot of domestic hot water. Average house 180sq.m built to modern spec should have a heat loss of ca. 50w/sq.m .= 9kW need. HPs cost ca. £1k /kW. A prudent architect would put in a 12kW. Your system would therefore cost somewhere around £12k. it will be 9k for a normal ground source HP and more like 13k for an airsource DX system. Payback will be between 7-12 years. The more heat you need/use the quicker the payback. I am not looking to argue with anyone, but simply try to give some insight into my own personal experience with and understanding of an airsource dx heatpump installed in the UK.
Dreamer
Just for clarity, HPs are between 2 -3.5 times more efficient that standard electric heaters (i.e COP 2-3.5). The reason people say you need larger radiators is that heat pumps circulate water at ca. 35'C and not at 55-65'C which is what a normal combi boiler does. You do not necessarily need bigger radiators, as HP's go off thermostats and not timers, but your HP will be circulating fresh heat as necessary (according to your temperature setting), this may be for 8-12 hours in any 24 hr setting (depending on your insulation quality in your home). Your normal boiler is usually on for 3-6 hrs (unless also regulated by thermostat only). In summary where your normal radiators are on for 4 hrs your HP needs to be on for at least 7 hrs to provide the same amount of heat. So if you don't upgrade your existing radiators, you just have to run your heating for longer if using a heatpump.