Air Source Heatpump – Daikin Altherma HT
Published September 14, 2011 by Sean
Being without gas in my village, and currently using storage heaters, I have been tempted to replace my heating system for a while. After trying 3 installers (one can’t be bothered to show, the other wants me to install Solar, then an electric boiler and rads!) I finally get a quote but for a high temperature system.
Air Source heat pumps extract between 2.5 and 5 units of heat whilst using one unit of electricity, but they get less efficient when it is cold, and when the output temperature is high. Typically, they generate water at 35 celcius and this is too low for radiators to be very effective. It is also not really very good for hot water.
The Daikin Altherma HT has a two stage heat pump, and runs at up to 80 celcius. A seasonally averaged COP of 3 (for an output temp of 60C) doesn’t seem too bad, but I need to compare this like for like against a cheaper, simpler low temperature system.
So far, the installer and Daikin have both failed to come up with any data. They are pretty stupid if they think this ought to be secret/proprietary because I am fairly sure even I can make a stab at estimating it with access to an installed unit.
See sidebar link for installation details of the system I finally bought
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So I found the specs, at http://www.daikinaltherma.eu/solutions/renovation/high_temperature/default.jsp
Ambient 7 Celcius (dry) 6 Celcius (wet)
LW 80, dT 10: 2.50
LW 65, dT 10: 3.08
LW 35, dT 5: 4.22
Well, the research I did suggested that this pump was significantly over-rated, and that although the units are able to modulate, the minimum is about 50%. That would mean the unit forever cycling on and off, or requiring a large buffer tank. I was also assured that 50C for DHW would be fine for showering.
As it turns out, a 7kW unit running at 50% output and set for about 44 degree flow seems to be enough for my house when it’s just above freezing – so one case of a cheaper unit being much more efficient.