20 Aug 2010
As a heat pump installer I've had similar experience with recommendations made by other companies.
As a general rule, there is no ground source heat pump out there that can efficiently heat the property you describe, the issues to be addressed would be:
Heat loss - likely to be as much as 100w/sq metre in aproperty of this age
Rads wouldn't be adequate to heat with circulating water , typically about 58 degrees - oil boiler produces about 85
Need 3 phase electricity for any heat pump over about 16kW, some manufacturers only do 10kW single phase
Collector type is irrelevant in view of the above, but if by slinky you actually mean straight pipe buried at a metre depth and a metre spacing that works well. If you mean coiled pipe - less efficient and we do not use it.
23 Sep 2010
Victorian properties are notoriously energy inefficient, in that I agree with Neil MacGregor. Setting that aside, you have asked how a heat pump is sized. We would usually look at one of two methods and, where both are open to us, cross-check between them. Firstly, if the property is not being significantly altered or extended, we would look at the existing energy consumption figures and deduce the annual energy demand in kWh. The second method is to calculate the gross internal area and to apply a heat loss rate which we would deem based on several years' experience of installing heat pumps into period and Listed property. This deemed rate varies according to age, construction and insulation levels.
Before starting the sizing exercise we would always seek to recommend ways to improve energy efficiency. In the case you have described this might include improving the loft insulation with sheep's wool and examining all doors and windows for draughts. If any re-plumbing is envisaged then migrating to low-flow-temperature devices would be good but not, in our experience, essential. If flow temperatures can be reduced then the heat pump can be operated more efficiently.
Like Neil, we do not use Slinkies or Compact Collectors as we believe that these carry a very high risk of over-extraction.
If the current energy source is oil or LPG then it may be possible to specify a heat pump solution which is justifiable. If the property is on the mains gas grid then the financial case is much more difficult to make because mains gas is so much cheaper than oil or LPG.
Does that help?