Buildings that are off the gas grid still have to conform to the same environmental standards as buildings anywhere else. This is a problem because the standards required by Part L 2013 are very strict and the efficiency of fuels like LPG and oil is a lot poorer than gas. This makes it hard to get a house to pass the SAP test in some rural areas.

One solution is to use a heat pump. There is often enough space around the building to have a ground source heat pump. It can’t go under trees and you need a large empty field. Heat pumps can be installed in small areas and need be no bigger than a single manhole cover but this requires vertical boring – drilling holes 100 metres straight down. This is very expensive. An ordinary ground source heat pump is cheaper than this but still, typically, tens of thousands of pounds. Air source is cheaper (the windmill in a box on the side of the wall) and would enable the house to pass the SAP test but it still costs a lot more than an oil or lpg boiler.

Some combination of different technologies can be used to achieve a lower cost solution. Is the property in an area where local logs are readily available? A log burning water heater is probably going to cost a lot less than a mixed fuel one and this would help pass the test. The price of solar panels is coming down and a simple system costs a lot less than a heat pump. If you fitted either of these you could probably pass the test with an lpg or oil boiler instead of a gas boiler.

Why can’t you pass the SAP test by putting as much insulation as you can into every corner and cavity of the property? This was possible up until SAP 2005 – to compensate for a less efficient heating system or fuel by using lots of insulation. This no longer works. You may be using as much insulation as is physically possible, advanced thermal bridging and advanced solar glazing to achieve the thermal envelope required to pass SAP 2012. Some houses are going to need all of these things to pass with a gas boiler. You can reach a point where the insulation is so efficient that an extra layer of insulation in the loft or the ground floor actually makes no difference at all. If the loft has a U value of 0.8 and the ground floor has 150 mm of PIR in it already, increasing the insulation here any further is usually a waste of time. There is so little heat going through the roof or the ground floor it makes no difference.

Passing SAP in Areas off the Gas Grid
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