There are two facts that we need to consider carefully when switching heat sources:
1) Oil or gas Boilers can heat water hotter and faster than renewable sources 
2) Oil and gas are also significantly cheaper than electricity, per unit of energy, in the UK

This leads to two things to watch out for when we switch to a heat pump
Output - Does the heat pump deliver enough energy to keep the house warm on the coldest day?
Affordability - Will the energy bills be more or less expensive after the switch?
Heat Pumps
Heat Pumps work like a fridge in reverse
   > A fridge takes heat from inside and puts it outside
   > A heat pump takes heat from the outside air and puts it into the house
There are two main types
  > Air source heat pumps
  > Ground source heat pumps
Ground source have higher efficiency and don't suffer from seasonal variations like air source.

Heat Delivery versus Pump Efficiency
The efficiency of a heat pump is determined by how hot it has to get the water
Radiators are usually designed for water at 65ºC
So as a rough guide:
Air Source Heat Pumps Heating radiators
250% Efficiency

Ground Source Heat Pump on Radiators
Underfloor heating
Underfloor Heating operates at lower temperatures, 35ºC, which can make a heat pump much more efficient
Air Source with Underfloor Heating
300% Efficiency

Ground Source with Underfloor Heating
500% Efficiency
Retrofitting Underfloor heating?
Retrofitting underfloor heating is not as crazy as you might think. Many people think of underfloor heating as being slow to respond and buried in a slab on concrete.
There are retrofit solutions that can be installed over your existing floor boards. These are usually sheets of insulation with grooves in, heating pipes run in the grooves and your floor coverings go right on top.
This can increase the efficiency of a heat pump and it responds quickly, unlike traditional concrete underfloor heating.
Obviously this is not an ideal for every home, but worth considering for the multiple benefits it brings.
Design Example
Take this example, a small apartment with a small garden in the city
It won't be possible to install ground source heating in this type of home, so an Air Source Heat Pump is the preferred option.
However, the property has period features which the homeowner is reluctant to insulate over.
There are possibly two options for meeting the goals of Output and Affordability:
1) High energy solution
Insulate the roof, windows, under the floor and make the building more airtight.
Install an underfloor heating system, replacing the radiators.
This will achieve an efficiency of 350% from the heat pump which will help get the energy costs lower. 
2) Lower Energy Solution
 Insulate the roof, windows, under the floor, make the building more airtight AND insulate the walls. Reinstate or replace the period features.
Retain the existing radiators
This will achieve an energy efficiency of 250% from the heat pump, but less energy will be required to heat the home, to the energy bills should be similar to the first solution
How we deliver heat to our homes has a big impact on the insulation levels we need.
Every home is different, so the only way to do this safely is to carry out a whole house assessment.
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