Air Sourced Heat Pump.

Now to the design of heating systems:


There are several, probably not obvious, requirements. These are the temperature to be achieved, the time to get to temperature, and the thermal mass to be heated. The aim temperature is easy to define, but we also need to know the outside temperature acceptable to achieve this, and I suggest that in the UK on average this should probably be accepted as -10C. If it is a large City, temperatures are always several degrees higher, but this depends on the heat loss from elsewhere so should get less with better insulation, stick to -10C. For this discussion, we will assume an inside temperature of +20C as the design temperature, comfortable but not hot. We will forget hot water for the moment. Next, we calculate the heat loss from the building envelope as a whole, windows, doors, walls, and roof, and the ground underneath. Then we need a ventilation loss, which curiously the Building Regs seem to assume is zero nowadays for some daft reason. Now it gets complicated (and this step is ignored by most of the alleged designers and a cause of failure of designs to be hot enough) we need to calculate the thermal mass of the entirety of the property. The air volume is some of this, but much more is the bricks in the walls, the wood, the plaster, etc, which will be several tonnes of materials of various thermal capacity. These must be assumed to start at -10C and finish at 20C. From this, we can calculate the dynamics of the heating system, how long it takes to heat up and cool, and make a decision as to whether we want quick heat or are prepared to wait for warmth. This will be a surprisingly big number but is often assumed to be zero. Imagine heating 5 packs of bricks through 30C with a blowlamp, this is the order of the number for conventional houses. Timber framed and similar modern buildings do not have so much mass, therefore, will be able to be heated (and to cool) much more quickly, along with better insulating values due to the construction. Next, the overall heat loss is added to the thermal mass and we can get the power required to achieve a temperature in a particular time. Most designs just use the heat loss at a steady-state condition and are therefore significantly under-designed unless run continuously.


For electric heating having 3kW of mains available, we need to look up the COP available (thus the available heat) at -10 degrees air temperature and whatever the output needs to be, perhaps 60 degrees for rads and 20 degrees for full air-con. This will undoubtedly be far too little to meet the design criterion above, and this is where we can decide on how to compromise, or perhaps how to get a lot more power! In a wet system, we can calculate the rad size for each room to achieve the right result and lay back knowing we have done a good design!


It is immediately obvious why most designs will not match expectations:

1. Air temperature assumption too optimistic

2. COP too optimistic

3. Achieved water temperature too optimistic

4. Rads too small because of 3 or space or both

5. Heat up time nearly infinite because the heating has forgotten the thermal mass

6. Insulation and ventilation numbers too optimistic

7. The cost is too high so something has to be pruned somewhere.

8. No allowance made for hot water, which adds both thermal time constants and significant extra heat. It is best not to heat the water during warm-up so the bath must wait a couple of hours.


The result, discomfort due to heating time and cold baths all round! The answer, You Know that one, get yourself a real Engineer to do the design. You will see that many of the problems map very well on to Electrical designs, supply, diversity, cable size, etc.


Thanks Mike, estimates are very difficult for an open-ended problem like this, but everything from the Government is far too low and WILL be a disaster waiting to happen. I have attempted to engage in several places, unfortunately, no one wants any bad news, and I am ignored completely. This is the typical client!

Now to the design of heating systems:


There are several, probably not obvious, requirements. These are the temperature to be achieved, the time to get to temperature, and the thermal mass to be heated. The aim temperature is easy to define, but we also need to know the outside temperature acceptable to achieve this, and I suggest that in the UK on average this should probably be accepted as -10C. If it is a large City, temperatures are always several degrees higher, but this depends on the heat loss from elsewhere so should get less with better insulation, stick to -10C. For this discussion, we will assume an inside temperature of +20C as the design temperature, comfortable but not hot. We will forget hot water for the moment. Next, we calculate the heat loss from the building envelope as a whole, windows, doors, walls, and roof, and the ground underneath. Then we need a ventilation loss, which curiously the Building Regs seem to assume is zero nowadays for some daft reason. Now it gets complicated (and this step is ignored by most of the alleged designers and a cause of failure of designs to be hot enough) we need to calculate the thermal mass of the entirety of the property. The air volume is some of this, but much more is the bricks in the walls, the wood, the plaster, etc, which will be several tonnes of materials of various thermal capacity. These must be assumed to start at -10C and finish at 20C. From this, we can calculate the dynamics of the heating system, how long it takes to heat up and cool, and make a decision as to whether we want quick heat or are prepared to wait for warmth. This will be a surprisingly big number but is often assumed to be zero. Imagine heating 5 packs of bricks through 30C with a blowlamp, this is the order of the number for conventional houses. Timber framed and similar modern buildings do not have so much mass, therefore, will be able to be heated (and to cool) much more quickly, along with better insulating values due to the construction. Next, the overall heat loss is added to the thermal mass and we can get the power required to achieve a temperature in a particular time. Most designs just use the heat loss at a steady-state condition and are therefore significantly under-designed unless run continuously.


For electric heating having 3kW of mains available, we need to look up the COP available (thus the available heat) at -10 degrees air temperature and whatever the output needs to be, perhaps 60 degrees for rads and 20 degrees for full air-con. This will undoubtedly be far too little to meet the design criterion above, and this is where we can decide on how to compromise, or perhaps how to get a lot more power! In a wet system, we can calculate the rad size for each room to achieve the right result and lay back knowing we have done a good design!


It is immediately obvious why most designs will not match expectations:

1. Air temperature assumption too optimistic

2. COP too optimistic

3. Achieved water temperature too optimistic

4. Rads too small because of 3 or space or both

5. Heat up time nearly infinite because the heating has forgotten the thermal mass

6. Insulation and ventilation numbers too optimistic

7. The cost is too high so something has to be pruned somewhere.

8. No allowance made for hot water, which adds both thermal time constants and significant extra heat. It is best not to heat the water during warm-up so the bath must wait a couple of hours.


The result, discomfort due to heating time and cold baths all round! The answer, You Know that one, get yourself a real Engineer to do the design. You will see that many of the problems map very well on to Electrical designs, supply, diversity, cable size, etc.


Thanks Mike, estimates are very difficult for an open-ended problem like this, but everything from the Government is far too low and WILL be a disaster waiting to happen. I have attempted to engage in several places, unfortunately, no one wants any bad news, and I am ignored completely. This is the typical client!