@kev-m

Resist the temptation to go lower as it will not meet your peak loads. The unit sized seems correct.

Do you actually need to replace all the rads?

We also opted for the largest tank we could squeeze in and it's made a world of difference - the increase in price was negligible given how much more hot water we now have.

Posted by: @bobbt9866

@kev-m

Resist the temptation to go lower as it will not meet your peak loads. The unit sized seems correct.

Do you actually need to replace all the rads?

Not replace, we don't have any rads currently.  Definitely sticking with the 14kW. 

@kev-m Wise move! Currently dealing with aproject that has an undersized unit and they have had major problems.

Hi Everyone,

There has been quite a few post today under the heading 'ASHP pricing justification' which I feel would be better placed under this heading, so with your permission I would like to move the discussion here, so that new followers are guided to the correct posts via the headings.

From my work with and knowledge of thermodynamics, one thing has become abundantly clear, heat energy is like a hamster or a gerbil, it is always trying to escape.

The faster that the heat energy escapes, then the faster it has to be replaced to maintain a constant temperature inside.

The main problem with efficiently heating a home is the air. Air is actually quite a good thermal insulator, hence its use in cavity walls, the only drawback in this situation is that air will insist on keeping moving, and slowly carrying the heat energy from the inside wall to the outside wall.

Of course, this movement that air persist in making, becomes useful in carrying the heat energy from the radiators out into the room.

Water is approximately 23 times better at conducting heat energy than air, so if we filled our rooms with water they would heat up much faster!!!

When I suggested this to my wife she said that the cats would not be too impressed.

Joking apart, the rate at which the heat energy can be transferred from the radiators into the room can soon become the defining factor. The amount of heat energy which can be transferred from the radiator to the air, is primarily a product of the radiator temperature and the available heating area of the radiator. 

Because water has a high heat capacity, it can be used to quickly and efficiently transfer a large quantity of heat energy from the heat source (boiler or ASHP), to the radiators. The radiators in turn quickly absorb some of this heat energy and become warm, the temperature being limited by the temperature of the water. The central heating water pump does not actually need to run continuously, since once the radiator is up to temperature, it will maintain this temperature for a period of time after the pump is stopped. The radiator will gradually cool as the heat energy is carried by the air into the room. Fan assisted radiators can obviously do this more effectively.

The heat output of a radiator is not linear with respect to the water temperature, but is derived from the following formula.

Actual Heat Output = ((Water Temp - Room Temp) / Specified Delta T) ^1.3 x Specified Heat Output

For a radiator with a Specified Heat Output of 2000W at a Delta T of 50C, the following Actual Heat Output could be expected.

Water Temp.     Room Temp.    Actual Heat Output (W).

      70                   20                    2000

      60                   20                    1496

      50                   20                    1029

      40                   20                      608   

      30                   20                      247

As can clearly been seen from the table above, the heating capacity of a radiator reduces markedly as the water temperature is reduced.

Before specifying the size of heat pump that may be required, it is first of all necessary to confirm that the radiators are of sufficient capacity.

As Bobbt9866 suggested, a simple method to try to confirm if the present radiators are of sufficient capacity, would be to operate your present boiler at lower temperatures and note the effect.

If it is not possible to operate your present boiler in this way, then alternative method is as described below.

You will require an electric heater (preferably convection, but a fan heater would probably suffice), a reasonably accurate thermometer, a notepad and pen and some time and patience.

Select a suitable room, turn off the radiator and close the door.

Place the thermometer near the centre of the room at a reasonable height, and allow the reading to stabilise and make a note.

Obtain a value for the outside ambient air temperature via internet or mobile phone etc. and make a note.

Place the electric heater in close  proximity to the radiator, plug in and switch On.

Note the time and the wattage of the electric heater and run for 5 minutes.

Leave the heater Off for 10 minutes, then switch On again for 5 minutes.

Repeat this procedure, noting the room temperature each time.

If the room temperature is increasing, then reduce the heater On time by 1 minute and increase the heater Off time by one minute.

Obviously, if the room temperature is reducing then do the opposite. 

Repeat the above procedure, increasing or reducing the heating time until the temperature stabilises around the desired temperature.

It should now be possible to approximate the heat loss for that particular room.

If the electric heater is rated at 1000W and the temperature is stabilised when the heater is On for 3 minutes in every 15 minute period, then the heat loss will be given by:-

3/15 x 1000 = 200W

If the outside temperature is 10C and the inside temperature is 20C, then this would give an approximate heat loss per degree of 20W.

Using this value it should be possible to estimate the probable heat loss at varying out side temperatures.

At -5C an expected heat loss of 500W could be expected, add say an extra 20% to be on the safe side would give 600W.

Using the radiator table above would indicate that a water temperature of 40C would be required to maintain a constant room temperature.

Obviously if one had the time, and inclination, the above procedure could be carried out on each room in the house, thereby give an overall heat loss for the property.

If the building fabric is the same throughout the property, then it should be possible to calculate the floor area or volume of the test room, and then apply the values obtained to the whole property.

The only remaining things to do would be to check the radiator sizes in each room are adequate for the estimated heat loss, and decide upon the size of ASHP required to meet the worst case heat demand.

Wow, now I need to go lay down in a darkened room (at the desired temperature). 

Thanks, very interesting.  I assume the MCS calculations the suppliers have to do calculate the heat loss for each room using the room sizes and construction materials rather than measuring it as you describe.  If I wanted to measure it as you describe I think I'd have to do more than one test room because we have rooms with one external wall and others with two or three. The way the surveyor described it he'll be doing the sorts of the calculations you describe for each room/radiator and I've asked to see them all before we finalise the radiators.    

@derek-m Good idea to move to this topic and great explanation of how to find the heat loss thank you 

Hi Kev M,

You are perfectly correct that my simplistic method may not be accurate for all the rooms, though if you were to choose the worst room as the test room then you would be certain to be on the safe side for the other rooms.

My intention was not to replace the need for MCS heat loss calculations, but to give homeowners a method of carryout some simple checks to bolster confidence in any calculations made.

I suppose it could also be used in situations where it is not 100% certain exactly how the property has been constructed.

You never know, in the future someone may develop the equipment to automate the process. All it would require is a controllable heat source, a data logger, a set of sensors and a programmable controller. Stick it in the room, set it running and come back later for the results.

@derek-m, you might be onto something with that automatic logging idea.  Maybe you should take out a patent!

Posted by: @derek-m

Hi Kev M,

You never know, in the future someone may develop the equipment to automate the process. All it would require is a controllable heat source, a data logger, a set of sensors and a programmable controller. Stick it in the room, set it running and come back later for the results.

Now there is a Raspberry Pi solution if ever there was !! with some simple sensors and instructions on telling the person to either switch off the heating or switch it back on of course if the heating is already automated then even better ! I think that could be achieved with an all in one box solution, even add a laser measurement tool for the size of room ..... total cost under £80 and sell for £150/250 

Kev M. & Derek M very interesting and informative expiation guys. I used the Spreadsheet Mars posted a few day ago and it addresses these things although I think the practical exercise outlined by Derek M would give a more reassuring feel to the exercise.

i think some MCS installers will use this spreadsheet but I note that they are not compelled to do so..

Posted by: @kev-m

@morgan Yup that's the same guy who came to us.  We had Mr Phillips himself for the survey. 

And how did it go?