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Air Source Heat Pumps: Insulation, Heat Loss & Design
Principles
Matching Heat Pump to Building
It is essential that the heat pump is sized correctly to suit the building load. Unlike a conventional boiler the heat pump needs to be sized correctly to match the building heat loss.
Pumps that are over sized will have problems with cycling; the building load will be too small as the return temperature will be reached a lot quicker. Due to this the compressor will stop/start reducing the life cycle and increasing running costs.
reduces the heat pump efficiency
reduces compressor life cycle
Under sizing could lead to freezing up due to the refrigeration cycle demand.
inadequate heating provided
will be unable to satisfy the load and will require addition heating
defrost will happen more often
Heat Loss Calculation
Accurate dimensioning is very important for heat pump systems because incorrect sizing may increase costs and have a negative effect on efficiency. The heat consumption (w/m2) is multiplied by the living space area to be heated; the result is the total heat consumption including both the transmission heat consumption as well as the ventilation heat consumption.
Because of the comparative capacity of air source heat pumps it is essential to do an accurate survey of a building and carry out a correct heat loss calculation.
Heat pump installation should be designed to run 24/7 The following points must be taken into consideration:
Location
Design temperature
Wall, floor and roof insulation levels
Windows and doors
Use an up to date heat loss calculator
Figure 1 Heat Loss from Building - Poorly insulated property with high U
values.
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Example of existing Building
Heat Loss for ground floor =15.59kw Design Temperature -3C
Walls Windows Doors Roof Floors
U values 1.6 4.7 2.0 2.6 2.0
Room Temperature Air change Area W/m² Heat loss (kW)
Hallway 18 1.5 13.1 142.2 1.87 WC 21 2.0 2.9 246.2 0.71 Kitchen 22 2.0 19.3 165.5 3.19 Dining room 21 1.0 23.9 98.0 2.34 Utility 18 1.0 10.1 102.2 1.03 Study 21 1.0 10.4 203.2 2.11 Living room 21 1.0 19.2 226.1 4.34 15.59kw
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The heated area (in m2) is multiplied with the following specific heating load:
Energy-efficient house 10 W/m2
Low energy house 40 W/m2
New build (good thermal insulation) 50 W/m2
House (standard thermal insulation) 80 W/m2
Older house (without specific thermal insulation) 120 W/m2
Example:
New build with good thermal insulation, area 180 m2: Calculated output requirement: 9 kW
Maximum off period 3 x 2 hours at minimum outside temperature (see EN 12831, previously DIN 4701).
For a 24 hour period, a daily heating demand of 9 kW · 24 h = 216 kWh results. To cover the maximum daily heating demand, only 18 h/day are available on account of the off periods of 3 x 2 hours. The building inertia means that two hours of the off period are not taken into consideration. 216 kWh/20h = 10.8 kW
Purely from a calculation standpoint, a heat pump with an output of 10.8 kW would be sufficient. In other words, the heat pump output would need to be increased by 17%, if off periods of 3 x 2 hours per day were to be applied.
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Supplement for DHW heating
For conventional buildings, a maximum DHW demand of approx. 50 litres per person and per day at a temperature of approx. 45°C are assumed. This represents an additional output of approx. 0.25 kW per person with an eight hour heatup time.
The precise calculation of supplements and the sizing of the heat pump are made in accordance with DIN 4708 part 2 (see DHW Heating Table).
DHW Heating Table
DHW demand at DHW temperatures of 45°C
(litres/day per person)
Specific available heat
(Wh/day per person) Recommended suppl.For DHW heating
(kW/person*) Low demand 15 to 20 600 to 1,200 0.08 to 0.15 Standard demand** 30 to 60 1,200 to 2,400 0.15 to 0.30 Or
At a reference
temperature of 45°C Specific available heat Recommended suppl.For DHW heating
(kW/person*) Apartment billing acc.
to consumption 30 Approx. 1,200 Approx. 0.15 Apartment (lump-sum
billing) 45 Approx. 1,800 Approx. 0.225
Detached house
(average demand**) 50 Approx. 2,000 Approx. 0.25 * With 8 hour DHW heat-up time.
** A higher performance supplement must be selected, if the actual heat demand exceeds the stated values.
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Insulation
Insulation, insulation, INSULATION! Solar panels, turf roofs, windmills, heat pumps -they are all wonderful, very fancy and visible. What really makes the difference is loads of insulation - not visible, not sexy, but vital. This is especially true in Britain where the sunshine is, to say the least, limited.
So, how much insulation?
Heat will always find the easiest way out so insulation needs to be as equally distributed as possible. There is no point in having a metre of insulation in the loft when all the heat is going out through leaky single glazed windows
In any existing property there are always practical limits to the thickness of insulation; the size of the cavity; the availability of head space in the loft; the space between the floor joists. You will have a lot more freedom with any new construction. In this case there is a beautiful logic to insulation-it costs very little extra to double or even triple the insulation required by current building regulations: the cost of the extra thickness and scarcely more labour. This additional insulation is one of the best investments you can make in your home.
Insulation
There are three ratings used for estimating heat loss: the k rating, the R rating, and the u value. Any manufacturer of building products and external fixtures such as doors and windows should know the R or u of their products. If you are looking for the lowest heat loss, all you need to know is:
R value - the higher the better
u value - the lower the betterArmed with this information, you can make direct comparisons between different products and materials. For example, normal off the shelf double glazing with aluminum frames has a u-value of 3.5. The high performance double glazing we used has a u-value of 1.6, half the level of heat loss. The difference in thermal performance between seemingly similar products can be dramatic, which is why it’s so important to keep an eye on the R and u values. Normal 75mm concrete blocks have an R of 0.07. Solar concrete blocks have an R of 1.36. A wall built of normal concrete blocks will
therefore lose nearly 20 times as much heat as a wall built of solar blocks.
Insulation Materials
The standard materials for insulation are glass and mineral fibre, and expanded foam sheet. Their insulation performance is excellent, they are highly water resistant, and are reasonably fire retardant.
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Insulation Priorities for Renovation
In an average house the main sources of heat loss are:Draughts 25%
Roof 15%
Windows 20%
Walls 30%
Floors 10%
These figures are averages and will vary greatly from house to house - a terraced house has half the area of external walls of a detached house and so roofs and windows are a greater priority. In a house with an unheated basement, floors will be less important, in a flat the windows may be the most important.
So the priorities for insulation will depend on the individual house and what has already been done to it- every time some improvement is made to the house, the remaining sources of heat loss will become relatively more important. Generally speaking the order of priorities for insulating an existing property is:
1.
Draughts
- a major source of heat loss that is very cheap to reduce substantially.2.
Roof
- it is very easy and cheap to insulate an unoccupied loft space (and vital for aconverted loft space). A well insulated roof makes a large difference to the comfort of sleeping in upstairs rooms.
3.
Windows
- secondary internal glazing or good quality double glazing. Replacing singleglazing makes a very significant difference to the internal comfort on cold nights. If you are not replacing the windows, a thorough draughtproofing will produce immediate gains. 4.
Walls
- certainly worth insulating if you have a cavity.5.
Floors
- a low priority but likely to become a major source of heat loss in an otherwise wellPage 10 of 10
TYPICAL HEAT TRANSFERENCES OF DIFFERENT MATERIALS
k typical thickness (mm) R u
Brickwork 0.84 100 0.12
Concrete Blocks 1.12 100 0.09
Solar concrete blocks 0.11 100 0.9
Clinker blocks 0.36 100 0.21 Plasterboard 0.16 12.5 0.06 Plywood 0.14 18 0.07 Chipboard 0.11 18 0.8 Timber 0.14 100 0.71 Unventilated cavity 0.18 Plaster/render 0.5 2.5 0.03
Celotex PIR insulation 0.019 50 2.63
Fibre glass/mineral wool 0.035 50 1.43
Thermal board 0.02 3 0.15
Brick & concrete block Cavity wall 0.6
Tiled roof with no felt 0.53
Tiled roof with felt 1.9
Single glazed window 4.5-5.5
Double glazed window 2.5-3.5
Solid timber door 3
There are three ratings for heat transmittance: the k-value, the R value and the u-value. The K value gives the comparative conductivity of materials of one metre thickness. R value gives the resistance of a specified thickness of material. The u-value gives the heat loss of one square metre of a specified thickness of combined materials. In each case the value given is for a difference on temperature between the two surfaces of one degree centigrade.
