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LeakyNick

Hi

I'm interested in these new-fangled Air Source Heat Pumps for my house but I understand you need to calculate the building load for the house to be able to size it correctly. Anyone got any pointers as to the best way to do this? I expect it's really complex and you need to get a 'consultant' in who will charge a fortune for doing some work on a computer! Pointers to avoid the 'consultant' would be good

Cheers

Leaky
 
What size is your house in M2, number of floors and how well insullated?

Eco
 
It's a refurb bungalow thats got underfloor heating, 500l thermal store heated by ASHP, Solar and leccy, which feeds off to underfloor. So, it will be very well insulated. It's kind of ideal for ASHP (no gas available). Thing is, I need to 'correctly' size the ASHP, so need to know building load for starters! Is the a rule-of-thumb method then?
 
look here to size your heat pump, Boiler Calculator

or have a look at this,

To enable you to understand the heat requirements of a space you need to understand how heat is lost from a space and how quickly this happens. This is to enable you to calculate the heat output you require and how quickly you need to heat a space to maintain the desired or design temperature.
Quick Rule of Thumb Route
Step One
Calculate the area of the space to be heated in m2
Measure and record the external or internal length in metres.
Measure and record the external or internal width in metres.
Example: Length 10m x Width 5m = 50m2
Step Two
Calculate the heat requirement of the space by using rules of thumb for different spaces.
Below are guides to heat requirements:
Living Room 60W/m2 21 Deg C
Bathroom 70W/m2 21 Deg C
Kitchen 60W/m2 16 Deg C
Bedroom 60W/m2 18 Deg C
Hall and Landing 60W/m2 16 Deg C
This is based on solid brick or block dwellings with cavity thermal insulation and double glazed windows.
Example:
Length 10m x Width 5m
= 50m2 x 60W/m2
= 3,000W
= 3 kW
This example can also be used to calculate the output required for a new boiler installation
Specific Heat Loss Calculation Route
To calculate the specific heat loss in a space you have to consider the temperatures of the outside space and the desired inside space. There is an accepted temperature criteria, which is - 4 Deg C minimum outside temperature and the desired temperatures as shown above in the Rule of Thumb step two.
To carry out the specific heat loss calculation you measure the space dimensions internally i.e. height, width and length in metres. Calculate the area of the windows in m2 and subtract this from the wall area and measure and calculate in m2 the ceiling and floor areas.
With the above carried out we can start to calculate the loss of heat through the material of the space e.g. Walls, floor, windows and roof or ceiling. In addition, calculate the heat requirements of the infiltration ventilation, from areas of the space that allow air into the space from outside from under doors or through windows with no draft proofing, etc...
This we use for air change rates within the space.
The way heat loss is calculated is by measuring the area and multiplying it by the known U value for the material. This is the thermal resistance of the material multiplied by the thickness of the material. There are tables available giving U values of materials e.g. an unfilled cavity wall has a U value of 1.6.
These values are calculated from the K value of the material multiplied by the thickness of the material.
Calculation example:
Space Temp 21/Outside -4/Air Changes 3/Differential Temp 25
Room: Living Dimensions x U Values = specific heat loss
Air 10 x 5 = 50 x 3 = 150m3 x 0.33 = 50
Glass 1 x 2 = 2m2 x 3.0 = 9.5
Walls (external) 10 + 5 x 3 = 45-2=43m2 x 0.45 = 19
Floor 10 x 5 = 50m2 x 0.10 = 5
Roof/Ceiling 10 x 5 = 50m2 x 0.54 = 27
Specific heat loss total = 110 x 25
Total Heat Loss = 2,750W HLoss Total
If we round up the result to 3,000w or 3kW you can see that we have lost 250W from our original rule of thumb calculation. This will not always be the case and if we had more window area, more outside walls or more air changes we would have more heat loss. In addition, it should be remembered that the rule of thumb is an approximation.
This can now be applied to each space and to radiator sizing calculations and schedules.
Historically a margin was always added to the calculation when radiator sizing of 11%. So, if we take our 3kW and add 11% it becomes 3.33kW say 3.5kW.
 
Thanks. So, are the boiler calculation methods interchangeable for ASHPs?
 
Once you know your total load required be careful when selecting your heat pump. The output quoted for ASHP are based on an outside temperature of 7deg C. There are only 2 ASHP worth considering, the Daikin Altherma is by far and away the best on the market with the Misubishi a bit further behind.
Go to the Daikin Altherma website and find your local installer they will be able to select a suitably sized unit for you.
 
Great. Thanks for the info. But I still don't quite understand. (Please feel free throw 'gentle' abuse). When I hear of boilers being replaced, we hear of a (say) 28Kw boiler, or a 30kW boiler. ASHP seem to be around 5kw, 11kw, 15kw. Why the output difference?
 
Thats for a combi boiler unless someone is realy over sizing the boiler, a detatched house can sometimes only need 6kw if well insulated.
 
Hi ecowarm

Thanks for your help. Much appreciated and much clearer now.

L
 
Tank Unit - SANYO EMEA nice, it comes with a nice 9kw back up immersion heater for every day use and another 6kw immersion back up for when it get cold?????? ÂŁÂŁÂŁÂŁÂŁÂŁÂŁÂŁÂŁ$$$$$$$$$ÂŁÂŁÂŁÂŁÂŁÂŁÂŁÂŁ
Earth save products dont use immersions.
 
Hi

I'm interested in these new-fangled Air Source Heat Pumps for my house but I understand you need to calculate the building load for the house to be able to size it correctly. Anyone got any pointers as to the best way to do this? I expect it's really complex and you need to get a 'consultant' in who will charge a fortune for doing some work on a computer! Pointers to avoid the 'consultant' would be good

Cheers

Leaky

You think the consultant is going to be expensive? Wait until you get the full quote for the heat pump! You need to get a company in to do a SAP rating of your house which might not be as expensive as you think, you need to get that part of it right as if you size the pump up to small it will not meet your heating / hot water demands for your house and size it up too large and you will pay higher electric bills.
 
I have had dealings with so-called consultants and you would be better off just dealing with a decent Heating engineer
 
Would add the hitachi yutaki units as well. I've been impressed so far. Also done the simplex training and now accredited with them and Viessman both good looking units apart from Viessmann is massive
 
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