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heat loss/

From postcode to heat pump advice.

You type in a postcode and house number. Year of construction and floor area come from BAG, the Dutch national register of addresses and buildings, and the dimensions of the building from 3D BAG, a 3D model of Dutch buildings based on that register. Fikst then calculates the heat loss. Next you choose a heat pump from your own products. If the advice is right, the unit is on the quote with one click.

The calculation is included in your subscription. No separate licence, and no module you add on per month.

Heat loss calculation according to ISSO 51:2023, ISSO 53 and ISSO 57, based on NEN-EN 12831, with all starting points visible in the report.

ISSO 51, ISSO 53 and ISSO 57 are the Dutch standards for calculating heat loss in homes, in non-residential buildings with room heights up to 4 metres, and in buildings with high rooms. NEN-EN 12831 is the European standard for the design heat load of buildings, in its Dutch edition.

See the ISSO publications and what each one covers

Book an introduction How it works

from enquiry to quote/

Everything in your customer's file.

A separate calculation package does not know your customer. You retype the address, save a PDF and put the unit on a quote by hand. Here it all happens on the same customer card as everything else.

  1. The enquiry comes in

    If the lead has an address, Fikst fetches the home profile from the public registers. If your customer fills in the questionnaire in the portal, their answers and photos are there too.

  2. You start the calculation on the customer card

    From the card of the lead or the customer. Address, year of construction and the dimensions of the building are already there, as a suggestion and with the source alongside.

  3. You calculate and choose a unit

    Quickly at building level or precisely per room. Then the advice: which heat pump, where the bivalent point lies and how much backup heating is needed.

  4. The quote

    With one click the recommended unit is on a draft quote for that customer. Is the heat loss unknown or out of date, or does the product have no selling price? Then Fikst refuses, and you see why.

  5. The report to the customer

    You send it from Fikst. The address of the customer or lead is already filled in, and earlier versions are kept.

Elite Klimaat had a separate package for the heat loss calculation. That package is gone. Read how Elite switched

Do you install heat pumps? How Fikst works from the site survey to the installation

two ways/

Quick first. Precise where it counts.

For a quote you want to know quickly which way it is going. For the installation you want it exact. You do not start again for that: they are two ways within the same calculation.

for the quote stage

Quick, at building level

Fikst calculates with the whole building. Year of construction and usable floor area come from BAG. Footprint, roof, facades, party walls and volume come from 3D BAG, as does an estimate of the number of storeys.

The year of construction comes with an insulation profile. Do you know more, because the cavity wall has been filled or the roof is already insulated? Then you adjust that per building element, with a U value or with the layers of material and their thickness.

A flat works too. You say what is below it and whether the homes next to it are occupied.

for the installation

Precise, per room

Per room, Fikst calculates transmission, ventilation and the heat-up allowance, with the temperature that belongs to that room. The U values come from the year of construction or you enter them yourself. The report shows which of the two it is.

Whatever comes from a register is a suggestion. You see where it comes from, and what you enter yourself takes priority. If there are several homes in one building, Fikst does not fill in the dimensions of the building; they do not belong to your home. And if a piece of data is missing, the outcome stays unknown until you add it. Fikst does not carry on calculating with a number that is too low.

the advice/

Which heat pump, and down to what cold it copes alone.

You choose the flow temperature the heating system runs on, from underfloor heating to radiators. And you choose a heat pump from your own products. Fikst puts the heat demand of the home next to the output the manufacturer gives for that unit.

Where those two lines cross is the bivalent point. When it is colder, backup heating is needed. How much that is at the design temperature is shown, with the beta factor. Does hot water count? Then you switch on an allowance per occupant.

Below the lowest temperature in the manufacturer's table, Fikst does not extend the line. There the output drops faster than a straight line suggests, and extending it gives a machine that is too small. If the heat loss is still unknown, there is simply no advice.

Heat demand and heat pump output against the outdoor temperature The heat demand of the home falls from 8 kW at minus 10 degrees to zero at 16 degrees. The output of the heat pump rises from 6.0 kW at minus 10 degrees to 7.4 kW at 2 degrees. The lines cross at around minus 5.3 degrees: the bivalent point. At minus 10 degrees, 2.0 kW of backup heating is needed. 0 2 4 6 8 10 -15-10-5051015 kW outdoor temperature in °C design -10 °C backup 2.0 kW bivalent point around -5.3 °C heat demand of home heat pump output
A worked example, not a real home. Heat demand 8 kW at -10 °C, heating limit 16 °C. According to the manufacturer, the unit delivers 6.0 kW at -10 °C and 7.4 kW at 2 °C. Bivalent point around -5.3 °C, beta factor 0.75 and 2.0 kW of backup heating.

more than the loss/

The questions that come next.

The heat loss alone does not tell you everything. Your customer asks whether it matches their gas bill, whether their radiators can cope and whether the neighbours will be bothered by the outdoor unit. Those checks are part of the same calculation, and whatever Fikst assumes is shown as an assumption.

  • gas use/

    Does it match the gas bill?

    Enter the gas use for twelve months. Fikst takes off cooking and hot water and calculates with the degree days of the KNMI, the Dutch weather institute, over the same months. If the outcome is more than twenty per cent away from your calculation, you see whether it comes out higher or lower. Every step is shown, with the sum.

  • cooling load/

    How much cooling each room needs

    Fikst calculates the peak per room on a clear July day, hour by hour, with the sun per facade direction, the people, the appliances and the ventilation. You get the sum for the indoor units and the simultaneous peak for the outdoor unit. You then choose the unit yourself.

  • emitters/

    Can the radiators cope?

    Per room you enter the radiators with the output from the rating plate, or the underfloor heating with pipe spacing, screed and floor finish. Fikst tells you per room whether it is enough and from which flow temperature. For underfloor heating it also works out the pipe length and the number of circuits.

  • sound/

    What the neighbours hear of the outdoor unit

    You enter the sound power, the distance to the property boundary and how the unit stands: free, against a wall or in a corner. Fikst works out the level at the boundary, during the day and at night, and the minimum distance you need. By default it checks against 40 dB(A). Which requirement applies to your job, you decide yourself.

  • hot water/

    Which cylinder fits

    Fikst suggests a tapping profile based on the number of occupants and whether there is a bath. With the output and the time the cylinder may take to charge, you get the smallest storage in litres, the next standard cylinder size and the heat-up time. If the demand does not fit in that time, you get a warning.

the report/

A report someone else can check.

For the quick calculation, the report is a PDF with your logo. At the top is the building itself, as a 3D model from 3D BAG or as an aerial photo. Then the temperatures used in the calculation, the insulation per building element and the outcome: transmission, ventilation, heat-up allowance and the total.

Whether the chosen heat pump goes in, you decide. The same goes for a paragraph on the Wet kwaliteitsborging (Wkb), the Dutch act on quality assurance in construction. That paragraph says honestly what the calculation is, and what it is not.

If you have filled in the checks, they go into the report too, with the assumptions: the gas use, the cooling load, the emitters, the sound and the hot water. If you change something afterwards, you make a new report first before you send it.

Sending is done from Fikst, to the address of the customer or lead. Earlier versions are kept, so later on you still know which report your customer received.

If you have the customer portal, you can also share the report there, without an email. Your customer finds it with their documents, and you take it out again whenever you want. You first switch on that sharing in the portal settings.

the honest limit/

What is not in it.

So you know where you still need something else.

No radiators from a database
You take the output of a radiator from the rating plate or the documentation. Fikst does not calculate pressure loss or balancing.
Calculating is not measuring
The sound at the property boundary and the cooling load are calculated from what you enter. The cooling load deliberately comes out on the generous side. If you want to be sure what reaches the neighbours, you measure it.

Questions about the calculation.

All frequently asked questions
Does the heat loss calculation cost extra?

No. It is included in your subscription. Fikst switches the module on for you; there is no separate licence and no price per calculation.

Which standard is the calculation based on?

Heat loss calculation according to ISSO 51:2023, ISSO 53 and ISSO 57, based on NEN-EN 12831, with all starting points visible in the report. The temperatures, the insulation per building element and the outcome per item are on the report, so someone else can check the sum.

Can I use the report for the Wkb?

You can include a paragraph on the Wet kwaliteitsborging (Wkb), the Dutch act on quality assurance in construction, with the starting points of the calculation. Which other calculations and documents belong in the quality file is for the responsible expert to judge.

Do I have to fill in all the data myself?

No. Year of construction and usable floor area come from BAG, the dimensions of the building from 3D BAG. They are shown as a suggestion, with the source, and whatever you had already filled in stays. You add the insulation per building element if you know more than the year of construction tells you. If there are several homes in one building, Fikst does not fill in the dimensions of the building, because they do not belong to your home.

Does it work for a flat as well?

Yes. You say what is below the home: another home, the ground or a basement. And whether the homes next to it are occupied.

Which heat pumps can I choose?

The heat pumps from your own products. The output the manufacturer gives at the design temperature and at 2 degrees sets the line in the advice. With that unit and your selling price, it then goes to the quote.

Can I compare the calculation with the gas use?

Yes. You enter the gas use for twelve months and say whether there is cooking on gas and whether the hot water comes from the boiler. Fikst takes that off and calculates with the degree days of the KNMI, the Dutch weather institute, over the same months. If the outcome differs by more than twenty per cent from your calculation, you see whether it comes out higher or lower. If a piece of data is missing, Fikst says which.

Can I also calculate the cooling load for an air conditioner?

Yes, per room. Fikst works out the peak on a clear July day, with the sun per facade direction, the people, the appliances and the ventilation. You get the output for the indoor units and the simultaneous peak for the outdoor unit. By default it calculates with 30 degrees outside and 25 inside; you can change that. Which unit you choose, you decide yourself.

Want to see it on a home from your own work?

Book an introduction