OpenHeatLoss - Free and Open source heat loss, system and pipe sizing tool, primarily for heat pump system design

[SimonD]

I'm actually not sure where the best place is to post this but as it's primarily about heat pump system design, I'll put it here. @marshian and @mads ,and maybe @MikeSharp01? you've expressed your interest so mentioning you here too. I hope the admins don't mind me putting this up here, but maybe if it is found to be useful, could be pinned to help so many of the people who come here struggling with poor heat loss calcs and designs and want to complete their own.

I've finally deployed the tool I've been working on for initial public use. It's available free and open source, so repository on GitHub if anyone wants to host locally. It's currently in a bit of a test mode so I can get some feedback and bug reports to refine it and add further important functions. Just bear in mind I've been developing this myself along with everything else in life and it's been quite a major piece of work since last summer. It is now based on the CIBSE 2026 Domestic Heating Design Guide implementation of BS EN 12831:2017 and BS EN12831:2017 so complies with MCS design requirements. I have been using this tool for MCS heat pump projects in house that I'm doing.

It's at https://openheatloss.com

Important user notes:

At the moment when you arrive, you can complete a whole project anonymously without logging in but this persists only for 48 hours or until you close the browser. The save a project, just register using name and email, nothing else. If you want to remain largely anonymous, you can just add the post code prefix to the installation address to set outdoor design temperature and Typical reference temperature. No need to put in loads of personal information.

The workflow design is to work you way from left to right across the app tabs.

Current limitations:

• I have not populated the database with standard wall build-ups and U-values. If you're a self-builder or doing major diy hopefully you'll have this info already for your project, otherwise you'll need to look it up manually. There is, however, a comprehensive floor u-value calculator and a simplified one in the room elements input too, so these can be calculated for you.
• Same thing with radiators - no standard sizes or outputs in a global database yet. I will do a scrape at some point. All outputs entered should be the Delta T 50 catalogue values and if you want system volume calcs, also input the radiator water volume. The UFH sizing calculates volume automatically based on your set pipe diameter, spacing and room area.

 

Text based design - I've tried some of the design tools that are trying to be like cad design software, so you've got do draw your project. Having used cad software, I didn't warm to any of them because they're not proper cad software but in house bespoke design. As I also found out they have limitations so you have to fudge some shapes - roofs in particular. Text based means you can input elements more flexibly according to your needs. Well, I hope so anyway.

 

What you will find different if you've used other tools, although I'm sure they'll either be doing it already or soon, is that the tool provides 2 different heat load figures.

1. is for the heat generator which calculates the whole house fabric and normal ventilation
2. includes a full fabric air infiltration calculation and is usually higher than the generator load

 

To explain this, the new heat loss calculation methods according to BS EN 12931:2017 and specifically those implemented in CIBSE 2026, with wind load under certain conditions, parts of the building and rooms may require higher heat outputs, whilst other parts may need less. This 'total' value comes to a higher result than the generator. In my own test projects that I've run through the new software, I can attest to this working quite well. For example, in one design and installation project of mine, one particular room in the house was designed for 23C but over the winter, whilst never cold, the room never managed more than 21C. I had thought it was a balancing issue, but no. When I ran this project through the new software it predicted that I would need larger radiators in this particular room, base on the new ventilation infiltration calculation, so that is what I will be installing before next winter.

Anyway, please have a go and let me know what you think, and ask any questions you have here, or email me at the tool - heatloss@openheatloss.com 

 

Does anyone need a user guide to the design workflow and inputs? Let me know..

 

 

Edited 21 hours ago by SimonD

[MikeSharp01]

This is a great contribution to self build and wider market @SimonD. It strikes me as taking the work @Jeremy Harris did way back and moving it up to a whole new level by both allowing the heat loss work to be undertaken and moving it through to the design of the accompanying heating system. It is also great that you chose to share it with us.

[SteamyTea]

I shall have a look at it when I have time. 

We need more 'calculators' like this.

[mjc55]

Do you want feedback on here @SimonD or would you prefer directly by email?

[SimonD]

1 hour ago, mjc55 said:

Do you want feedback on here @SimonD or would you prefer directly by email?

 

Either or, whichever is easier for you. I'm happy to be fully transparent about any issues or problems as well as suggestions for improvement but email certainly helps me to collate them into relevant patterns. Thanks!

[marshian]

One quick question - what platforms does it work with - ie phone as an app or windows PC with browser?

[Dillsue]

I've not looked at it and have no need to but good on you for doing it and making it available

[SimonD]

48 minutes ago, marshian said:

One quick question - what platforms does it work with - ie phone as an app or windows PC with browser?

 

Is this for installing locally from Git or using the hosted version? If installing locally, it's currently designed to work as a web stack using React/Express/Postgresql, so you'll need to install those and access it through your browser. Initial development was on windows, so it can be installed on a windows machine, but the majority of the development has been on Linux as that's what all the hosts use. The hosted version is best through PC & browser, but it will work on both tablet and phone through browser. I'm currently completing the Readme for Git which will guide you through the setup for self-hosting.

[marshian]

5 minutes ago, SimonD said:

 

Is this for installing locally from Git or using the hosted version? If installing locally, it's currently designed to work as a web stack using React/Express/Postgresql, so you'll need to install those and access it through your browser. Initial development was on windows, so it can be installed on a windows machine, but the majority of the development has been on Linux as that's what all the hosts use. The hosted version is best through PC & browser, but it will work on both tablet and phone through browser. I'm currently completing the Readme for Git which will guide you through the setup for self-hosting.

 

I was planning on trying

 

"hosted version" on Android Tablet, iphone, Windows 11 Laptop (Firefox or Edge)

 

Installed locally on stand alone Windows 8.1 Desktop (windows doesn't have internet access) and on the same PC running Linux as  web based access either as installed locally or hosted 

[SimonD]

59 minutes ago, marshian said:

 

I was planning on trying

 

"hosted version" on Android Tablet, iphone, Windows 11 Laptop (Firefox or Edge)

 

Installed locally on stand alone Windows 8.1 Desktop (windows doesn't have internet access) and on the same PC running Linux as  web based access either as installed locally or hosted 

Ah okay. The finished Readme is now up on Github so should contain all you need to know for Linux install: https://github.com/OpenHeatLoss/OpenHeatLoss You'll notice in the Readme that while the UI is completely functional there is room for improvement especially for mobile and accessibility. With the amount of input for a design, I'd hesitate to use it on a phone unless I was just accessing it for reference and using the survey checklist.

 

But obviously it's all ready to go with the hosted service

Edited 5 hours ago by SimonD

[marshian]

OK few questions

 

Rads

 

Adding radiators to the database - is that to a central database so others can look up or just for the project in question

 

What would I select as type for a 1800 x 578 double flat tube vertical rad

 

I know the kW rating at T50

 

I know the volume of the rad (because I measured it on a drain down - call me sad if you like) but how would I calculate the litres/metre

 

U-value (W/m²·K)

 

I know the U-value for my external walls

 

Brick 102mm Mineral wool in cavity 50 mm 100mm std aerated block 13mm plaster is 0.45 (well that's the closest approximation to what I have)

 

So on external walls I should select a U Value of 0.45 but I can't seem to change it from the default 0.10 am I muddling my units?