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 Sunday at 22:46 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 14 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?

[SimonD]

1 hour ago, marshian said:

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

 

Any radiators you add go into your own 'company' radiator database linked to you as a user, so if you had several projects, you'd build up your own database of radiators. Nobody else has access to those or can edit/change them, they're yours. In the wider version for managing several projects, there's even a project settings page where you can add edit and delete you own database. If you want access to this wider version with multi-project capability, ping me your registered email address and I'll change your settings to 'beta' test engineer in the system.

 

1 hour ago, marshian said:

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

 

I'll add flat tube radiators to the type as a system addition, but for now, two column would work fine - the type doesn't really matter that much, it's more for your own reference. The important bits are output at DT50 and water volume really. And then size if you're putting a new one into a space...

 

1 hour ago, marshian said:

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

 

 

Not at all, I've done that to check volume on existing rads myself, but luckily most rad manufacturers now provide l/m figures in the tech spec. Stelrad actually even provide Kv. To get the l/m min take your total measure volume and divide by length in meters - so in your case the 0.578m

 

1 hour ago, marshian said:

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?

 

Best thing to do in the workflow is add all your standard elements into the U-value tab and create your U-value library. Do this especially for all your floor U-values by using the Floor U-value calculator as floor U-values vary according to the Perimeter/Area ratio. Once that's in you'll get an automatic drop down when you add an element in the rooms tab. These will then auto-populate and add the thermal bridging addition. I've been back and forth on the editing of value directly in the room as if you skip creating the U-value library and you're working on a large project, inputing into individual room elements gets very tedious, which is partyl why I locked it to force users back to the U-value library. I ought to put a user hint in there about this.

 

The other bit to bear in mind here is that if you add a door or window to the room, you'll get another automatic drop-down where you can select which element area this door or window needs to be subtracted from. So if you have an external wall added, this will be listed in the dropdown and you just apply it. The reason to do this is because this will apply the correct calculation for fabric loss, but at the same time used the full external element area for the ventilation & infiltration calculation.

 

To make things easier to follow, I tend to add the wall element, then add any corresponding window or door to that element and subtract it. Then move onto the next element.

 

Then in the elements also remember to change any internal element's Design Delta T (e.g.partition wall) to the difference between the two adjacent internal spaces and not leave it at default.

Elements roof, ground floor suspended, external wall are all automatically added to exposed envelope so if you have a ceiling that is the external envelope, then you need to add this to exposed area by selecting Exp.

 

I have on my to do list a load of videos to walk new users through the tool. If you spend a lot of time inputing this stuff, it follows pretty much a normal design workflow but it's not obvious at all for someone not in the industry.

 

Edited 8 hours ago by SimonD

[marshian]

20 minutes ago, SimonD said:

Any radiators you add go into your own 'company' radiator database linked to you as a user, so if you had several projects, you'd build up your own database of radiators. Nobody else has access to those or can edit/change them, they're yours. In the wider version for managing several projects, there's even a project settings page where you can add edit and delete you own database. If you want access to this wider version with multi-project capability, ping me your registered email address and I'll change your settings to 'beta' test engineer in the system.

 

email sent - to be honest adding rads has been my biggest issue - doesn't seem to want to save them!

 

20 minutes ago, SimonD said:

I'll add flat tube radiators to the type as a system addition, but for now, two column would work fine - the type doesn't really matter that much, it's more for your own reference. The important bits are output at DT50 and water volume really. And then size if you're putting a new one into a space...

 

I know all my rads are pretty much spot on size wise for flow temps I'm using and house heat loss - I've made all the mistakes and learnt from it - I'll use two column 

 

20 minutes ago, SimonD said:

Not at all, I've done that to check volume on existing rads myself, but luckily most rad manufacturers now provide l/m figures in the tech spec. Stelrad actually even provide Kv. To get the l/m min take your total measure volume and divide by length in meters - so in your case the 0.578m

 

So even with a vertical rad that is 1800 mm high you divide the volume by the width?

 

I get that for a panel rad it would work that way but thanks - good learning point

 

20 minutes ago, SimonD said:

Best thing to do in the workflow is add all your standard elements into the U-value tab and create your U-value library. Do this especially for all your floor U-values by using the Floor U-value calculator as floor U-values vary according to the Perimeter/Area ratio. Once that's in you'll get an automatic drop down when you add an element in the rooms tab. These will then auto-populate and add the thermal bridging addition. I've been back and forth on the editing of value directly in the room as if you skip creating the U-value library and you're working on a large project, inputing into individual room elements gets very tedious, which is partyl why I locked it to force users back to the U-value library. I ought to put a user hint in there about this.

 

Ahh this makes sense now - thank you I'll do that next time I'm back at it

 

20 minutes ago, SimonD said:

The other bit to bear in mind here is that if you add a door or window to the room, you'll get another automatic drop-down where you can select which element area this door or window needs to be subtracted from. So if you have an external wall added, this will be listed in the dropdown and you just apply it. The reason to do this is because this will apply the correct calculation for fabric loss, but at the same time used the full external element area for the ventilation & infiltration calculation.

 

To make things easier to follow, I tend to add the wall element, then add any corresponding window or door to that element and subtract it. Then move onto the next element.

 

That was my approach after I screwed up the first room and didn't know which external wall I was adding a window too

 

20 minutes ago, SimonD said:

Then in the elements also remember to change any internal element's Design Delta T (e.g.partition wall) to the difference between the two adjacent internal spaces and not leave it at default.

Elements roof, ground floor suspended, external wall are all automatically added to exposed envelope so if you have a ceiling that is the external envelope, then you need to add this to exposed area by selecting Exp.

 

I have on my to do list a load of videos to walk new users through the tool. If you spend a lot of time inputing this stuff, it follows pretty much a normal design workflow but it's not obvious at all for someone not in the industry.

 

As I said - I like it and I haven't broke it yet which is what I'm normally paid to do 

 

[SimonD]

36 minutes ago, marshian said:

mail sent - to be honest adding rads has been my biggest issue - doesn't seem to want to save them!

 

Thanks for letting me know. That's my fault. I was playing around with users and authorisation earlier to create the new beta engineer test and in doing so seemed to break how radiators are saved to specific users - it was saving fine for anonymous but not any registered users! Anyway, it should be fixed now 😊 

41 minutes ago, marshian said:

So even with a vertical rad that is 1800 mm high you divide the volume by the width?

 

I get that for a panel rad it would work that way but thanks - good learning point

 

Yeah, the width is actually termed length in catalogues so you always have dimensions = height by length e.g. 600x1200 or 1800x445

42 minutes ago, marshian said:

As I said - I like it and I haven't broke it yet which is what I'm normally paid to do 

Glad to hear you're getting on with it okay!

[marshian]

1 hour ago, SimonD said:

Thanks for letting me know. That's my fault. I was playing around with users and authorisation earlier to create the new beta engineer test and in doing so seemed to break how radiators are saved to specific users - it was saving fine for anonymous but not any registered users! Anyway, it should be fixed now 😊 

 

Ahh good to know - I tried various methods and none seemed to work (As a registered user) 

 

1 hour ago, SimonD said:

Yeah, the width is actually termed length in catalogues so you always have dimensions = height by length e.g. 600x1200 or 1800x445

 

OK I get you but for me heat emitters are all about surface area (especially at low flow temps), flow rate and volume of water

 

Take my 1800 x 578 vertical it has a volume of water of 10 Litres

 

10 litres divided by 0.578m width = 17.3 l/m

 

10 litres divided by 1.8 height = 5.5 l/m

 

Compare that to my K33 1400 x 700 with a capacity of 14 Litres

 

14 litres divided by 1.4 = 10 l/m

 

14 litres divided by 0.7 = 20 l/m

 

OK now I bloody get it - ignore me I'm stupid - it's basically looking at how much volume you have per m as a factor and you need to always use the width...................

 

 

 

1 hour ago, SimonD said:

Glad to hear you're getting on with it okay!

 

You've clearly put a lot of effort into it - it seems quite polished already - I do like the "not to current regs" pointers (I've got an 80's house - extended in naughties so stand no chance of being to current regs but I've improved it where I can