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

[marshian]

1 hour ago, SimonD said:

 

Yes, of course, share the numbers.

 

66.2 m2 floor area (this includes the internal brick walls)

 

32.6 m External Walls

 

So 0.67?

 

[SimonD]

1 hour ago, marshian said:

 

66.2 m2 floor area (this includes the internal brick walls)

 

32.6 m External Walls

 

So 0.67?

 

 

32.6/66.2 (although you can just use room internal measurements) = 0.492

[SimonD]

3 hours ago, marshian said:

Using firefox

 

Windows or Linux machine? I'll test on my windows 11 machine to see if I can replicate. And put it in the bug list for further investigation.

[marshian]

8 minutes ago, SimonD said:

 

32.6/66.2 (although you can just use room internal measurements) = 0.492

 

Glad I checked - will have to get my head around that.

[marshian]

7 minutes ago, SimonD said:

 

Windows or Linux machine? I'll test on my windows 11 machine to see if I can replicate. And put it in the bug list for further investigation.

 

Windows

 

Just about to try it on android tablet (but I let the battery get flat so it needs a charge first)

[marshian]

On 13/04/2026 at 14:34, SimonD said:

The hosted version is best through PC & browser, but it will work on both tablet and phone through browser. 

 

Ref phone comment - yes it works but it's not optimised for the small view (may not help my phone is set for larger text view) - it's possible just not as easy as PC or Tablet

[marshian]

One question - might seem a bit stupid

 

In a two storey house Ground Floor gets the downstairs floor does it get the ceiling as well or does the upstairs floor get the ground floor ceiling as it's floor and the ceiling between it and the roof structure?

 

At the moment ground floor has ground floor and ceiling, first floor has floor and ceiling - concerned that this may be double counting?

Edited 5 hours ago by marshian
clarify

[SimonD]

32 minutes ago, marshian said:

One question - might seem a bit stupid

 

In a two storey house Ground Floor gets the downstairs floor does it get the ceiling as well or does the upstairs floor get the ground floor ceiling as it's floor and the ceiling between it and the roof structure?

 

At the moment ground floor has ground floor and ceiling, first floor has floor and ceiling - concerned that this may be double counting?

 

No, not stupid at all, just something you need to get your head round for heat loss calcs. In the 2 story:

- Ground floor gets 1 x Ground Floor (Slab) or (Suspended) + Ceiling

- 1st Floor get 1 x Floor + Ceiling

You're not really double counting because if both rooms have the same design internal temp, the Design dT is set to zero on both the ceiling of ground floor and floor of 1st floor so there are no losses between them. If there is a difference in internal temp, you just need to keep in mind that the Design dT becomes either + or - the difference. So if one room is 21C and the other 18C, it's +/-3 either way on floor & ceiling. 

You'll not know this now because the standard U-values aren't in the global database yet, but one thing to keep in mind is that the floor always has a lower U-value than the Ceiling for the same buildup - The CIBSE standard for plasterboard, joists, floorboards, with 100mm insulation is 0.32 I think. The same buildup as a floor is 0.25.

 

Hope that clarifies it? 

 

Edited 4 hours ago by SimonD

[marshian]

OK Finished

 

 

 

Two Warnings

 

I think I understand both are due to the internal nature of the Hall and Landing being centre of house so no external wall ticked

[marshian]

U Values used

 

[marshian]

So now I need to understand why the result (if I am reading it right) at 12 kWh is 3 times my current heat loss 🙂

 

Current heat loss is just under 4.0 kWh at -2.5 Deg C

 

So I've definately made an error somewhere

[marshian]

23 minutes ago, SimonD said:

You'll not know this now because the standard U-values aren't in the global database yet, but one thing to keep in mind is that the floor always has a lower U-value than the Ceiling for the same buildup - The CIBSE standard for plasterboard, joists, floorboards, with 100mm insulation is 0.32 I think. The same buildup as a floor is 0.25.

 

heat rises easier than it falls??

[marshian]

Rad Evaluation at my Target Flow Temps

 

 

 

Edited 3 hours ago by marshian
Fix missing rad

[SimonD]

4 minutes ago, marshian said:

Rad Evaluation at my Target Flow Temps

 

 

 

 

14.83kW emitter sizing load? That seems massive. Something doesn't seem right there. Do want to show some of the room inputs? Or PM me screenshots?

[marshian]

4 minutes ago, SimonD said:

 

14.83kW emitter sizing load? That seems massive. Something doesn't seem right there. Do want to show some of the room inputs? Or PM me screenshots?

 

Sent you a PM

[marshian]

Amusingly for me anyway the total actual Rad output is spot on for my heat loss

 

I know I've cocked up somewhere just can't see where 🙂

[SimonD]

2 minutes ago, marshian said:

Amusingly for me anyway the total actual Rad output is spot on for my heat loss

 

I know I've cocked up somewhere just can't see where 🙂

 

Here you go. This is a lot better. Not quite there yet, but about 1/2 of what you were seeing, but as you can see the calculated fabric losses of 4.22kW are much closer to yours, but this is at -4.6 which is the conservative MCS post code lookup:

 

In the rooms editor, there is a column labelled Design dt (C). When you add a new element, it default to the Design dT of room temp - outdoor design temp. Then next to it is another column for ref dT which is the default delta T between room temp and Typical Outdoor temp.

For all internal elements this needs to be set to the difference between internal rooms, or 0 if there is none. I've just gone and updated all your internal settings to this, which has made the difference. I may have incorrectly amended some of your insulated ceiling which may be external?

I've then gone and change the outdoor design temp from -4.6 to -2.5 and as you can see, the fabric loss is now down to 3.84kW and generator sizing is 5.26kW:

 

The total emitter sizing is quite large compared to what you're experiencing with your low flow temps etc. which means I need to dig into the calculations to make sure I haven't got something wrong in there, but I suspect that because of the way it has been implemented, there's been a cautious approach taken by CIBSE in how they've implemented the new method. It wouldn't surprise me and is actually a good thing as it's better to upsize rads for worst case scenarion and reap the benefits of lower flow temps over the long term. But I will need to check.

I'd also recommend you go over your element inputs just to check the assignment of U-values from your U-value library and some didn't quite match.

 

Hope that helps?

 

[marshian]

13 minutes ago, SimonD said:

 

Here you go. This is a lot better. Not quite there yet, but about 1/2 of what you were seeing, but as you can see the calculated fabric losses of 4.22kW are much closer to yours, but this is at -4.6 which is the conservative MCS post code lookup:

 

In the rooms editor, there is a column labelled Design dt (C). When you add a new element, it default to the Design dT of room temp - outdoor design temp. Then next to it is another column for ref dT which is the default delta T between room temp and Typical Outdoor temp.

For all internal elements this needs to be set to the difference between internal rooms, or 0 if there is none. I've just gone and updated all your internal settings to this, which has made the difference. I may have incorrectly amended some of your insulated ceiling which may be external?

I've then gone and change the outdoor design temp from -4.6 to -2.5 and as you can see, the fabric loss is now down to 3.84kW and generator sizing is 5.26kW:

 

The total emitter sizing is quite large compared to what you're experiencing with your low flow temps etc. which means I need to dig into the calculations to make sure I haven't got something wrong in there, but I suspect that because of the way it has been implemented, there's been a cautious approach taken by CIBSE in how they've implemented the new method. It wouldn't surprise me and is actually a good thing as it's better to upsize rads for worst case scenarion and reap the benefits of lower flow temps over the long term. But I will need to check.

I'd also recommend you go over your element inputs just to check the assignment of U-values from your U-value library and some didn't quite match.

 

Hope that helps?

 

 

Bloody amazing Simon - thank you

 

I'll go in and check out where you've made the changes

[SimonD]

30 minutes ago, marshian said:

 

Bloody amazing Simon - thank you

 

I'll go in and check out where you've made the changes

 

No worries. Thank you!

[marshian]

OK checked out the changes - made a couple of tweaks now I understand the logic

More stupid questions 6 rooms upstairs all have doors then I treated the landing as a room and put 6 doors in there..........

 

Ceiling for all upstairs rooms is plasterboard & then 300mm Insulation and over boarded - temp in the loft is always 2 to 3 deg above OAT should I be lowering the delta from room to ceiling

 

Same with the temp below the suspended ground floor it's always higher than OAT

Edited 1 hour ago by marshian
spelling

[marshian]

Last point - pretty sure it's ventilation losses that are driving the higher calculated heat loss

 

Fabric is now 4144 W

Ventilation is 3090 W

 

Anyway to reduce that ventilation factor? I guess I see a blower test in my future

[SimonD]

2 minutes ago, marshian said:

More stupid questions 6 rooms upstairs all have doors then I treated the landing as a room and put 6 doors in there..........

 

That's actually what I do when I have a space like a hall or landing and when I have rooms with large open plan openings, so I create a non-existent door - as long your room dTs match, and set to 0 then it doesn't impact the fabric losses. It also shouldn't affect the ventilation losses because the exposed envelope is set and the calculation uses that.

 

6 minutes ago, marshian said:

Last point - pretty sure it's ventilation losses that are driving the higher calculated heat loss

 

Fabric is now 4144 W

Ventilation is 3090 W

 

Anyway to reduce that ventilation factor? I guess I see a blower test in my future

 

It's a bit prescriptive on the ventilation, so there isn't anything immediately available to make adjustments to this ventilation factor. There are still some questions about how BS EN 12831:2017 treat ventilation and the CIBSE guide uses a bit of SAP in there. I'm going to have a think about this one. It's really the way it's calculating infiltration leakage which is giving you the high figure, but you've got to be a bit careful how you interpret the Ventilation - Emitter Sizing. This calculation isn't saying that you are going to use this amount of energy at a given time, which is your Fabric loss and Ventilation - Generator Sizing. The Ventilation - Emitter sizing, although provided as a total, should really be looked at in relation to individual rooms because it's essentially calculating the effect of high winds on each room based on the exposed area. So at any given time the rooms won't be losing all heat to the outside, but transferring to other rooms within the building. But high infiltration will increase the heat load within the room subject to high infiltration and so the standards are encouraging us to install slightly larger radiators and thus improve efficiency. Does this make sense? 

[SimonD]

32 minutes ago, marshian said:

Ceiling for all upstairs rooms is plasterboard & then 300mm Insulation and over boarded - temp in the loft is always 2 to 3 deg above OAT should I be lowering the delta from room to ceiling

 

Same with the temp below the suspended ground floor it's always higher than OAT

If those are known quantities, then yes, change the dT. The standards just ask for default values, so an MCS design requires the use of the default Design Outdoor Temp as the difference with suspended floors. There are some aspects where deviation is okay for MCS designs but you have to be prepared to justify them if asked.

[marshian]

6 minutes ago, SimonD said:

That's actually what I do when I have a space like a hall or landing and when I have rooms with large open plan openings, so I create a non-existent door - as long your room dTs match, and set to 0 then it doesn't impact the fabric losses. It also shouldn't affect the ventilation losses because the exposed envelope is set and the calculation uses that.

 

OK that makes sense - I considered removing all the doors from the landing 

 

the whole landing, hallway stairwell area is also a bit tricky I broke it into chunks to make it easy for me as they are all linked but only one radiator (albeit a big one) is in the lower hallway (so the tool says it's oversized) however if I add all the reqts together it's in line with the other rooms

 

It used to be a 550 by 800 T11 - it's now a 500 x 1400 T22 

 

6 minutes ago, SimonD said:

It's a bit prescriptive on the ventilation, so there isn't anything immediately available to make adjustments to this ventilation factor. There are still some questions about how BS EN 12831:2017 treat ventilation and the CIBSE guide uses a bit of SAP in there. I'm going to have a think about this one. It's really the way it's calculating infiltration leakage which is giving you the high figure, but you've got to be a bit careful how you interpret the Ventilation - Emitter Sizing. This calculation isn't saying that you are going to use this amount of energy at a given time, which is your Fabric loss and Ventilation - Generator Sizing. The Ventilation - Emitter sizing, although provided as a total, should really be looked at in relation to individual rooms because it's essentially calculating the effect of high winds on each room based on the exposed area. So at any given time the rooms won't be losing all heat to the outside, but transferring to other rooms within the building. But high infiltration will increase the heat load within the room subject to high infiltration and so the standards are encouraging us to install slightly larger radiators and thus improve efficiency. Does this make sense? 

 

Makes perfect sense - before I insulated between the joists of the ground floor with 75mm PIR you could feel a draft in most rooms when the wind got up - now it can be a bloody good gale outside and there is no drafts at all........ I know I've got the house reasonably well sealed hence the installation of a PIV unit to manage humidity and ACH's (But it's not MVHR  )