air change rates, rooms with open flues

[mike_s]

Hi, I've just done heat loss calculations from the domestic heating design guide book. I'm very happy with the method and the resulting rad sizes sound about right but I think over generous allowances for air changes might be oversizing the rads. In the living room as I've put a hole in the wall gas fire in, it's a flue box with a 5 inch flue liner.

The heating design guide states that living rooms should normally be 1.5 air changes per hour but for rooms up to 40 M3 with an open flue up to 200mm x 200mm then it should be 3 air changes/hour.

The room is 4.2 x 4.2 x 3m. It's a top floor flat with double glazing and insulted roof. It states the heat loss is 2,600 watts allowing 15% extra for heat up times. This means if I want to set my system up with a DT of 39, then I need a radiator with a listed output of around 3,500 watts at DT50. Thats equivalent to a 2000 x 600 k2.

It doesn't sound too over the top I suppose but I'm interested to hear what you allow for air changes as these can have a big impact on the required rad size.

For my other rooms the design guide states 1 air change/hour for bedroom, 2 for kitchen, 2 for hall and 3 for bathroom.

These air change rates sound a little dubious and not likely in most cases. Do you apply your own air changes rates from experience? I think common sense says I could change it for 2 to the living room or even 1.5 all around.

What rates do you use? I don't want to use online or mears calculators as I feel this way is best and just as easy.

 

[josh]

design temp of 39? that being the tempreture of the radiator or am i wrong?
i know its better to run temps lower to increase condensing but sounds a bit low to me and will obviously make the rad a much bigger one.
We just use a mears calculator, never had any comebacks with people saying their cold and the rads never are to small/to big for the room.

 

[Deleted member 32524]

plumb centre has an easy one. Gives you it room by room breakdown.

 

[plumber0808]

Hi, I've just done heat loss calculations from the domestic heating design guide book. I'm very happy with the method and the resulting rad sizes sound about right but I think over generous allowances for air changes might be oversizing the rads. In the living room as I've put a hole in the wall gas fire in, it's a flue box with a 5 inch flue liner.

The heating design guide states that living rooms should normally be 1.5 air changes per hour but for rooms up to 40 M3 with an open flue up to 200mm x 200mm then it should be 3 air changes/hour.

The room is 4.2 x 4.2 x 3m. It's a top floor flat with double glazing and insulted roof. It states the heat loss is 2,600 watts allowing 15% extra for heat up times. This means if I want to set my system up with a DT of 39, then I need a radiator with a listed output of around 3,500 watts at DT50. Thats equivalent to a 2000 x 600 k2.

It doesn't sound too over the top I suppose but I'm interested to hear what you allow for air changes as these can have a big impact on the required rad size.

For my other rooms the design guide states 1 air change/hour for bedroom, 2 for kitchen, 2 for hall and 3 for bathroom.

These air change rates sound a little dubious and not likely in most cases. Do you apply your own air changes rates from experience? I think common sense says I could change it for 2 to the living room or even 1.5 all around.

What rates do you use? I don't want to use online or mears calculators as I feel this way is best and just as easy.

600 high 1400 long double should suffice.

 

[AWheating]

last time i did cals i used 1.5 all round and 3 for rooms with mechanical extraction from memory. its been a while since i did them by hand as ive not fitted a full system on my own work for a long time. ive never increased the air change for an open flue tbh.

 

[mike_s]

Sorry to clear up I meant a Delta T of 39 which is a boiler flow at 70 and the return at 50 making the mean water temperature 60. The design temp is 21 making it 39 Delta T. Rads are usually listed with outputs calculated at 50 Delta T which is a bit high for a condensing boiler. 39-45 is more like it so you divide the listed output by 0.735 to find the output which will be lower. Hence needing a bigger radiator than listed to get the required output.

Like I said I'm very happy with the methods in the design guide, apart from I think an air chage rate of 2 per hour is more realistic than the advised 3 for a living room with an open flue, especially as in my close it is only a 5M 5 inch liner.

Any more comments on air change rates or other reputable methods?

I wouldn't increase the air change rate in the calculation for a cooker hood in a kitchen as the heat lost through the extractor is made up for with the heat from cooking, common sense that one really.

 

[dancinplumba]

a cooker hood, if piped outside will still take air out of a room even if cooker not on imo

 

[chris watkins]

Hi Mike, Like you I do proper heatloss calc's, I think, as you say you have to use a bit of common sense as a designer with these figures. You really have to look at it on a case by case & what appliances are going to be installed (a lot of different between a DFE & a small room heater, even if fitted into a 9x9) I just work between the fig's in the DHDG range.
P.S have you tried the Myson heatloss program very good & free from them, similar to DHDG as to the inputs, worth a go.

 

[chris watkins]

a cooker hood, if piped outside will still take air out of a room even if cooker not on imo

I think thats why the recommended ACR for kitchens is 2 & not 1 / 1.5 as for most other rooms, but you are right some of these very big one with 150mm duct must do more ?

 

[mike_s]

Cheers Chris, well last night it was quite windy and I could hear the draught up the flue so maybe an air change rate of 3 isn't too far off. I wouldn't argue with people who would suggest a maximum of 2 which I may do myself in future if the radiator looks unnecessarily large.

I have ordered my radiators now, they are modern black vertical ones. Only the living room one is a little short on heat output but went for the largest I could afford in this design, an 1800 x 500 double. Seeing as I've been generous with extra allowances on my calculations I'm sure it will be enough, especially with the other rads all sized and positioned well. I've got the fire for back up and can always whack the boiler on maximum if I need to throw my 39 delta t out the window.

Interesting discussion on kitchen rates too. I will probably duct my hood to outside when I put my kitchen in, I thought you could close them when the cooker isn't on? I've used an ACR of 2 for kitchen anyway.