Commercial ventilation question

[mcspoo]

If you have ducted ventilation to a boiler room, does the difference in height between high and low level ducts, in the boiler room, need to be replicated outside?
i.e. does the high level, external, grille need to be further up the wall than the low level external grille.

Cant find any info to confirm this but my manager is adamant.

 

[Bernie2]

I would assume so. The lower is for cool air and the higher for hot. On the principle that heat rises and cold falls then the highest vent should vent out highest and the cold air intake be lowest.

 

[chris watkins]

I would say not, so long as they have the height difference inside & the grilles are not side by side on the out side (not short cycling between in & out) then there will be air flow through the system. The warmer air will be removed from the room by the cooler air being ducted down to low level.

 

[Dotty]

No. Providing the ducting's been sized properly taking into account the resistance of its length and that the grills are allowing the correct amount of free air.

Your manager gsr?

Or one of the ones who thinks knowledge is infectious because he manages a few gsr's?

 

[Bernie2]

Hmm!

Interesting but what about cross flow if both vents are level? What is to stop the hot air exhaust being drawn into the cold air intake except heat is drawn to cold so it is assumed the hot kitchen air would vent to the cooler outside? But then the cooler outside air would not be drawn into a hot kitchen anyway so perhaps it would be pointless having two vents, just increase the size of the one.

I have been in commercial kitchens where it has been so hot through a lack of cold air cooling the cook has virtually melted on a hot day.

Surely we are not talking about dismissing the basic laws of science here. If of course the system had a means to cool the incoming air by all means they could be sited side by side and the intake cooled physically instead of naturally, but what if the fan breaks down and the cost of the power to run the cooling system?

No the basic principles of hot air rising and cool air falling must rule.

 

[chris watkins]

I would say not, so long as they have the height difference inside & the grilles are not side by side on the out side (not short cycling between in & out) then there will be air flow through the system. The warmer air will be removed from the room by the cooler air being ducted down to low level.

Hi Bernie2
Already done the short cycling bit & as far as dismantling the laws of science I think we are just getting some of them confused, heat doesn't flow out of the hot kitchen (or any other room & for that matter gasses up a chimney) because they are attracted to the cold outside, they are pushed out by the cooler more dents air coming in. The warmer air is lighter, as it has expanded, so we can see that they lie to us at school when they say it rises co's it don't, it is pushed by the cold air falling.
We have to take account of friction as croppie reminds us for the system to work but what ever you do, do not do away with the two ducts or vents as it will not work. I have seen this first hand a large single duct with high & low grilles cut into it & no air flow whats so ever.

 

[stevetheplumber]

got to agree with chris it the position in the boiler house that will cause the flow how they terminate would have no effect

 

[Dotty]

Just double checked my logic book. And I quote!

"Balanced comprtments: For open flued appliances only, air is entrained via a duct located adjacent to the chimney outlet terminal, this air is dispersed at low level, a 'room sealed' effect is created."

As for drawing warm air in were there the two terminations side by side, providing the ductwork and grills were sized properly and accordingly then the maximum permissable temperatures in the boiler house wouldn't be reached -

Floor level 25degC
Mid level 32degC
Roof level 40degC

 

[Bernie2]

I was of course quoting the simple explanation of hot air rising not displacement.

I know its misleading but in general it is probably more easily understood that way. One also has to remember that nature abhors a vacuum so hot air rising causes cool to rush in. You could say its the cold air pushing it out but the displacement would not even start if the hot air did not start to rise. And of course it rises through colder air. Hot air can only rise through colder air, if the hot air never rose then the cool could not rush in. Same happens in a cylinder full of hot water, the cold comes in only if the hot is displaced first. It does of course have to do with the lighter density of hot air, that is why hot air rises in the first place. In general though heat is attracted to cold. It is why we calculate using thermal loss not gain and measure loss going outwards through the fabric. I

Its not attracted to colder air in a physical way its just that you have to have temperature differences to make it rise.

A fires flue gasses get hot and so lighter they then rise up the flue to be replaced by air drawn from the room. If you do not have air with oxygen entrained to replace it you get vitiation because of a lack of oxygen which is well known. Generally though hot air is said to rise and colder sink.

In the case of ventilation you want cool air at the bottom of the room and hot air taken out at the top. So a high and a low vent are required. Do not forget the air in a ventilation duct is usually for cooling purposes so you want a good temperature difference.

 

[Bernie2]

I was of course quoting the simple explanation of hot air rising not displacement.

I know its misleading but in general it is probably more easily understood that way. One also has to remember that nature abhors a vacuum so hot air rising causes cool to rush in. You could say its the cold air pushing it out but the displacement would not even start if the hot air did not start to rise. And of course it rises through colder air. Hot air can only rise through colder air, if the hot air never rose then the cool could not rush in. Same happens in a cylinder full of hot water, the cold comes in only if the hot is displaced first. It does of course have to do with the lighter density of hot air, that is why hot air rises in the first place. In general though heat is attracted to cold. It is why we calculate using thermal loss not gain and measure loss going outwards through the fabric.

Its not attracted to colder air in a physical way its just that you have to have temperature differences to make it rise.

A fires flue gasses get hot and so lighter they then rise up the flue to be replaced by air drawn from the room. If you do not have air with oxygen entrained to replace it you get vitiation because of a lack of oxygen which is well known. Generally though hot air is said to rise and colder sink.

In the case of ventilation you want cool air at the bottom of the room and hot air taken out at the top. So a high and a low vent are required. Do not forget the air in a ventilation duct is usually for cooling purposes so you want a good temperature difference.

 

[chris watkins]

Bernie
Sorry but I can't let this one lie on the mistaken fact that somehow warm air or water is attracted towards cold. I Have no problem with heat flow, hot to cold & heat loss all of which are conduction.
What I think you need to have a look at it convection, hot or warm air in it self will not move (rise) the motive force is the more dents cooler air rushing in to push it up, if you have an open flue system from our fire but it is in a sealed room i.e. no cooler air can get in, then the flue gases will not leave the flue pipe no matter how hot they become.

 

[Bernie2]

But we are not talking about a sealed room we are talking about high and low ventilation and how it should work. And the cross flow you can get if two external grills are placed side by side. We are not talking balanced flue boilers either, for all intents and purposes they are sealed rooms. Ventilation air is meant for atmospheric cooling and breathing as well as air supply.

And of course an open flue fire in a sealed room would not start the process because there is no cooler air to replace the hot air so no vacuum for the cooler to fill. Then what air there is, the open flue appliance removes the oxygen from it in the burning process and it becomes oxygen reduced or as its called vitiated.

But hot air cannot rise without a temperature difference. Its also why they give a short warm up time to an open flue before its smoke tested. The warming helps the flue gases rise. But you must have temperature differences for that to happen, it is most easily explained in hot flows to cold.


If you used a hot air balloon and warmed it only to the ambient air temperature would it rise? No probably not, the air in it would have the same density as the surrounding air it would not be lighter. Its because warm air is lighter than cold that the warm rises. As said its not attraction in that cold attracts to hot like a magnet attracts ferrous metals its temperature difference.

Actually I think we are both talking about the same thing Chris but coming at it from different angles.