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 large amount of glass surface, more radiating surface is necessary. In such cases, and especially where a large amount of ventilation is desired, it is necessary to see that the quantity $$V\,$$, as obtained above, is equal to the amount of air required for ventilation. It will be found sufficient in all ordinary cases to change the air four times per hour, which is generally satisfactory for private houses; but where much entertaining is to be allowed for, six times per hour is better. In designing indirect radiators it is necessary to be very careful in the proportion of flues, but such details of construction will be considered in the next chapter.

In proportioning direct-indirect radiators the same rules apply as for the indirect type, although their action as direct radiators may be counted on to some extent. Where this kind of radiator is used in connection with an exhaust ventilating system very good results are obtained by using the author's formula for direct radiators, with an addition to the $$0.008 C\,$$ of $$2/3 K \div 55\,$$, where $$K\,$$ is the cubic feet of air per hour required for ventilation. This gives additional surface necessary to heat $$2/3 K\,$$ cubic feet of air from the outside temperature to that of the room. The author figures on $$2/3 K\,$$ (in some cases ¾), as in extreme weather the degree of ventilation may be somewhat reduced. For these radiators also, the factor $$a\,$$ in the formula may be taken as 1.9 or 2.