Dry Bulb Temp (ASHRAE) 1

[Zeroman]

I am trying to size a heater for a building in a cold environment. I am not sure what outdoor temp I should use from ASHRAEs "Heating and Wind Design Conditions". Should I use "Heating DB 99.6%" or use "Coldest Month MDB 1%"

Thanks for your help

 

[walkes]

We typically use the 99.6% Heating Dry Bulb condition. There may be some lee-way depending on how well you know your climate. Design the base system around the 99.6% mark and allow for supplementary heating for those days when the temperature is colder. eg. The 99.6% DB for Winnipeg, Manitoba Canada is -27°F, during the few weeks in January when we may hit -40°F the supplementary heating condition may be to reduce the ventilation to the space, the bodies in the space and the lighting during occupied hours may also be sufficient to provide supplementary heat.

As long as the 99.6% condition is designed for assuming no internal heat gains you should be ok.

 

[Zeroman]

Thanks Walkes,

I appreciate your help very much. The system is for Fort Mac, a 99.6%DB of -34 deg C.

Dave

 

[MintJulep]

The introduction of Chapter 27 of ASHRAE Fundamentals explains what each of the columns is typically used for.

The MDB information in the Coldest month column should be used in conjunction with the associated wind speed and direction data provided.

Is this a new or existing building? Is it tight or leaky? Leaky building envelope and high winds will increase the load.

 

[quark]

I like Walkes idea. No. of hours the heating DB is offset during a year is 35.04 and total hours coldest month DB ocuuring is 87.6. So you will have problems with 87.6-35.04 = 52.56 hours or 2.19 days. Just check the enthalpy difference for the both ambient conditions and if the difference is not significant, you can go for any reference temperature. If it is substantial then go for 99.6% value with additional heaters during problematic hours.

Regards,

 

[ChrisConley]

I agree in principle with Walkes, but this last winter where I am, we had a week long stretch of -44 deg C (our 99.6% Design is -35 deg C) Many of our customers with higher ventilation requirements (restaurants, etc.) called to complain that it was cold inside...

Usually we responded; well, it's really cold outside eh?

We then recommended that they drop their ventilation rates for a little while until the cold snap passed.

**If you are not part of the solution, then you are part of the precipitate.**

 

[MintJulep]

-35 C design temp? Crap, that's freaking cold.

Just go someplace warm for the winter