Cold Corner Office 1

[ChrisConley]

Hello All,

I have a question with many potential answers and I'd like to get any opinions that I can. I have a situation where a corner office is reporting that it is continuously too cold.

First off the climate is Canada, so our winter design temp is usually -40deg (C or F). The office is supplied ventilation air from an air handling unit that supplies tempered air to all the offices in that wing. The heating for the particular office comes from a fan-powered terminal unit with an electric coil. The room is around 65 square feet (6.04 m2), two windows 8 ft2 each(double paned, argon filled) facing east and south. It is ground floor, slab on grade, conditioned spaces above and on two sides. Wall construction is R20. Dimensions are W- 19.5 ft (5.96 m) L – 12.95 ft (3.95 m) H- 10.8 ft (3.28 m).

The terminal unit varies the tempered air (which is used for cooling in the summer and ventilation only in the winter) with a modulating damper. In the winter the damper closes to min position. At this point a call for heat will energize the fan in the terminal unit and energize the heating coil (single stage). The terminal unit then recirculates air in the space while adding a small amount of ventilation air.

The reason I'm asking the question is that there is no good reason for this room to be cold. The terminal unit provides 200 cfm (95 l/s) of air, and the electric coil is 2.5 kW (8530 Btu/hr). This means that we have over 130 (Btu/hr)/ft2 of heating (414 W/m2).

Does anyone have any thoughts on the matter? I've already examined a dozen possibilities, but I'd like to see if anyone could think of anything that I can't. The installation contractor, the controls contractor and the unit manufacturer have all inspected the situation and say that the system is working as designed, and that the design is to blame…I disagree.

Any thoughts?

 

[rworthen]

First. I don't understand the 65 SqFt if the room is 19.5 ft by 12.95 ft. Also you neglect to mention the ceiling but if you assume it is a heated second floor, the capacity of the system seems large enough. Here are some possibilities.

1. With a coil capacity of 8250 Btu/Hr, and 200 CFM you should have a discharge air temp of around 108 Degrees if the air into the coil is around 70 degrees. If the supply air coming from the main air handler is too cold - say 55 degrees - then the coil will produce supply air of around 92.5 degrees maximum. At 200 CFM this equates to only 4500 Btu/Hr of heating into the space. If this is the case check the control logic and how the contractor set up the minimum position on the dampers. Damper opening and air flow are not linear relations.

2. The supply air may be short cycling to the return air. Where is the diffuser located in relation to the return air inlet? Are they too close?

3. What is the wall construction. Were metal studs used?
If so the actual wall R value is much lower than the R20 you might expect.

3. What is the actual complaint? Is the room air near 72 but the room still "Feels" cold? It could be the Cold slab and you might want to look at removing any night set back to keep that slab warm.

Good luck and let us know what you finally figure out.

 

[ChrisConley]

I'm out to take measurements this afternoon to confirm those exact points. I dropped a couple of numbers when posting area and heat/area. Those of you that were paying attention caught that pretty quick (thanks rworthen).

1. First thing I confirmed was that supply air was not to cold. According to controls contactor supply temp is 70 or higher. What I plan on confirming is that the terminal unit closes properly on call for heat so that 450 cfm isn't being pushed through the diffuser.

2. That is on my list to confirm. As short cycling has been a problem elsewhere.

3. I need to confirm wall construction, but R20 was a conservative estimate.

4. Air temperature in the space can't seem to get over 60 F (16 C).

I'm out to physically verify dimensions/temperatures/flow rates/current draws this afternoon and I will update when I can. My hope was that someone out there could offer advice on things to check, which someone did, thanks rworthen, enjoy the star.

 

[DouginMB]

I has a similar situation once. It turned-out that the contractor had installed the wrong VAV box for the space. Also, you might want to verify that the space isn't under a negative pressure for some reason. Or that there is any source of infiltration.

 

[anandHVAC]

Hi,
This is anand here. I am working with Consultancy in INDIA
and responsible for HVAC system design.
Here in INDIA we only do cooling system. But, I am working
on one project where I have to provide heating system for
winter. Can u provide me information on how to calculate
winter heatload with CFM requirement.
Pls. do the needful.

 

[ChrisConley]

Anand,

I recommend that you repost this question as a unique question. However I do have a tip or two:

Winter design is usually much easier than summer design. You use many of the same formulae, but get to ignore a few factors. Solar heat gain is the main one you ignore, you also generally ignore an effects of people, equipment and lighting.

What I do is break the space into loads: conduction through walls, floor, windows and ceiling. And then add infiltration and heat required for ventilation. I have a little check sheet that I could email to you if you'd like.

Once you have the heat load for the space CFM is a factor of supply air temperature and room temperature. For instance if you require 130,000 Btu/hr of heating (38kW) and are supplying air at 110 deg F (43 deg C) and are trying to maintain 70 deg F (21 deg C) then you will require CFM = Q/(1.08 X delta T (deg F)) .... CFM = 130,000/1.08 * 40, CFM = 3010 cfm.

If this wasn't clear enough, then please repost your question.

 

[anandHVAC]

Chris,
Thanks for your respond. Can U e-mail me that check sheet u have maintioned.
My e-mail ID is anand.khadse@jacobs.com