Maintain Temperature vs Colling Off & Reheating

[SilverGhost]

I'm hoping to sit down and run some calculations as soon as I have more information about how to system is running, but I was wondering if anyone had a quick, general answer as to which would be more efficient: Maintaining a set temperature or letting a room cool to an unoccupied set-point and reheating it back up again when necessary.

I'm working on a new LEED building and this is concerning the control of the temperature for 2 floors of a hotel that are filled with different ballrooms/boardrooms.

The rooms aren't occupied 24/7 (nor are they currently being used every day), yet the current approach is to maintain the rooms @ 70F rather than to let them cool down (to ~65-68) and then reheating them prior to meetings.

These are large spaces, roughly 3,500-10,000 sq ft w/16-20 ft ceilings, with the HVAC boxes pumping air out of the ceiling. There will be at least 8 hrs of down time at night (if not more like 10-15). The constant air flow that isn't heated or cooled maintains between 55-65.

Any quick thoughts?

 

[SilverGhost]

Also, any documentation surrounding this topic would also be of great use. I've tried using a couple search engines, but wasn't able to find anything too reputable or sound.

Thanks

 

[berkshire]

The general answer would be " it depends"

Factors involved would be the recovery time of the room after the command for heat, versus the down time or idle time on the room. Another factor would be ajacent rooms drawing heat or cool through thin dividing walls.
B.E.

 

[KiwiMace]

The likely answer is that you will save energy. Setback is used throughout the industry as an energy saving scheme. The frequency of occupation will have a big effect on any savings to be made.

Ideally this should be managed with a room booking system feeding data to the BMS. The BMS can optimise the restart, saving the human factor of getting someone up to restart the system manually at the right time.

Consider your requirements for adequate purging after each use.

 

[IRstuff]

Probably...

It will depend on several factors:
> How well is the volume insulated and isolated in an absolute sense from its environmental surroundings.

> What is the ratio of heat loss relative to total thermal mass.

> What is the relative temperature difference between the room and its environment



TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[walkes]

The thermal mass of the space will have an effect as well. We have had intances with night setback on a school that would take until noon on Monday to get back to setpoint after the weekend.

 

[Drazen]

Precise answer can only be reached by thermodynamic modelling, as you have several confronting factors involved: additional heat losses occur during reheating, and you also have internal heat losses from neighboring spaces, so neighboring spaces heat loss actually increases during setback period.

You have several simpler cases when decision is less complex: school building is out of operation for the whole weekend and whole building is on setback temperature, so things are easy.

At home, you can define yourself fixed period of night setback, and reaheat time as well.

In hotels, there could be many different users or persons with authority to operate HVAC controllers, which can make things complex.

You have commercial software that can calculate "good guess", but you can go easiest with monitoring in actual practice.

Designer needs to specify controllers with setback temperature setting option, that is where his responsibility lays and building managers should do the rest: apply different scenarios and monitor actual energy consumption.

I would not recommend that designing includes calculation for additional capacity for quick reheat, that itself leads to much larger equipment than needed (often 20% larger). German standard for heat loss had that for years, but they gave it up several years ago.

Because years of cost-benefit analysis showed: if you cannot reheat with wyour standard equipment quickly enough, setbeck is overkill in investment-payback terms.

 

[IRstuff]

One additional factor that I left off my list is that many hotels have their meeting rooms on the second or third floor, so a temperature setback actually does nothing from a heating or cooling perspective, because the rooms are completely immersed with the infrastructure of the hotel's environment. What you would save on, in that case, is strictly the AHU operational cost, since there would be no need to provide much in the way of fresh air, or remove humidity.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[cry22]

For any HVAC post, people must give their climate condition to get proper analysis from folks in here.

Large occupancy spaces are some of the most difficult systems to size and control, because they are so unpredictable.

First, It's a good idea to have dedicated system for each large ballroom, you boardrooms should be on a different system as they require different occupancy and schedule.

ballrooms should have CO2 sensor to control OA as a first things to reduce coil load, this DCV can be modeled and will give you more energy savings than any control scheme.

The best approach would be to downsize your system considerably, as much as 50% (you definitely need to undersize, how much is your choice), there will always be diversity of some kind in your system, then have your system started a minimum of 4-hours prior to occupancy, say 4:00 AM for an 8:00 AM occupancy, and run till midnight

Relying on reheat could be trouble. In summer, your boiler plant if OFF (boiler disabled when OA temp above 60F in any typical sequence of operation), so no heat is available.

In addition, simulatenous Cooling and heating is a violation of ASHRAE 90.1. and since ASHRAE 90.1 is a pre-requisite in LEED, I would not mess with it if I were you.

So let the heat from people bring the space to the desired 70F after an hour or so. the temperature may rise as much as 74/75, but by then, the people will have most likely exited the room, at least for luch break.

Downsize by using a cooling load factor of 0.4 or 0.5 (whatever you feel comfortable with) on people and lights for straight occupancies under 4-hours (look up CLF table in ASHRAE fundamentals).

But when you do downsize this much, you will need to let the unit run continuously during occupied mode, whether occupied or not.

Of course, when the building is unoccupied (use occupancy sensors to know that), you can close your OA damper, more energy savings.

As for pull down load (per walkes post), i.e Monday morning cool-down mode, just make sure that your system is running 24/7 anytime OA temperature is 90F or higher, even during the week-end.

I like Kiwi's suggestion of interlocking fuction schedule with BMS for optimum start/stop, but when you do downsize substantially, you will need continuous operation anyway. I hear that sophisticated hotels interlock their room key program with the BMS to shut-down the room FCU's.

I have designed systems for large lecture halls with 50% downsizing and the systems worked perfectly.

All that said: If you are is a hot and humid climate, forget about turing the AC off at all during the Hot and Humid season.