Reheat Controls for Swimming Pool

[natalie_p]

Hi all,
I am working on a swimming pool application and this is my first swimming pool application.
This unit is locate in a cool and low humidity area. During winter time (OAT <55F), mechanical cooling (DX Cooling) is locked out. So when unit is in dehumidification mode, good condition outside. They want to use outside dehumidificationby modulating the outside air damper to maintain the indoor humidity setpoint. This could over cool the space so they want to use Gas Heat as reheat. Assume that unit is in cooling mode while unit goes into dehumidification, so outside air damper override the cooling economizer. When space temperature drops below the cooling setpoint, gas heat enable to reheat the space. Assume it is very cold outside because outside air damper open to maintain the humidity setpoint, indoor temperature keeps dropping and it drops below heating setpoint. That put the unit into heat mode, current setpoint now is heating setpoint instead of cooling setpoitn. My question is, what setpoint should the gas heat now should maintain? Cooling or heating setpoint? or should I disable the econ dehumidification mode when space temp drops below the heat setpoint?
Any help is much appreciate.
Natalie

[nikko]

Sounds like you may want to control from SAT, not space temp. Have the SAT setpoint as a reset from Space temp (or OAT) and let the OA dmpr function as a dehumid. as needed. Understand your delta T across your heating coil so you know the lowest OAT the system will still be able to run at (100% OA) and still be able to heat. If the OA is below that, control it to a MAT sp instead (as your econ, should be in cooling mode anyways) and have the econ. override the humidifier call.

[Cagey57]

Do you have an exhaust fan serving the pool area ? If so what controls it, Temp, Rh, building pressure ? Each type of EF control has a different strategy for using the economizer to dehumidify.

I agree with nikko that you need to have the reheat control at 100% OA, my guess would be to set it up for a minimum DA Temp somewhere close to the pool water temp setpoint so you don't waste energy.

More info would be helpful if you can.

[natalie_p]

Sounds like you may want to control from SAT, not space temp. Have the SAT setpoint as a reset from Space temp (or OAT) and let the OA dmpr function as a dehumid. as needed. Understand your delta T across your heating coil so you know the lowest OAT the system will still be able to run at (100% OA) and still be able to heat. If the OA is below that, control it to a MAT sp instead (as your econ, should be in cooling mode anyways) and have the econ. override the humidifier call.

Thanks for your respond.
The system doesn't have a mixed air temp.
This customer wants the unit to be controlled from space temperature and limit the outside damper with supply air temp at 45F. So if the supply air gets below 45 F, the outside damper start to close off regarless of what indoor humidity. To me, this doesn't prevent the space to drop below heating setpoint though. So if this the only option, I have to bring on the gas heat to reheat the space but I am not sure what setpoint I should maintain when this happens.
I will see if I can convince the customer to change the control temp to supply temp instead of the space temp. If that happens, should I have 2 reset setpoints? 1 for cooling and 1 for heating? This unit doesnt have a Mixed Air Temp so controlling from Mixed Air Temp isn't a option here.
Can you please tell me more about how to control this unit from supply air reset setpoint(s).
Thanks again.
Natalie

[natalie_p]

Do you have an exhaust fan serving the pool area ? If so what controls it, Temp, Rh, building pressure ? Each type of EF control has a different strategy for using the economizer to dehumidify.

I agree with nikko that you need to have the reheat control at 100% OA, my guess would be to set it up for a minimum DA Temp somewhere close to the pool water temp setpoint so you don't waste energy.

More info would be helpful if you can.

Thanks for your respond.
This unit has a ERV and Exhaust Fan, the Exhaust Fan is controlled from Buiding static pressure. ERV runs ALL the time (not excepting econ mode).
thanks

[natalie_p]

By the way, this unit is located in New York State Area. The indoor humidity setpoint is reset from 40% to 60% base on outdoor condition. (Lower dewpoint reset to lower setpoint and higher dewpoint reset to higher setpoint).

[nikko]

If you use OA as a dehumid , you'll be firing the gas heater at any OA temp below some "normal" space temp regardless.

Is it variable or staged heat? I'll assume it's staged (variable would make this a LOT easier). And I'm going to over simplify here, but you should get the idea.

If you have to stick with space temp control and the dehumid ctrl is controlling the OA dampers, you're going to have to incorporate a MAT sensor in order to figure out when to fire the stages of heat in a pro-active fashion, rather than the regular reactive typical to most systems. This is to prevent slugging cold air onto the swimmers.

Let's assume a 40*F delta T across one stage of heating. And for this purposes of this, let's assume a 75*F space temp setpoint (and no losses). This means that if the MAT is > 35*F, you'll only need one stage of heating in order to maintain space temp. As the OA damper closes (because of the dehumid cycle) and the MAT starts to approach the space temp SP, you'll have to start cycling off the heat. For example, if the MAT is at 55*F, you'll be cycling the heat on a 1/2 duty cycle. conversely, as the MAT drops below 1x HC delta T, you'll have to leave one stage on all the time, and start to cycle the second stage. I'll leave the minimum on/off cycles up to you, but you get the idea.

[ddcfreek]

I think you guys are over complicating this more than it has to be based on the assumption of what the indoor environment is like (ie. whether airflow can be felt by any occupants). Most of the pools I have seen the outlets are placed so that if the discharge air is cold, it wouldn't be blowing on any people. I'm not saying this is the case here, but we are assuming.

Besides that, she stated the owner wants the unit to be controlled by space temp only. So then simply control the gas reheat to the space heating setpoint and the oa dampers to the space cooling, low limit and dehumid setpoints. If there are any complaints of cold drafts then bring out the upgrades.

I run into alot of customers that think they know how they should control thier equipment and alot of them are not right. Since you can't tell them that you just prove it.

40% seems to be an unrealistic setpoint for a pool. I have never seen a reset in this type of application.

[nikko]

Cost of doing it right isn't a lot more than the cost of doing it wrong, but it's a lot cheaper than doing it twice. I'm hired because I understand controls. It's my job to put forth a solution to the customer. If the customer wants it done their way, I either walk, or they sign away my responsibility and I work T&M until no one's happy. I've never resorted to the latter.

re: cold air dumping - ddcfreek, you may be right, you may be wrong. Why take the chance when you don't have to?

I agree that a humidity reset is not required. A simple setpoint will suffice.

[ddcfreek]

I'm not saying I'm right and your wrong here. We each have different views.

When ever I see staged heating or cooling I ALWAYS control to space temp regardless of what the spec says. I have never had anyone complain about that. I have several new customers that dumped the original installer because they controlled to discharge temp and within two years thier equipment was crapping out. Granted you can add min and max runtimers but everything is still cycling more than it should.

In this case they would have been much better off with a water coil and face bypass but alot of the time the customer expects the controls to make up for mechanical deficiencies.

I understand your point of "do it right or don't do it at all" but alot of customers around my area would figure if you don't want to do it thier way then find someone who will. At some point you have to figure "the customer is always right"?? Maybe....

[Carnak]

the biggest heat loss from a pool is evaporation.

venitlating with dry air in the winter is like putting gasoline on a fire from an energy stand point.

the dry air makes water evaporate so you lose pool heat, then you ventialte to get the humidity down. It is just a never ending spiral.

money changes every thing though, a dry-o-tron for example is a mechanical dehumidifier, it dumps its heat of rejection back into the pool, along with the condensate.

It can even partially heat the space.

[Cagey57]

natalie_p,
First, Is this an older Pool Pack type unit ?
Second, are these problems New problems, problems that have developed over time or is it a case of "It has Never Worked right" ? If it is one of the first 2 then see if you can find the original control sequence and determine what is not working as designed. If it's a case of "Never Worked Right" then you have your work cut out for you. The original design/equipment will dictate 'What' can or cannot be done.

I agree with the 40% setpoint being quite low for a pool, we usually run them aroung 50-60% but Colorado doesn't have humidity, really !

If we assume a typical pool that's 50'x150' with an average depth of 7' we get 52,500 cu.ft. x 62# per cu.ft we end up with 3.2 million # of water (mass). The space setpoint has to be relatively the same (or higher) as the pool temp or you will accelerate evaporation. Adding 45 deg air (too cold) into the space with a high(er) Rh but lower temp than the space only compounds your Rh problem. The unit has to be setup to satisfy "The Pool" not the people.

The primary function of the unit should be to temper the air, the exhaust fan and economizer should be used as the ventilation function. There are several ways to apply control functions but you have to indentify "The primary" problem you have to address. Is it Lack of ventilation (OSA), lack of Humidity control (Exhaust) or the unit's failure to condition the air to meet setpoint (Htg/Clg/ventilation) ?

I didn't notice if you said what kind of controls are existing. Are they electric, electronic, pneumatic or DDC ? Each requires a different approach to get the same general result.

[natalie_p]

If you use OA as a dehumid , you'll be firing the gas heater at any OA temp below some "normal" space temp regardless.

Is it variable or staged heat? I'll assume it's staged (variable would make this a LOT easier). And I'm going to over simplify here, but you should get the idea.

If you have to stick with space temp control and the dehumid ctrl is controlling the OA dampers, you're going to have to incorporate a MAT sensor in order to figure out when to fire the stages of heat in a pro-active fashion, rather than the regular reactive typical to most systems. This is to prevent slugging cold air onto the swimmers.

Let's assume a 40*F delta T across one stage of heating. And for this purposes of this, let's assume a 75*F space temp setpoint (and no losses). This means that if the MAT is > 35*F, you'll only need one stage of heating in order to maintain space temp. As the OA damper closes (because of the dehumid cycle) and the MAT starts to approach the space temp SP, you'll have to start cycling off the heat. For example, if the MAT is at 55*F, you'll be cycling the heat on a 1/2 duty cycle. conversely, as the MAT drops below 1x HC delta T, you'll have to leave one stage on all the time, and start to cycle the second stage. I'll leave the minimum on/off cycles up to you, but you get the idea.

The unit has modulating gas heat and adding a mixed air temp sensor is not an option because unit is already out in the field.

[oldhweller]

The unit has modulating gas heat and adding a mixed air temp sensor is not an option because unit is already out in the field.

What's so difficult about adding in the field? It's done all the time. You ask for the best solution and then say you can't do it!!

Good Luck!

[nikko]

If a MAT is truly not possible, and you still want to use OA to control humidity, And you have modulating heat (which is good) we're back to discharge air temp control. You'll want to do a simple reset table (space temp = 65*F, DAT = 85*F, Space temp = 80*F, DAT = 70*F for example) and control the heat on a PID loop to maintain that setpoint. Reset the setpoint every ten minutes or so. In theory, you're still controlling to space conditions, but you're not allowing the unit to swing wildly as pure space temp control would do. This will also prevent slugging cold air into the space.

[ddcfreek]

If a MAT is truly not possible, and you still want to use OA to control humidity, And you have modulating heat (which is good) we're back to discharge air temp control. You'll want to do a simple reset table (space temp = 65*F, DAT = 85*F, Space temp = 80*F, DAT = 70*F for example) and control the heat on a PID loop to maintain that setpoint. Reset the setpoint every ten minutes or so. In theory, you're still controlling to space conditions, but you're not allowing the unit to swing wildly as pure space temp control would do. This will also prevent slugging cold air into the space.

Now that there is modulating reheat I could definately agree that dat reset is required. It would have been nice to have known that before. It makes a big difference with what you can do here.

If you reset the dat with space temp, can you still call it zone control to satisfy the customer? I would kind of think you could...

I would still think you should control the dampers to the clg and dehumid setpoints and the reheat to the htg setpoint on dat. Maybe still keep the dat low limit in there in case the reheat can't keep up.

[justjohnson]

Keep in mind that the ideal space temp setpoint is 2-3 deg greater than the pool water temperature. This will help make the pool water evaporate more slowly and make the pool occupants not feel freezing when they get out of the pool. I definitely agree with the supply air reset from space temp control. You are still ultimately controlling to space temperature but you keep the supply air more stable instead of wild swings. I have even used a PID loop for space temperature demand and used that to control the Supply air setpoint instead of just a straight reset. Dials temps right in.

[natalie_p]

If a MAT is truly not possible, and you still want to use OA to control humidity, And you have modulating heat (which is good) we're back to discharge air temp control. You'll want to do a simple reset table (space temp = 65*F, DAT = 85*F, Space temp = 80*F, DAT = 70*F for example) and control the heat on a PID loop to maintain that setpoint. Reset the setpoint every ten minutes or so. In theory, you're still controlling to space conditions, but you're not allowing the unit to swing wildly as pure space temp control would do. This will also prevent slugging cold air into the space.

Hi Nikko,
That is exactly what I was thinking to do for this unit. Thanks

[crab master]

I hope the unit is rated for this application - stainless heat exchanger/other so it can handle the cold outside air coming across it. You may have to install a MAT sensor to maintain a high enough MAT.