Reheat on GSHP 1

[dbill74]

I have an existing dormitory building with rooms that open onto balconies in South Carolina, USA (near coast & humid environment). During a recent renovation, contractor was given a design to use through-wall PTAC room units with electric reheat. It was later decided have the contractor replace the PTAC units with GSHP's. Question is should the GSHP units have reheat for humidity control?

In a normal application where the supply air is ducted through out having some reheat to reduce the RH in the SA stream makes sense to prevent condensation inside ductwork. (in the past I've allowed fan heat to provide this reheat.) However, when the AHU is located inside the space such as with a hotel room PTAC, mini-split or GSHP, is this reheat really necessary since the air stream enters the occupied space almost immediately. The electric heat coil on an air cooled PTAC is reasonable to provide general heating.

 

[tys90]

Do the GSHP have any sort of capacity control? Modulating or multiple compressors? I'm guessing for just dormitory rooms that they don't. Humidity issues in the room (along with other issues) arise from short-cycling heat pumps in small rooms because they are generally oversized. I wouldn't worry about condensation, if you have any ductwork just make sure it's insulated.

Anytime I could add a hot gas reheat coil to a heat pump, I would. Depends on budget but it's usually not much more expensive and uses very little extra energy. I wouldn't go cheap in a humid environment like that.

 

[EnergyProfessional]

At minimum I hope you would use hot gas bypass and not electric Resistance heat. Reheat inherently is inefficient.

 

[dbill74]

GSHP's are non-modulating. Average capacity is 1 ton for 310 SF rooms; no ductwork.

 

[Drazen]

reheat is not a process to control humidity, it is only inevitable addition to wet cooling that serves for humidification.

you would need to do psychrometric calculation, any decision without it is blind-folded one if you want to be aware of what are you doing.

the other story is how much data you are provided with for "simple" units planned for residential use. such units normally serve for partial, uncontrolled humidification and the question is which data you can obtain for the unit - supply air temperature for instance.

if you have limited data available, the best what you could do is what tys mentioned - performing detailed calculation of cooling loads and taking care not to oversize units.

 

[cry22]

small capacity heat pumps do not come with Hot gas reheat packages.

simultaneous cooling and heating (or de-humidification and humidification) is not allowed by ASHRAE 90.1 - one would say that reheat is out of the question, unless it is used as primary heat for winter heating only - reheat should not be enabled in summer.

As tys90 mentioned, OVERSIZING is generally the problem. Consider downsizing the AC unit, it will work like a horse all day long and keep dehumidifying continiously - Use Manual J calculations instead of running Trace for such system.

 

[tys90]

Many small capacity heat pumps DO come with hot gas reheat nowadays as an option. In fact, I would wager that it would be hard to find one not offering hot gas reheat. Just to be clear - using hot gas reheat is allowed by ASHRAE 90.1. Using electric or hydronic reheat would not be allowed.

 

[dbill74]

Not getting a good clear answer I was looking for, perhaps because I jumbled it up with some unnecessary information. So let me rephrase:
Why do we put reheat in an AHU?
I have my own thoughts on it, but I want to hear from others in the community to confirm or rethink the way I design and review systems in the future.

 

[tys90]

Comfort. It's to prevent over-cooling a space. There are many situations where over-cooling can occur (such as dehumidifying cycles) but that's the main reason.

 

[Drazen]

tys, are you talking about hot gas bypass for defrosting or about reheat? some manufacturers make mess with terms in their catalogs, especially non-native english speakers who translate it into english.

if you have some link for small heat pump that uses hot gas for space reheat, i would be interested to see it.

 

[DRWeig]

Imagine the AHU provides the required ventilation air. When the cooling load in the zone is less than what the minimum required ventilation flow demands, overcooling happens. Reheat solves this. That's in a VAV system. If it's constant volume, the reheat provides all temperature control.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[tys90]

McQuay, Waterfurnace, Trane. That's just the first three I looked at, they all have heat pumps down to 1/2 ton or 3/4 ton that have hot gas reheat coils.

http://www.daikinmcquay.com/McQuay/ProductInformation/WSHP/SmartSourceSingleStageHorizontal

http://www.waterfurnace.com/products.aspx?pl=60&prd=Versatec_Ultra

http://trane.com/COMMERCIAL/DNA/View.aspx?i=2354

 

[Drazen]

thanks, i rarely have opportunity to study that scope as simply noone within my range can afford it, rebates for gshp were never introduced.

it's revelation to me to see so small units quipped with dedicated hot gas reheat, together with ec fans. I will try to find time to study how minimum outdoor air can be achieved.

 

[chicopee]

I suspect to lower the relative humidity is one factor for reheat in a AHU.

 

[urgross]

Lowering RH would be a reason for sizing a preheat, and used frequently for on-the-fly RH control for OR's, AALAC or other applications with RH control requirements. Reheats control sensible load, unless paired with an additional cooling coil.

I'd recommend looking at AAON literature, as they write much gooder English than me.

http://aaon.com/literature.aspx?id=1

 

[CarolinaPE]

I think you can get one with a condenser water reheat coil, I had a project with Climatemaster units and they worked well.

 

[walkes]

The condenser water reheat coils can work well, for sub-zoning or better temperature control with dehumidification. Taking the water right after the heat pump can be used to reheat or recool the air depending on what mode the heat pump is running in. Since you are not using additional energy to reheat mechanically cooled air it would be acceptable to ASHRAE.
McQuay has heat pump design manuals on their website that will show this configuration.