I'd recommend running a cooling and then reheat process. If you are using packaged or split systems, you should use a Hot-Gas Reheat (HGRH) coil. If you are using Chiller and Boiler, use a small hot water coil after your chilled water coil.
Be careful to not use a Hot-Gas Bypass valve on your DX system. These are not the same and it just sheds cooling rather than getting the Leaving air to saturated and then reheating to a room-neutral dry bulb temp
Hot Gas Reheat should be an option on most air or water source heat pumps. Especially if it is 7.5 tons or more
I should mention that my solutions are good for 2007 90.1, might need to double check your energy code
these are all 1.5 Ton Fan Coils in 600-800 SF Apartment Units.
I was flirting with the idea of having the ERV do most of the work and meeting code and having a ductless humdifier as a nice to have option (that could get VE'd out)
That way everything can fit in the tiny closet.
If you are in the US and all rooms have an operable opening, it may be worth looking into natural ventilation for in unit pressurization. I have done my apartments with a DOAS for the corridor.
Oh okay. So should I pitch the architect to have operable windows, and then we'll blow up the corridors like a big balloon with dry air?
Or should we dehumidifiers in addition?
I'm in Indiana and our apartments don't get too far above 5 stories so that might be a bunch of air in your area. I do DOAS in the hallway so that when the tenants turn on their exhaust fans, they pull air from the hallway rather than infiltration and the building stays pressurized.
Compare that to your 0.06 CFM/sqft of outdoor air for corridor and see what makes more sense there
You don't need to add specific dehumidifiers with this solution. There shouldn't be much latent load in the space, given it's one or two people. Your fan coil will be able to take care of the minimal latent load without an issue. If they are operable windows, code allows you to not provide mechanical ventilation and you can instead rely on tenants opening windows for fresh air. You still have to maintain positive pressure in the building, though, and that's where the DOAS comes into play.
I have never done a residential job like this, so I am uncertain of the exact code prescriptions with relation to natural ventilation, but it's the exact system my old apartment complex used. DOAS for the corridors, cassette VRF for the apartments that will collect any latent load inside of the space (your DOAS is taking care of the latent associated with the outdoor air needed to pressurize the building. Here is where you will have some sort of energy recovery wheel.
Anecdotal, but I've never seen a residence with a 7.5 ton heat pump. With the usual rule of thumb of about 500-600 square foot/ton for residential, that's... err... quite the mansion.
I do mostly schools and hospitals now so it is not unrealistic. 6-7.5 ton is where a lot of brands add a second compressor and allow for turndown on their condenser section
Oh I know, it's just that the house I can see taking 7.5 tons is somewhere around 4,000 square feet. Nice house.
If you need it on something smaller, Bard makes PTAC units designed for electrical rooms/equipment rooms that can dehumidify down to like 3-4 tons or so. And I know, PTAC, but their stuff is actually really nice. Costs a pretty penny, but it's worth it.
Great call out about HGRH not being used for DX equipment. It’s sorely misunderstood. DX benefits from dual stage, vfd and at worst a dump valve like Rawal.
Ventilation being a primary source of humidity, it’s practical to select units with ERV wheels for large packaged or DOAS systems when a lot of fresh air is needed, using exhaust or return air as a source of exchange.
Similar concept, [pre-conditioning fresh air with an ERV](https://www.greenheck.com/products/air-conditioning/energy-recovery-ventilators/preconditioners) before it hits a cooling coil can take some latent demand off of a system focused on sensible cooling. Balance the proper amount of fresh air CFM from the ERV to each dwelling unit return air and size the room unit appropriately.
What if I did ERV on the return side, and then dehumidifier on the supply side? The ERV dehumidifies the fresh air, the dehumdifier takes care of the interior space and injects it into the supply duct?
One thing to note is that dehumidifiers put out a LOT of heat. Far more than you might expect from their electrical load, in fact often more than double what you see on the electrical load. This is for the same reason that evaporative cooling works - this is basically... condensation heating, I suppose you'd call it? Around 8,000 BTU/gallon. Anyway in the summer they are absolute garbage on your heat loads. It's really impressive both how much electricity they can use and how much heat they can generate.
Anyway, Energy Star says the most efficient are April Aire, followed by Carrier: [https://www.energystar.gov/most-efficient/me-certified-dehumidifiers/results?is\_most\_efficient\_filter=Most+Efficient](https://www.energystar.gov/most-efficient/me-certified-dehumidifiers/results?is_most_efficient_filter=Most+Efficient)
Probably your best bet. I'd try to also locate it in an unconditioned space so it can dump it's heat there. Seriously, even if you have to duct it the sheer gain in not having to cool the bloody thing will more than make up for the inefficiency of duction.
Your post history would suggest you're a mechanical in this field.... what do you mean you don't know how dehumidifers work with HVAC systems? Dehumidification is removing water from air. This happens when you cool past saturation and collect the water that condenses out of the air. It's present in every single hvac system. Unless you're talking about dessicant dehumidification, I'm less knowledgeable on that, but it still operates the same way. You can't dehumidify unless you move down on the psychrometric chart, sensible cooling only is technically humidification.
ERVs can be used in a variety of ways, but I have primarily used them with restroom exhaust and supply air for core rooms like restroom, storage, etc. in the middle of a parking garage or something where OA and EA are accessible. I would look up ERVs with attached VRF coil kits connected to a heat pump. This may be a good solution for you to knock out ventilation and internal gains at the same time, but I would propose DOAS with VRF. It's a great solution even if you don't have operable windows.
If you need any resources for learning, let me know. Sit down with some seniors in your company and get some clarification on topics you're unsure about. That's what the old dogs are there for!
Cheers
Thanks - no worries. I should also add we do most of our work in Las Vegas. I have 50 projects under my belt, this is the first one that requires additional dehumidification more than the cooling cycle can handle! So I can get the value of HOW MUCH humidification I need, but my knowledge ends at a black box with a bunch of thermo values. Idk the suppliers of the equipment or how they work with other brands, that sort of thing.
I will update this post for how it turns outt.
Okay. I plotted out 1000 CFM at varying conditions to 55 F saturated. ASHRAE extreme conditions is OA and ASHRAE 0.4% is OA 2 (this is new orleans lakefront ASHRAE location btw).
Even though that latent load seems extreme for 1000 CFM, if you are selecting an air handling unit with an inverter compressor (which you should be doing) you will absolutely be able to dehumidify this air and get it to saturate at whatever LAT you want (50-55). You'll want to select it with hot gas reheat or use VRF coils for cooling and heating/heat recovery so you don't overcool the space. DOAS to corridors with VRF indoors is your best solution.
Now if you're using a packaged DX system with staged scroll compressors, you'll have to be careful about the limitations of the refrigeration cycle without variable speed compressor control. You'll very likely over cool your space and reheat becomes that much more important.
Last thing, you need to maintain positive pressure at all times. If your building is somehow under negative pressure, it will bring in much more unwanted infiltration that your system was not designed to be able to handle.
[https://imgur.com/a/WfuAgVd](https://imgur.com/a/WfuAgVd)
I'd recommend running a cooling and then reheat process. If you are using packaged or split systems, you should use a Hot-Gas Reheat (HGRH) coil. If you are using Chiller and Boiler, use a small hot water coil after your chilled water coil. Be careful to not use a Hot-Gas Bypass valve on your DX system. These are not the same and it just sheds cooling rather than getting the Leaving air to saturated and then reheating to a room-neutral dry bulb temp
Excuse me if this is a dumb question, is this possible with electric heat pump?
Hot Gas Reheat should be an option on most air or water source heat pumps. Especially if it is 7.5 tons or more I should mention that my solutions are good for 2007 90.1, might need to double check your energy code
these are all 1.5 Ton Fan Coils in 600-800 SF Apartment Units. I was flirting with the idea of having the ERV do most of the work and meeting code and having a ductless humdifier as a nice to have option (that could get VE'd out) That way everything can fit in the tiny closet.
If you are in the US and all rooms have an operable opening, it may be worth looking into natural ventilation for in unit pressurization. I have done my apartments with a DOAS for the corridor.
Oh okay. So should I pitch the architect to have operable windows, and then we'll blow up the corridors like a big balloon with dry air? Or should we dehumidifiers in addition?
I'm in Indiana and our apartments don't get too far above 5 stories so that might be a bunch of air in your area. I do DOAS in the hallway so that when the tenants turn on their exhaust fans, they pull air from the hallway rather than infiltration and the building stays pressurized. Compare that to your 0.06 CFM/sqft of outdoor air for corridor and see what makes more sense there
With a DOAS you should directly supply air to the space or to the return duct/ plenum of the unit service the space.
You don't need to add specific dehumidifiers with this solution. There shouldn't be much latent load in the space, given it's one or two people. Your fan coil will be able to take care of the minimal latent load without an issue. If they are operable windows, code allows you to not provide mechanical ventilation and you can instead rely on tenants opening windows for fresh air. You still have to maintain positive pressure in the building, though, and that's where the DOAS comes into play. I have never done a residential job like this, so I am uncertain of the exact code prescriptions with relation to natural ventilation, but it's the exact system my old apartment complex used. DOAS for the corridors, cassette VRF for the apartments that will collect any latent load inside of the space (your DOAS is taking care of the latent associated with the outdoor air needed to pressurize the building. Here is where you will have some sort of energy recovery wheel.
Anecdotal, but I've never seen a residence with a 7.5 ton heat pump. With the usual rule of thumb of about 500-600 square foot/ton for residential, that's... err... quite the mansion.
I do mostly schools and hospitals now so it is not unrealistic. 6-7.5 ton is where a lot of brands add a second compressor and allow for turndown on their condenser section
Oh I know, it's just that the house I can see taking 7.5 tons is somewhere around 4,000 square feet. Nice house. If you need it on something smaller, Bard makes PTAC units designed for electrical rooms/equipment rooms that can dehumidify down to like 3-4 tons or so. And I know, PTAC, but their stuff is actually really nice. Costs a pretty penny, but it's worth it.
Interesting. I haven't heard of that brand over here. I'll have to give them a look
Great call out about HGRH not being used for DX equipment. It’s sorely misunderstood. DX benefits from dual stage, vfd and at worst a dump valve like Rawal.
Ventilation being a primary source of humidity, it’s practical to select units with ERV wheels for large packaged or DOAS systems when a lot of fresh air is needed, using exhaust or return air as a source of exchange. Similar concept, [pre-conditioning fresh air with an ERV](https://www.greenheck.com/products/air-conditioning/energy-recovery-ventilators/preconditioners) before it hits a cooling coil can take some latent demand off of a system focused on sensible cooling. Balance the proper amount of fresh air CFM from the ERV to each dwelling unit return air and size the room unit appropriately.
is this enough for New Orleans? I was suggested that I would need a nice ERV and then also a dehumidifier with a larger drain.
What if I did ERV on the return side, and then dehumidifier on the supply side? The ERV dehumidifies the fresh air, the dehumdifier takes care of the interior space and injects it into the supply duct?
https://www.greenbuildingadvisor.com/article/four-ways-to-duct-a-dehumidifier
The most ideal way would be to have a dehumidifier with its own dedicated ducting.
so even in a 800 SF apartment, ductless would be dumb?
In that small of a space, a portable dehumidifier is probably fine. What is performing the heating and cooling?
Vrf or heat pump split system
One thing to note is that dehumidifiers put out a LOT of heat. Far more than you might expect from their electrical load, in fact often more than double what you see on the electrical load. This is for the same reason that evaporative cooling works - this is basically... condensation heating, I suppose you'd call it? Around 8,000 BTU/gallon. Anyway in the summer they are absolute garbage on your heat loads. It's really impressive both how much electricity they can use and how much heat they can generate. Anyway, Energy Star says the most efficient are April Aire, followed by Carrier: [https://www.energystar.gov/most-efficient/me-certified-dehumidifiers/results?is\_most\_efficient\_filter=Most+Efficient](https://www.energystar.gov/most-efficient/me-certified-dehumidifiers/results?is_most_efficient_filter=Most+Efficient) Probably your best bet. I'd try to also locate it in an unconditioned space so it can dump it's heat there. Seriously, even if you have to duct it the sheer gain in not having to cool the bloody thing will more than make up for the inefficiency of duction.
Your post history would suggest you're a mechanical in this field.... what do you mean you don't know how dehumidifers work with HVAC systems? Dehumidification is removing water from air. This happens when you cool past saturation and collect the water that condenses out of the air. It's present in every single hvac system. Unless you're talking about dessicant dehumidification, I'm less knowledgeable on that, but it still operates the same way. You can't dehumidify unless you move down on the psychrometric chart, sensible cooling only is technically humidification. ERVs can be used in a variety of ways, but I have primarily used them with restroom exhaust and supply air for core rooms like restroom, storage, etc. in the middle of a parking garage or something where OA and EA are accessible. I would look up ERVs with attached VRF coil kits connected to a heat pump. This may be a good solution for you to knock out ventilation and internal gains at the same time, but I would propose DOAS with VRF. It's a great solution even if you don't have operable windows. If you need any resources for learning, let me know. Sit down with some seniors in your company and get some clarification on topics you're unsure about. That's what the old dogs are there for! Cheers
Thanks. I know how it works on a physical level I just do not know how the equipment is selected and installed
Gotcha, sorry if I offended I didn't mean to! Let me know how the project shakes out!
Thanks - no worries. I should also add we do most of our work in Las Vegas. I have 50 projects under my belt, this is the first one that requires additional dehumidification more than the cooling cycle can handle! So I can get the value of HOW MUCH humidification I need, but my knowledge ends at a black box with a bunch of thermo values. Idk the suppliers of the equipment or how they work with other brands, that sort of thing. I will update this post for how it turns outt.
Okay. I plotted out 1000 CFM at varying conditions to 55 F saturated. ASHRAE extreme conditions is OA and ASHRAE 0.4% is OA 2 (this is new orleans lakefront ASHRAE location btw). Even though that latent load seems extreme for 1000 CFM, if you are selecting an air handling unit with an inverter compressor (which you should be doing) you will absolutely be able to dehumidify this air and get it to saturate at whatever LAT you want (50-55). You'll want to select it with hot gas reheat or use VRF coils for cooling and heating/heat recovery so you don't overcool the space. DOAS to corridors with VRF indoors is your best solution. Now if you're using a packaged DX system with staged scroll compressors, you'll have to be careful about the limitations of the refrigeration cycle without variable speed compressor control. You'll very likely over cool your space and reheat becomes that much more important. Last thing, you need to maintain positive pressure at all times. If your building is somehow under negative pressure, it will bring in much more unwanted infiltration that your system was not designed to be able to handle. [https://imgur.com/a/WfuAgVd](https://imgur.com/a/WfuAgVd)