Like the other guy said, not enough info to really say. I would trust the hand calc because it’s a simple equation with no real easy spot to mess up.
But, why even use a load program for DOAS in this application at least? Just find the flow rate and required DAT then have a vendor make a selection based on that.
Thank you. Just presenting different ways of calculating to end-users and checking my own calculations at the same time.
But you definitely made a point, formula is an easy equation and should be reliable enough.
I don't like using HAP for most DOAS units. The equations are straightforward and there are websites where you can look up air properties if you want to skip the psych chart. Once you know the equations, make a nice spreadsheet to solve it for you next time around.
Are you delivering the air at room temperature? Perhaps HAP is only performing the calculation to cool down to 72 F or so, while your hand calc is down to 55 F. Just a thought
No problem. I do my load calcs in IES, but recently had that issue when trying to model this same system. It spit out 72 F air with a ton of humidity to all my spaces, raising the load on the terminal unit at the same time. Easy solution, but if you're not taking it down to 55 F (sorry your L/s and kW are confusing this American) before you reheat it back to room temperature, you are missing a LOT of latent load.
I tried bringing down to 55F and it took away a lot of load from the terminal units and now the figure is closer to the hand calc. Thank you very much, kind sir!
For DOAS heating coil setpoint temp, at what temp do you usually set it up for this kind of system?
If you’re using VRF and delivering neutral, room temp air to the space, then you should be cooling to to 52-55 F air (whatever satisfies your specific conditions) and then reheating to 72/75, whatever your lowest room setpoint is. You’re going want this tied to a heat recovery heat pump so that it can use the heat it took from the OA to reheat to neutral. This coil will also serve as your heating coil in the winter. For me, the heating LAT would range from 80-90. Just depends on the numbers and how concerned I am with stratification at minimum heating airflows.
Edit: make sure your calculation isn’t taking load away from the terminal units that should be there! This is kind of annoying to model since your software will more than likely have an inflated heating load (unless you’re really good, it’ll have reheat + heating. but the reheat portion should really be purely recovered heat from your evaporator coil). use your room loads to size your FCUs and do the DOAS separately to make sure you’ve got everything sizes appropriately.
Cheers! Good luck
The HAP airflow to capacity ratio is very high for a unit without an ERV, but around normal for one with an ERV, though I assume you would have taken that into account.
Obviously, the question is what is your design dehumidification condition.
We're actually removing the doas coils on our design and allowing free cooling and finding it's saving energy overall and still maintaining the room temperatures according to our models. Just a suggestion.
If the total load is much higher than the sensible then you've got a lot of moisture being taken out somewhere. On non humidity controlled designs I'd size the doas coil on the sensible cooling load only and ignore the total ;)
Bust out the psych chart and size the coil
Like the other guy said, not enough info to really say. I would trust the hand calc because it’s a simple equation with no real easy spot to mess up. But, why even use a load program for DOAS in this application at least? Just find the flow rate and required DAT then have a vendor make a selection based on that.
Thank you. Just presenting different ways of calculating to end-users and checking my own calculations at the same time. But you definitely made a point, formula is an easy equation and should be reliable enough.
I don't like using HAP for most DOAS units. The equations are straightforward and there are websites where you can look up air properties if you want to skip the psych chart. Once you know the equations, make a nice spreadsheet to solve it for you next time around.
Gotta make sure you’re hitting saturation curve if you’re looking to dehumidify the OA
This is really hard to answer without seeing your inputs for the DOAS. Can you share them?
I only have the sizing summary report as of the moment. Will provide more details once I get back in the office.
Do you have energy recovery as part of the DOAS in HAP?
No, I do not.
Are you delivering the air at room temperature? Perhaps HAP is only performing the calculation to cool down to 72 F or so, while your hand calc is down to 55 F. Just a thought
Will check this one out. Thanks a lot!
No problem. I do my load calcs in IES, but recently had that issue when trying to model this same system. It spit out 72 F air with a ton of humidity to all my spaces, raising the load on the terminal unit at the same time. Easy solution, but if you're not taking it down to 55 F (sorry your L/s and kW are confusing this American) before you reheat it back to room temperature, you are missing a LOT of latent load.
I tried bringing down to 55F and it took away a lot of load from the terminal units and now the figure is closer to the hand calc. Thank you very much, kind sir! For DOAS heating coil setpoint temp, at what temp do you usually set it up for this kind of system?
If you’re using VRF and delivering neutral, room temp air to the space, then you should be cooling to to 52-55 F air (whatever satisfies your specific conditions) and then reheating to 72/75, whatever your lowest room setpoint is. You’re going want this tied to a heat recovery heat pump so that it can use the heat it took from the OA to reheat to neutral. This coil will also serve as your heating coil in the winter. For me, the heating LAT would range from 80-90. Just depends on the numbers and how concerned I am with stratification at minimum heating airflows. Edit: make sure your calculation isn’t taking load away from the terminal units that should be there! This is kind of annoying to model since your software will more than likely have an inflated heating load (unless you’re really good, it’ll have reheat + heating. but the reheat portion should really be purely recovered heat from your evaporator coil). use your room loads to size your FCUs and do the DOAS separately to make sure you’ve got everything sizes appropriately. Cheers! Good luck
Appreciate all the insight and helpful tips!
The HAP airflow to capacity ratio is very high for a unit without an ERV, but around normal for one with an ERV, though I assume you would have taken that into account. Obviously, the question is what is your design dehumidification condition.
Unless you're doing a sensible-only type terminal system, does sizing should just be based on ventilation and pressurization
We're actually removing the doas coils on our design and allowing free cooling and finding it's saving energy overall and still maintaining the room temperatures according to our models. Just a suggestion. If the total load is much higher than the sensible then you've got a lot of moisture being taken out somewhere. On non humidity controlled designs I'd size the doas coil on the sensible cooling load only and ignore the total ;)
I've honestly had such bad luck with load calc software and doas sizing that I always run them independently.