Hot water makeup air unit

[gman994]

The scenario: 100% outdoor air unit, 2 position OA damper, 2 way modulating HW valve, glycol content minimal. Unit will be located indoors with OA ducted to it. It will be shut off when exhausts are shutdown. I'm speculating that the supply air temp can be a constant 60-70 deg. Not a Face & Bypass unit, it's a basic Fan Coil arrangement. I was approached to provide an operating sequence for this platform, but I'm not sure of a foolproof way to do it that will provide reliable operation. I'm aware of freeze protection concerns, and I am confident I could devise a control scheme to operate it, but reliably ? No nuisance freeze trips? I'm not real confident that this is the best equipment scheme for the application. Once I commit to this, I own it. I welcome any opinions or advice on this situation. Thanks --- gman

[ascj]

The scenario: 100% outdoor air unit, 2 position OA damper, 2 way modulating HW valve, glycol content minimal. Unit will be located indoors with OA ducted to it. It will be shut off when exhausts are shutdown. I'm speculating that the supply air temp can be a constant 60-70 deg. Not a Face & Bypass unit, it's a basic Fan Coil arrangement. I was approached to provide an operating sequence for this platform, but I'm not sure of a foolproof way to do it that will provide reliable operation. I'm aware of freeze protection concerns, and I am confident I could devise a control scheme to operate it, but reliably ? No nuisance freeze trips? I'm not real confident that this is the best equipment scheme for the application. Once I commit to this, I own it. I welcome any opinions or advice on this situation. Thanks --- gman

Is the exhaust a fixed cfm? Meaning they are all "On" or "Off"?

Is the MUA the properly size to the fixed exhuast?

Is the HW actuator 135ohm control?

If all Yes then I would just control the discharge air with a proportional stat. Add a freeze stat to the discharge air. Install a end switch to the inlet damper to control the fan. And make sure the MUA is interlocked to the exhuast fans.

[Pneuma]

The scenario: 100% outdoor air unit, 2 position OA damper, 2 way modulating HW valve, glycol content minimal. Unit will be located indoors with OA ducted to it. It will be shut off when exhausts are shutdown. I'm speculating that the supply air temp can be a constant 60-70 deg. Not a Face & Bypass unit, it's a basic Fan Coil arrangement. I was approached to provide an operating sequence for this platform, but I'm not sure of a foolproof way to do it that will provide reliable operation. I'm aware of freeze protection concerns, and I am confident I could devise a control scheme to operate it, but reliably ? No nuisance freeze trips? I'm not real confident that this is the best equipment scheme for the application. Once I commit to this, I own it. I welcome any opinions or advice on this situation. Thanks --- gman

I prefer no water valves and face bypass dampers. But the other way is a pump, sized for the head and gpm of the coil. It is piped to circulate water through the coil and a three way valve, the idea is the velocity of the water will prevent it from freezing. You have it set to cycle on when the valve is nealry in full bypass. It's a pretty common application and you should be able to find a drawing of it somewhere.

[james mo]

Plate heat exchanger and separate pump. Glycol mix as needed and standard control package.

[duke of earl]

Would you like a full sequence of operation or just as it applies to the hw coil and freezestat? There's no such thing as no nuisance trips with a freezestat unless you jump it out.