Sizing of Hydro Coils: Can you go too big?

ILikeEmOlder

My sense is that “go big or go home” applies to the sizing of hydro coils if you want to supply at low temps while still maintaining comfort.

If this is the case, how much would you oversize the coil for the heat load? Let’s assume the heat load calc is aggressive.

EBEBRATT-Ed

Yes but bigger coil means a bigger box to fit it in =bigger air handler.

You want when heating to keep the discharge air temp above 110 minimum or you will feel like cold air movement.

EdTheHeaterMan

The coil should have a BTU rating based on CFM (air flow) and water temperature. Look at that information to select the proper size coil. If you want to keep the air temperature above 110°F you will need at least a 140°F water in with a 10° Drop to 130° on the return water temperature. That may be a very big coil for you. For the most part I would size based on a 180°F input with a 20° Drop. That gives you 125° to 140° air temperature based on other parameters (like GPM flow, air flow). I hate the 100°F air from my heat pump. After you get the design you are looking for, and turn it on… you may end up increasing the water temperature to make the air from the ductwork more comfortable. Then you will need to have a 2 temperature system to accommodate the reason you want the lower temperature water in the first place.

I'm not goos with GPM and CFM calculations, I am more into the electrical diagrams that make that stuff happen. @hot_rod knows more about this stuff.

Kaos

Look at the data sheet for the air handler. Most will list output at lower temps. For example a Rinnai CAH050CE will do 26kBTU @ 3GPM 120F SWT and 50% blower (800CFM).

Delta is 30F, so supply temp of 100F. That is a bit on the low side. So you either want to run it a bit hotter SWT or lower CFM.

Either case, you will probably loose about 1/2 the nameplate capacity at low temps.

The one thing to watch is you want something with an ECM blower. These use significantly less power, much quieter and can be turned down more then PSC ones (good turndown is needed if you want to oversize).

hot_rod

If you run low temperatures you might want vents up high so you blend the air before it hits the occupants. I know air to air heat pumps can blow low temperature un-comfortable air on people.

From Idronics 29, # 3&4

ILikeEmOlder

Kaos,

I’m convinced of the worthiness of the ECM vs PSC motor.

The hydronic air handlers I have easy access to have PSC motors. I would be replacing with an ECM.

EBEBRATT-Ed

ECM motors are great until you have to pay to replace it.

ILikeEmOlder

Hot Rod,

I am inclined to spec hydro air for the upper stories of houses only.

I know I am fighting against the tide here, so I’m trying to implement as many best practices as possible in duct design (to overcome the limitations of hydro air versus conventional forced hot air).

Locating vents at the ceiling seems like one of the “musts” to avoid this cool air effect.

ILikeEmOlder

Ed (the heater man),

There’s no doubt you like your heat sir. I definitely know the high temp heat feeling you get from a big ole gas furnace. Most folks I work for know it as well, and have become very used to it.

If it takes SWTs near 180F to give folks that feel then I’ll give it to them.

My goal is to try to use my experience as a home improvement contractor of many years to give them a feel they like with much lower SWTs.

I have become pretty darn good at reducing heat loss through envelope improvements. My forte is air leakage improvements. My experience is that if you can significantly reduce drafts then you really don’t need or even want the high temp heat that we grew up with and became accustomed to.

ILikeEmOlder

Ed (EBEBRATT),

I wouldn’t expect to replace any ECMs in my systems for a long time. If the static pressure of the ducts is low (say 0.5” WC or less) then your ducts should not be eating up your ECMs.

I have learned to always test for the TESP of existing systems before recommending any new forced air equipment.

The lifetime energy cost savings of an ECM vs a PSC motor are hard to ignore.

Kaos

100F air is not cold. The key to comfort is for the air handler to be running at some low speed pretty much all the time. Low speed means low approach temp, so air supply will be near boiler SWT. Nobody will complain about 120F air. It should only kick into higher speed for setback recovery.

The best location for heat supply in cold climate is bellow windows. Don't even think about ceiling unless you have a well sealed and insulated house, even there you want some decent throw to mix well with colder air near the ground.

Under windows also means generally away from people.