Dehumidification with chiller.

drooplug

I recently installed a hydronic heat pump. For air conditioning, I have an air handler and a hydronic coil. The system is meeting air temperature set points, but the relative humidity remains in the mid 60s. I've tried reducing the fan speed on the air handler with no effect. The coldest supply temp to the coil is 45F and seems to get as high as 55F. I would appreciate any advice as to how I can increase my dehumidification.

Jamie Hall

Keep in mind how dehumidification occurs: the air has to be cooled down so the excess moisture condenses out. At an air temperature of 70, the dew point is 55 with 60% relative humidity, so you have to cool the air to 55 to get to 60%. If you want it drier, you have to cool the air lower.

But... that only applies to the air that is cooled. Unless all the air in the building is cooled to that, the mixed air will have a higher humidity.

And furthermore -- you then need to reheat that air back up to the desired temperature, without introducing more moisture.

Unless you have reheat on your system, then, what you need to do is to drop the air temperature through you fan coil to a much lower temperature -- near freezing -- and put as much air as possible through it. You may simply not have the capacity.

drooplug

Well, I definitely can't get the water temp lower than 42F. So it would seem that I may need to add a dehumidifier to the system.

mikeg2015

What are you supply temps? Common to use 45F water and 50F supply temps to dehumidify without condensation on the ductwork. Also check water flows. What is your delta T? Should be no more than 10F. Probably 5F for dehumidification. THis of course hurts chiller efficiency.

What’s the total installed capacity and total capacity of the indoor units? Having too much coil area compared to the total load will hurt as well. You might also be able to restrict water flow to keep part of hte coil cold and sweating.

If you had seperate hot water coils you could do reheat.

Let me guess.... you have a foamed house? Closed cell or open cell? Even if closed cell, moister is pushed into the house by vapor diffusion through the roof deck and foam. IF open, cel foam, or worse fiberglass or blown in without a good vapor barrier, and it acts almost like a desiccant driving moisture into a house, drying the outdoors. I’ve seen 80% RH and 70F in a foamed attic on a 20F sunny day with an outdoor dew point of 5F. I the living area it was 30% RH, about a 40F dewpoint.

You only option may be a good ventilating dehumidifier.

EBEBRATT-Ed

Does the equipment cycle on temperature? If the equipment is oversized it can shut off on temp and not remove enough humidity.

Look into a larger (thicker coil) for the AHU if need be

ratio

Does the unit produce condensation at all when it's running? If it doesn't produce any at all, coil temp isn't low enough. If it produces lots when it does run, run time is too short. If it produces some but not lots, both issues exist.

ChrisJ

@drooplug You do not mention the indoor temperature you are maintaining.

Since your coil is a fixed temperature this is the other part of the puzzle. The lower the indoor temperature the colder you need to run your coil.

SuperJ

Check you piping to make sure it's counter flow. As in the coldest water is on the leaving air side of the coil, with the warmer water on the return air side.

Reducing the fan speed was the right thing to do, keep it on the lower setting and investigate if you can go lower. Some ECM fans have multiple jumpers and dip switches to adjust the fan speed fairly precisely.

Make sure your fan shuts off between cooling run cycles or the condensed water on the coil will evaporate back into the air stream. the effect is significant.
https://www.energyvanguard.com/blog/76674/This-Thermostat-Setting-Can-Cost-You-Money-and-Make-You-Sick

Some thermostats like the Ecobee allow a controlled amount of overcooling to attempt to control humidity. There is a setting to specify how much overcooling you'll allow to get longer run cycles to dehumidify more, as well as a rh% setpoint.

The chiller maintaining a pretty steady temperature, or is it cycling and fluctuating? You really want your supply air to stay below 55f max for dehum.

drooplug

Supply temp is 45 with a Delta t of 5 or 6 degrees. It does fluctuate. I haven't watched it long enough to get the exact behavior, but the supply will get as warm as 55. However, the leaving temperature doesn't seem to rise much above 55 when this happens.

I have about 6-7 GPM of flow. The coil is pipied with the supply water and the leaving air side of the coil

I believe my heat gain is 28,000 to 36,000 btu. The coil is sized for 3 ton. There is a buffer tank between the coil and heat pump. The coil does generate condensation, but not a ton.

I am trying to maintain 72 degrees in the home. I've currently have the thermostat set at 70 to see if that changes anything and it hasn't made a noticeable difference.

My home is closed cell spray foam in the upstairs and blown in cellulose downstairs.

The air handler does have long runtimes. I've tried blocking part of the coil with foil to decrease it's size. No improvement in humidity.

Did I miss any questions?

ChrisJ

drooplug said:

Supply temp is 45 with a Delta t of 5 or 6 degrees. It does fluctuate. I haven't watched it long enough to get the exact behavior, but the supply will get as warm as 55. However, the leaving temperature doesn't seem to rise much above 55 when this happens.



I have about 6-7 GPM of flow. The coil is pipied with the supply water and the leaving air side of the coil



I believe my heat gain is 28,000 to 36,000 btu. The coil is sized for 3 ton. There is a buffer tank between the coil and heat pump. The coil does generate condensation, but not a ton.



I am trying to maintain 72 degrees in the home. I've currently have the thermostat set at 70 to see if that changes anything and it hasn't made a noticeable difference.



My home is closed cell spray foam in the upstairs and blown in cellulose downstairs.



The air handler does have long runtimes. I've tried blocking part of the coil with foil to decrease it's size. No improvement in humidity.



Did I miss any questions?

If the system isn't running long enough, it's not going to pull enough humidity out. Especially with a discharge temp of 55 degrees if you're trying to maintain 70F.

Here's a simple dew point / RH calculator. Mess around with it a bit.
http://www.dpcalc.org/


A dew point of 55 at 70 degrees best case gives a RH of 60%.

I maintain as low as 68 in my house but my evaporator gets colder as the air gets colder. I'm sure it's been near freezing a few times as my discharge temps were 41-42 degrees.

Notice a dew point of 41 gives a RH of 38% @ 68F.

You either need to raise your indoor temperture or lower the coil temperture. Most likely the best solution is a separate dehumidifier. This will keep things good regardless.

SuperJ

Where is your air-handler? Is it in an attic? Any duct work in unconditioned spaces? It's common to have AHU driven infiltration from duct leaks (both supply and return). Any pressure imbalance is going to up infiltration/exfiltration.

That's great that you have a buffer tank. Perhaps you can lower the dead band if you're getting nice long heat pump cycles? Does it have an aqua-stat to control the heat-pump? Does the heat pump compressor cycle while the coil is calling, or run continuously?

How is that tank piped? Is it dedicated to cooling only? For cooling you want to draw off the bottom to the coil, or do a 2/3 pipe buffer to get direct access to the cold discharge from the heat pump.

How does the actual coil entering water temp compare to the controller sensor for your buffer tank.

These are heating diagrams but the flow characteristic apply to cooling as well (except you want to use the bottom as your supply and top as return).
https://www.pmmag.com/articles/100544-the-finer-points-of-applying-a-2-pipe-buffer-tank
https://www.pmengineer.com/articles/92398-different-ways-to-pipe-a-thermal-storage-tank

drooplug

@chrisj

My leaving water temp is 55, my leaving temp is around 67, I think.

drooplug

@superj

The air handler is in the attic. However the attic is conditioned.

What do you mean by dead band?

I have a single buffer tank. The flow is reversed for cooling so the heat pump supplies the coldest water to the bottom of the tank and the coil draws from the bottom of the tank.

The way the tank temp is maintained for cooling is totally wonky. The heat pump monitors the entering water temperature of the heat pump. It will turn on the compressor when the water temp is 2C above set point and turn off when 2C below setpoint.

The heat pump comes set up to continuously run the circulator so it can monitor water temp. I changed it so it would cycle every 3 minutes to check.

drooplug

What's the best way for me to size a dehumidifier? It seems like to smallest units offered will do, but I want to make sure I get it right.

EBEBRATT-Ed

The problem look like a coil problem to me. If your leaving water is 55 and your leaving air is 67 that is an issue. You leaving air temp should be closer to the leaving water temp. You not getting the air cold enough to dehumidify properly.

Make sure that no air can bypass the coil without going through the fins. Also check the duct work. a disconnected supply or return will drive the system nuts. Also, to control humidity the space should be as tight as possible

mikeg2015

I agree the airflow is too high from the output of the chiller. at 7GPM and a 6F temp rise, that’s only 21,000 BTU. IF you used “nominal” chiller ratings for sizing the AHU, then you made the classic engineer mistake I see all the time. You need to match specific conditions. The chiller output at 45F LWT, 55 EWT and 95F OAT

Actually, if you slow the airflow and water flow, you’ll get more humidity removal, higher EWT, which results in more chiller capacity.

I think you are slightly over pumped (want 8-10F delta T) and too much airflow.

Best setup is a dehumidistat that stages AHU airflow (if it’s an X13 or VS ECM, or can use a fan relay for PSC).

The bad news is that you could be short of sensible capacity on the hottest days, but I suspect you may have enough mass to ride through peak loads. Load calculations assume you have no mass and use peak heat loss not a rolling 6 or 12 hour average. If you use a rolling average, actual peak heat loss can be 25-50% lower than load calculations show.


For example, conventional calculations... and my electric bill, show that I need 6 tons of cooling for my house. Built because it’s brick, high mass, I get away with 4 tons and it just runs a long, long time (RH is 40-45% upstairs and 50-55% downstairs (ballon framing & pocket doors downstairs infiltrate air by reverse stack effect from attic).

I bet you have enough total capacity, but just need to reduce water flow and air flow to get the latent capacity and increase chiller output slightly. Again, shriller output will be a function of Entering Water temp, and flow rate, but affected more by EWT once a minimum flow is achieved. SO for example, at 5 GPM you might get 55F EWT, but at 6GPM you get 53EWT. In that scenario, you get 1000BTU more output from the chiller.... if my theoretical numbers are correct.