Humidity Control in Mild Climate - San Diego

[Offsite]

Dana, et. al.

I am hoping to have my existing Trane HVAC residential system replaced with one that can help control humidity better on days when humidity is high but temps are near or below the set point.

There is not enough space in the 'furnace room' for a whole house dehumidifier to be installed in parallel with the HVAC system.

To me it looks like a system comprising a Trane air handler (for reasons given below) with an A/C coil and a hydronic heating coil fed by a tankless W/H could work in our environment. It's rarely very hot or very cool... just humid above 50% RH most of the time.

It seems to me that, if some sort of smart thermostat exists that can control this system appropriately, the system can be made to function as a whole house dehumidifier on these 'shoulder days', as I've learned they are called.

In theory, when in dehumidify mode, the A/C coil would remove water (but also cool the air below the set point) and the hydronic coil would rewarm the air back up to the set point (actually lowering the RH ever further).

I've scanned brochures for other major brand systems, but the only one I found so far where the hydronic coil is downstream of the cooling coll is the Trane Hyperion air handler.

So, my two questions are: 1) can the system described above really act as a whole house dehumidifier during shoulder days, and 2) are there additional brands where the (re) heating coil is inserted after the cooling coil?

Thanks!

-rb-

[Dana]

To be efficient at dehumidifying without excessive sensible cooling the system needs to be able to run the air handler at a low speed. If you're then burning natural gas or something to restore temp to large volumes of cool air it's a power-and-fuel hog. You're essentially pumping the heat-of-vaporization of the extracted humidity, but also that of the reheat coil out of the house via the AC condenser. The better the sensible-cooling efficiency of the AC system is, the more of a fuel & electricity hog it'll be running it as a dehumidifier.

There may be continuously variable speed air handlers that can be controlled for maximum dehumidification/minimizal sensible cooling, but I haven't looked into it. It's easier to pull off if both the compressor and air-handler are continuously variable over a wide range, where the air handler is running a miniscule volume of very dry (but also very cold) air. Mini-splits with "dehumidify" modes run the blower at the minimum speed, and modulate the compressor speed to keep the coil as cold as possible without frost-up. While this lowers the temp of the room somewhat if the duty cycle is high, the air in the exit stream is quite dry (dew points below 40F.) In normal air conditioning operation the exit air of the same system is in the 50s or higher, with higher dew points similar to or higher than that of your central-air system. (Very efficient for sensible-cooling.)

In modest to mid-sized homes a 65-70pint room dehumidifier with a 1.8liter/kwh rating would be enough to handle the latent load, and would be more efficient than running the AC & hydronic coil for net-zero sensible cooling, if not as efficient as a whole-house dehumidifier. Smaller versions tend to have much lower efficiency- it's worth the extra $50-100 up front, even though a 30-pinter would still be able to handle the load. If place the dehumidifier in a room with a large return-air register and cycle the air handler ~5-10 minutes per hour to redistribute & mix the air it can keep the RH & temps relatively even throughout the house.

The temperature of the output stream of a room dehumidifier should be well above the room temp, since it's extracting the heat-of-vaporization of the water it's condensing and adding it to the air stream. When it's running a high duty cycle this will raise the temp of the room somewhat.

On larger commercial systems there are many methods of restoring the temp of the dehumidified airstream with high efficiency using heat pipe technology similar to that used in evacuated tube solar thermal systems but I've never heard of residential sized systems. See: http://www.epa.gov/oaintrnt/energy/hpipe.htm and http://www.heatpipe.com/abouthpt/heatpipes.htm

Above 50%RH isn't a disaster from a health & comfort point of view, but it may aggravate those with dust-mite allergies. Above 65%RH is a hazard from a mold spore and fungal infection point of view. I try to hold the line at 60% RH at my house, and running a room air conditioner set to 60% RH in the basement (the coolest room in the house most of the time) is usually enough to keep the rest of the (warmer) house in the 50s without running the central AC at all. I have it set up to drain in to a sump rather than manually emptying the bucket. It uses between 400-500kwh/year (based on monitored performance) which is as much as a full sized refrigerator (without ice-maker), but that's largely due to a very low sensible AC load and extremely intermittent use of the central AC (fewer than 20 hours/year most years.)

[jadnashua]

In an old Trane add, they compared two identical houses and HVAC systems, the only difference between the two was one had a variable speed fan and the other the more conventional single speed one. Their add claimed the variable speed fan equipped unit would draw enough extra moisture out of the air over it's life to fill a typical backyard swimming pool. I'd look into that as a first option. Re-heat is done, but it is very energy intensive. If you tend to open the doors and windows at night, you're letting all that humidity back into the house, so keeping it closed up can help, too. Once all of the walls, carpets, furniture, etc. in the house get saturated, it takes a long time to dry them all out again.

[jimbo]

Maybe I am just too easy to please. but I would not even consider spending money on dehumidification equipment here in San Diego. Are you near the ocean?

[Dana]

Maybe I am just too easy to please. but I would not even consider spending money on dehumidification equipment here in San Diego. Are you near the ocean?

In a tight house humidity must be mechanically controlled even in relatively-dry San Diego. Dew points there are low enough that most of the time it could be achieved via ventilation with outdoor air (preferably with a heat recovery ventilator to preserve sensible heating/cooling). Short of that a coupla Franklins for a room dehumidifier is going to be much more cost effective than a whole house dehumidifier. Variable speed air handlers can help maximize the latent cooling, but you'd still only be running it when you have a sensible cooling load unless it's coupled with a variable-speed compressor (the mini-split case.) In the shoulder seasons when there's no or very low sensible cooling load, indoor humidity can still dreep up, even when outdoor dew points are relatively low.

The dew point of 50% RH 70F air is about 55F, which is about the average dew point of outdoor air in San Diego in May or October. From June<==> September the average outdoor dew point is a bit higher, but so is the sensible cooling load and a lower speed on the air handler could deal with it. From November-April the average is below 50F, and nearly all of the dehumidification could be handled with outdoor. It's really just the shoulder seasons where a dedicated dehumidifier is called for.

[jimbo]

Again, I must be easy to please. We ran the air conditioning maybe 10 days in 2011 so far. Usually just from about noon to 5:00 pm or so. The rest of the time ...the windows are OPEN. Window open at NIGHT ~365 days a year.
We use very little heat as well, just sometimes to take the chill off in the morning or evening.

FRESH AIR...that's the secret! The "secret magic climate" is why we are willing to pay $4 for gas and god only knows how much for a little house! Don't let the secret out..everyone will want to move here. For our east coast friends...WE do NOT have earthquakes, floods, hurricanes, or tornados!

[Offsite]

Thank you all for the thoughtful replies!

We are on a peninsula between ocean and bay... so, most days, we do see higher outside humidity than one might otherwise expect. We were shooting for RH levels of 40 - 50%, but after reading your remarks, we now believe that 50% plus or minus is a much better target... and one that is more easily achieved.

As the result of your information we will try to get the following installed by a local contractor:

1) Variable speed air handler that can run as low as 40% of max air flow rate.

2) Twin compressor roof unit designed to support that low flow air handler.

3) Hydronic heating coil to run in heat mode only (abandoning the concept of 'whole house dehumidifier')

4) Tankless (or possibly tank type) water heater to supply hydronic coil water plus heated tap water.

Our thinking on the hydronic coil is that it eliminates the 2nd combustion chamber (gas furnace) from inside the house, and assures no combustion chamber heat exchanger is in the room air stream. Further, if we pick a condensing tankless unit, combustion air and exhaust for that are essentially isolated from room air by sealed intake and exhaust pvc tubing (unlike the tank type, which draws it's air from the room and exhausts into a vent across an open air gap).

Also, abandoning the 'whole house dehumidifier' quest dramatically simplifies the thermostat specification and eliminates likely an order of magnitude of complexity from the overall system design.

So, again, thank you all for this valuable help!

-rb-

Btw, As a fresh air fanatic, I would love to sleep with windows open, but we are having a series of home invasion burglaries here (silverware, mostly), so everyone has started locking up tight 24/7.

[Dana]

jimbo: Unlike Offsite, you're OK with running interior RH above 60% occasionally (which will happen with open windows in summer), whereas he's holding the line at 50%, so in that regard you ARE easy to please.

Offsite: A condensing tankess can be run at low temp/high efficiency with hydronic coils in air handlers, as can condensing boilers and condensing tank HW heaters. Some HW heater vendors will void the warranty in space heating applications, and others will shorten the warranty period. Both Navien and Rinnai have come out with dedicated combi- units that might be more appropriate, but a 50-80KBTU condensing boiler and an indirect tank will likely outlast them. Non-condensing tank heaters take a bit of design work to keep them from self-destructing from flue condensation when used in this type of application, but the Bradford-White Combi-2 is purpose built for low-load space heating (but will have a much lower AFUE than a condensing tank in the same application.)

BTW: Being on the peninsula doesn't affect the dew point of the outdoor air (unless the salt-spray is literally hitting the house), but it does moderate the outdoor air temp which means the AC will run a lower duty cycle and dehumidify less. Most of the year you would be able to efficiently dehumidify to 50% or lower using outdoor air using an HRV.

[jimbo]

Btw, As a fresh air fanatic, I would love to sleep with windows open, but we are having a series of home invasion burglaries here (silverware, mostly), so everyone has started locking up tight 24/7.

Cornado?

Fortunately I live in a second floor condo, so we can leave some windows open in relative security. And no one in our neighborhood even owns any silver, so we are less of a target!!