Running My Heat Pump at 73℉

[Mr_Luke]

I had a talk with an HVAC man today. I told him how running my heat pump at 77-78℉ is a problem this summer and has been hard to keep the RH in the comfort zone at this temp. He recommended just what I thought has been helping lately, is running it at a lower temp. I turned it down from 76 to 73℉ and the RH is now well into the zone now, at under 50% for once. It feels like I'm in a refrigerator but I guess I'll get used to it. I bought a dehumidifier that I should have in a few days. But I don't know, running the heat pump at a lower temp might be more cost effective that running it higher and having the dehumidifier kick in once in a while. Thoughts? I always functioned better in dry air, not humid air. I'm in East Tennessee but lived in New Hampshire for over 50 years. I can't afford to move to a drier climate but I can get rid of the humid air in my house.

 

[greenjp]

Lower set temp equals longer run times which increases moisture removal. Some thermostats that have a dehumidification feature will over cool from your temperature set point by a few degrees if you're a particular amount above the humidity set point.

The dehumidifier will really help in moderate weather when the humidity is high but the temperatures aren't. If it's 78 and humid you won't be able to run the AC long enough to do much about it, but the dehumidifier which is designed to remove a higher percentage of the latent heat (humidity) will still be able to work.

jeff

 

[Mr_Luke]

The HVAC man suggested I put a fan in my basement, which is 80% crawl space, to move the air around and get rid of the excess moisture but to be honest, I don't know if I will do that now that lowering the temp settings and the dehumidifier seem to be working well. Refreshing the air in the basement he says will lower the amount of moisture in the HVAC ducts that may be getting blown into the house at higher settings.

 

[Mr_Luke]

The RH is on it's way up again. It seems to go up every night, It's 62% and I can feel it. It's been in the 70s often when I get up in the morning.
We'll see. I'm experimenting with it.

 

[MParr]

Do you have a hygrometer to measure your indoor humidity?
I run my heat pump set at 74 during the daylight hours. I will back it down to 73 between 11:00pm & 6:00am. My indoor humidity stays around 41%. Many so-called experts claim that between 30% & 50% humidity is the ideal range.

 

[MParr]

I see you have a basement/crawl space. Is it a usable basement or is it just a crawl space? If it’s just a crawl space, you need more ventilation. My home is built off of the ground with cinder block piers and sills. Thick plastic is laid on top of the soil and there is plenty of venting holes around the outside. It’s never wet under there. However, my duct work is in the attic and vents through the ceiling.

 

[SZR48207]

Great case for a an inverter compressor system like they use in other countries (Mitsubishi, Daikin, etc). Variable compressor output that can keep humidity at bay while keeping the temps from dropping too low.

 

[Mr_Luke]

Do you have a hygrometer to measure your indoor humidity?

Yes, I got a hygrometer a few weeks ago and am kind of obsessed with watching the changes in RH.

 

[Mr_Luke]

I see you have a basement/crawl space. Is it a usable basement or is it just a crawl space? If it’s just a crawl space, you need more ventilation.

It's just for the water heater, heat pump blower, and circuit breaker box. The floor is dirt.

 

[samven]

I have a very similar situation and found that running the ceiling fans have mostly solved it. I have fans in all the bed rooms and the living room and they run on low all summer long. It allows me to keep the temp set at 77 F and for some reason the humidity stays down. When I do get one of those days when it drizzles all day and the outside temp sits at 76 or 77 or the worst when it rains in the morning and the sun pops out and you can see the steam coming off the driveway I may have to bump the temp down a couple deg.

 

[MParr]

It might be to your benefit to check your local building codes. Also, contact a local crawl space expert. There may be something that can be done to the existing crawl space that can remedy some of your problems. If your dirt floor isn’t covered, with heavy plastic sheeting and overlapping seams, that’s part of your problem.

 

[KJSmith]

The RH is on it's way up again. It seems to go up every night, It's 62% and I can feel it. It's been in the 70s often when I get up in the morning.
We'll see. I'm experimenting with it.

If the HVAC isn't running, it wont remove moisture.
When the sun goes down there is less heat load so the unit reaches setpoint and shuts down.
You can off set this somewhat by lowering your night time set point.

FYI, Humidity problems are often caused by oversized units.
How many sq ft?
How big is the unit?

In the hot humid south, we tend to oversize the evaporator by 1/2 ton to help remove moisture.
( think surface area)

 

[Mr_Luke]

How many sq ft?
How big is the unit?

In the hot humid south, we tend to oversize the evaporator by 1/2 ton to help remove moisture.
( think surface area)

My house is 968 SF. I don't know how big the unit is offhand.
I've been here over 16 years and can't recall a Summer when I felt the RH go up in the evening. The RH seemed more consistent in past years.

The unit shuts down as the outdoor air cools slightly, then a fan runs and blows damp air in the house and will not shut off unless I shut it off via the circuit breaker box. The HVAC man told me to leave the heat pump on, not off, but the only way I can combat the damp air is by lowering the set point.
I want to cover the crawl space with heavy plastic. That sounds like an amazing idea.
Plus maybe a fan to move the air around more. .

 

[MParr]

Mr. Luke
Here are some guidelines that will address some of your humidity problems.

http://agr.georgia.gov/Data/Sites/1/media/ag_plantindustry/structural_pest_control/files/2012ircsectionr4084underfloorventilation.pdf

 

[Ws6]

I am probably hell on HVAC things. I leave "fan on" and set to 70. If it gets too humid, I will run it at 78 until it attains temp, and then drop back to 70. The coils kick in after a 2* diff between set and actual, so it does run the furnace (electric) a bit, but this little maneuver takes about 1 hour to complete, and will drop my RH 15-30% depending on the day.

1600sf, 3.5t unit.

 

[krismoriah72]

Everything will change once you get the dehumidifier going. My solution was to put the dehumidifier in the opposite side of the house of the ceiling fan. No idea of the physics of it but it works for me.

 

[Mr_Luke]

I now have a good idea what my problem has been : a faulty switch in the heat pump base unit that was causing my fan to run after it went out of the cooling mode and pumped moist basement air into the house, causing the RH to get up to 84% RH this morning after getting down to 49% last night. But today after I turned it back on at the breaker box, it ran perfect and for some reason that fan doesn't run after the cooling cycle. If it would keep up like this I'll be all set and wouldn't even need the dehumidifier for right now. The switch in the unit is probably corroded due to the basement moisture. I can get it replaced for about $100 and be on my merry way. He's going to stop by in about a week to do it.

Right now the RH is 60% and the temp is 73.2 ℉. I'm fine with that temp and RH. The RH would be lower than that if not for the fan pulling that moist air in last night. But for 16 years this heat pump has kept me comfy here.

It will be good to get the dehumidifier to keep on hand. I appreciate nice air that isn't too humid.

 

[greenjp]

Correct you do not want the fan running all the time during the cooling season, because it'll just blow a lot of the condensed moisture off the evaporator and back into the house. Better to leave the fan setting to "off". Some systems will have a "circulate" type setting that'll run the fan in a low speed mode for some fraction of the off time to move the air around a bit which can be good if you have a multi-story house or uneven temps for some reason, that can be OK.

jeff

 

[Donald]

The HVAC man suggested I put a fan in my basement, which is 80% crawl space, to move the air around and get rid of the excess moisture but to be honest, I don't know if I will do that now that lowering the temp settings and the dehumidifier seem to be working well. Refreshing the air in the basement he says will lower the amount of moisture in the HVAC ducts that may be getting blown into the house at higher settings.

You need to put a vapor barrier in crawl space if it does not have one and a dehumidifier and get the crawl space down to 45% to 50% RH. Otherwise lots of bad things can happen starting with mold. For the most part venting the crawl space will not help as the outside air is higher than 50% RH. So seal all vents. Got to have a dehumidifier, with a condensate pump. Some install a small vent fan in addition to the dehumidifier to get rid of soil gases, radon, etc.

 

[Mr_Luke]

Better to leave the fan setting to "off". Some systems will have a "circulate" type setting that'll run the fan in a low speed mode for some fraction of the off time to move the air around a bit which can be good if you have a multi-story house or uneven temps for some reason, that can be OK.

jeff

I leave my setting on "auto fan". I tried setting the fan to off but due to the faulty switch it ran anyway. That's why I want to get the switch replaced, although it's working again at least for the time being. Setting the fan to "off" did not stop it from running all the time. That's why I had to resort to turning off the heat pump at the circuit breaker box.