HVAC circulation timing

[Ashful]

We have a boiler with 8 zones for heating the main part of our house, but our AC is broken into only three zones. One of these zones covers three floors in the oldest part of our house, with the thermostat on the middle level. This works quite well when it's hot enough outside that the AC is running frequently, but on days when it's less than maybe 85F outside, the system does not cycle frequently enough to keep the floors equalized. It's common for the middle floor (bedrooms) to be cool, while the upper floor is very hot, and the lower floor gets a little uncomfortable due to TV's, computers, etc. on that level.

If I could just have my thermostat set to cycle the blower on for a minute or two every 20 minutes, it would completely solve the problem, but my thermostat (Honeywell RTH7500D) does not appear to have that option. I am wondering if I'm just missing it. If I have to go to another T-stat to get that capability, I'd hope to find another with the same user interface, so as to not confuse the wife and kids (we use 5 of the same, on the heating zones that we auto-cycle).

I've thought about just adding the timer separately to the blower itself, but then I'd have to manually enable/disable it throughout the season, and it would not be sync'd at all to the t'stat (no reason to have the timer cycling on/off (possibly short-cycling blower) if the t'stat is already doing the job on a hot day. It's best to have this built right into the t-stat.

Ideas? Thanks!

 

[heat seeker]

My 'stat has an option to keep the fan running all the time. It runs at a low speed, so is quiet and uses little electricity. Perhaps that would work for you. I have "AUTO" which cycles the fan with the AC, and "ON" which runs the blower at low speed, and runs at higher speed when the compressor is running.

That moves enough air to keep it from stratifying between the floors.

 

[Ashful]

Same here, except our fan is single-speed. My thermostat does have the capability to control a multi-speed rig like yours, but my house wiring doesn't support it. It's usually handled thru an extra wire (or two).

 

[TradEddie]

I'd doubt that you'll find a regular thermostat to do this, but its been a while since I shopped for one. I can program "fan only" periods when the system is "off" (not set to cooling or heating). but I am limited to four periods per day. My thermostat is a 12 year old Honeywell, so I'm sure yours can do this too.

If you could find a suitable timer with relay, it would be very simple to put it in parallel with the thermostat switch wiring for the fan. Finding a a suitable affordable timer might be a bigger challenge than the wiring.

If you do ever find yourself replacing the furnace, don't feel limited by the house wiring. There are now wireless thermostats where you can mount a receiver at the furnace which receives signals from a thermostat than can be located anywhere in the house. I was tempted to buy one of those and put it upstairs in the hot rooms, but the cheapest was about $500 when I last looked, and they were very difficult to search for because of all the Nest type wifi devices, which are totally different to what I'm describing.

TE

 

[woodgeek]

The EcoBees have that option. In the heating shoulder seasons I set mine to run 'at least xx min per hour', and set xx to 15. It ignores the request during normal cycling, but runs 5mins out of every 20 when there hasn't been a cycle in the last hour or so.

In cooling season, this may not be satisfying since running the handler between cycles ends up killing the dehumidification. The coil 'holds up' a pound of liquid water that has not drained yet, and when you air cycle with a warm coil, you re-evaporate it. Kinda a limitation of existing AC tech. I use a ceiling fan in a two story space to keep things equalized summer and winter in my 1 zone house, and it helps.

If you change a stat and get periodic circulation, but excess humidity becomes a factor, there are a couple options. On my ecobee you can set a min AC runtime per cycle. In my house, the (oversized for heating) HP would cycle for like 3-4 mins only. By setting the min run cycle to 8 mins, I get a lot better dehumidification. So, perhaps even with cycling you could get an acceptable result. Another option (if you have a variable speed air handler) would be to drop the cfm on the handler (requiring jumpering the control board). The lower cfm would give you higher dehumidification that could compensate the cycljng effect.

But of course, you know the real answer is to airseal and insulate the attic. Unless the upper story has a wall of glass or skylights, it should not be heating up if the attic is airsealed and code-insulated.

 

[Corey]

Cheap / easy stuff I can think of: Fan to circulate hot air to second floor, and cool air to other floors? Close registers on second floor forcing more cool air to first / third floors? Replacement thermostat which does have a 'circulation' option.? When I lived in a 2-story house, it was not uncommon to completely reverse the open/closed state of all registers in the house based on summer / winter operation, but doing so would almost completely even out the temps.

 

[Ashful]

Good info, guys. But problem is not our second floor as much as the... fourth. I was simplifying the problem in the OP.

Our lowest level is treated as basement, it's just a recreation room that we don't use too often. It's mostly above grade, actually walk-out on one side, but we call it the basement. Living space on second level, bedrooms on third and fourth. This fourth floor was finished maybe 10 years ago, so insulation is "modern", as far as can be done in 2x6 vaulted ceiling configuration. About 1/4 of the fourth floor is un-finished, and that's where the air handler resides.



We do have a ceiling fan on the fourth floor, but not directly over the staircase. This is an old central-hall four-square, with completely enclosed stairs on each floor, so that doesn't help with air movement.

Of all the ideas, the EcoBee might be the answer, but I did not consider the humidity problem of cycling the air handler without cooling. Hmm...

Setting the cooling to run every so many minutes, whether needed or not, might make the place mighty cold some days... no?

Again, no troubles on hot days, when the system is cycling. It's on those marginal days, when the system is not cycling enough but high pollen levels have us keeping the windows closed, when things get stuffy in here.

 

[woodgeek]

The xx minutes per hour algo on the ecobee was for the air circulation, not the AC or heat. That is still by setpoint.

 

[Ashful]

I think I'll play with manually cycling the blower a few times each day, and see how that works. If it seems to do the job okay, then maybe I'll swap that Honeywell for an EcoBee. I liked having all the same T-stat throughout the house, but that's not top priority.