08:38PM | 08/30/03
Member Since: 08/30/03
1 lifetime posts
Hello to experienced DIY folks

I have a an upstairs/downstairs temperature control problem for a pretty new 3500 sf house that has 2 heating/ac systems – one for the upstairs (UP) bedrooms, and one for the downstairs (DN) part of the house that includes servicing a 2 story great room in the middle of the house. Here is my issue: I have a very difficult time balancing the 2 systems and stat’s to get the desired effect of a house where the rooms are all about the same temp throughout winter and summer. The location of UP stat is in a central hallway bridge over the great room; the location of the DN stat is in a hall way about 5ft away from the great room. Both stats are located below the return air vents at the correct height.

Here is typical winter time problem (the colder it is outside the worse the problem): the DN system runs to keep the DN rooms warm, but in doing so heat rises up (sometimes >10 deg differential) to the UP stat such that it is never triggered on and the UP bedrooms get pretty darn cold. Of course to compensate for this, I move the temp on UP stat higher in order to get it to kick on. One way I have solved the issue of all the heat gathering in the middle of the house at the great room ceiling is to periodically run upstairs and manually turn on the UP fan from Auto to ON to distribute the hot air collecting around the UP hallway to the UP bedrooms. In winter, the DN system cycles on much more often than the UP system (2 or 3 types more often). The winter differential is worse than summer problem, and the colder it gets outside the worse the issue.

Here is the typical summer problem: the UP system runs much more since the UP gets hotter than the DN. However, the UP ac unit is about 2/3 the size of the DN unit (a 36 vs. a 60). If both stats are set the same, then the smaller UP unit runs much more than the DN unit. The DN unit has much more capacity and does a better job of cooling the house, so to help out the UP system, I set the DN stat 1 or 2 deg lower than the UP stat. This helps keep the UP from running so much, but if I’m not careful the DN rooms get too cold and the UP rooms get VERY hot if the DN system is set to cold since the UP system runs less do to the great room hallway being cooled by the DN system keeping the air around the UP stat cool. Again, as in winter, it can help to run the UP fan continuously to distribute and level the air temp UP. The only problem with raising the DN system stat to the same as the UP stat is that the UP ac system then runs all the time. This I believe will wear out the smaller UP unit. The UP unit appears to be failing already anyway (its only 3 year old carrier) as I had to have it charged this summer.

So, what are my options to fix this? Here is my thinking:
1) Interconnect the 2 systems so that when the DN unit runs for heat or ac, the UP fan kicks on to distribute air. I like this option since this is how I control the house now – it’s just a pain to run up and down to turn on/off the UP fan, and not something I do in the middle of the night. However, the Carrier units needs some sort of a separate switch relay to do this – so it seems like a kluge. I looked into a direct wiring scheme between the 2 units that did not work out. I have also considered a setup where if either the UP or DN system comes on to heat or cool, the other system cycles its fan.
2) See if a 2-stage stat could help.
3) This seems simple enough – move the UP stat into one of the bedrooms; however, there must be a good reason for having the stat near the return air intake. And it won’t take care of the heat collection problem in the winter
4) Get the wireless stats – 2 of them, one for up and one for down – but at $300 each (e.g. the Honeywell), this seems expensive. However, if I can get a 2-stage wireless, then possible find a good location in the house for the wireless stat – this may work even better than option 2. (I assume a 2 state stat can run to separate HVAC systems).
5) Get a 2 HVAC system bi-directional X10 HVAC controller, and the remote stats and write programs/macros to control the systems. This is pretty expensive too – about $400 for the approach – and do I really want to spend that much time messing around with X10 and programming? Not really! (I have 4 sets of X10 security lights right now, so I understand the X10 basics).
6) Look into a commercial 2 system controller

Please send all any and all bright ideas to [email protected]

Thanks, Jeff.

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