thermostat change

[crs7200]

I have had a outdoor boiler for 5 years. I have 2 thermostats. One is for the forced air furnace that isn't used unless the boiler is off before heating season. The 2nd was added by the installer and all it does is turn the fan on and off in the forced air furnace to push the heat through the house . Of course there is a heat exchanger in the furance that the hot water travels through.

My question is this?????
The thermostat that they installed in not a low voltage one like on the oil furnace. It is just a plain old dial thermostat ( I beleive a line voltage thermostat, but not sure) and is completely inaccurate and very touchy. You move it just a bit and it competely changes the range where the heat turns on and off.

What can I use to replace this thermostat that will be easier to set and have more accurate temp. control with a 3-5 degree temp. differenctial?

Thanks for the help :ahhh:

 

[rkusek]

I would recommend a Honeywell FocusPro 5000 series. These are non programmable ones so the prices are pretty cheap. I picked this model # (#TH5110D1006.) on ebay for $23 still in box. This model was the basic one of the series having only 1 stage heat/ 1 stage cool. That's what the two 1's in 5110 stand for. This is what I use to turn the blower on when I using my wood boiler. My heat pump thermo is the same series but a 5320 or something like that meaning 3 stage heat / 2 stage cool I guess. I wanted it to be similar because they both read either the same room temp all the time or the only differ by 1 degree at most. This allows me to set the boiler t-stat at 71 and the heat pump at 69. I've never had any t-stat keep the temp right at the setting like these do. when the room temp drops to 70 and I have hot water at the aquastat the blower and circ come on and run until its back to 71...My house is either 70 or 71 all the time when I have hot water available. Same is true with heat pump, either 68 or 69. They still have a setting for minimum off time, I think mine are set for 5 minutes or something like that. Contrary to gas furnace, this is how they want you to use a heat pump I guess and I find it works very well with the wood heat too. My blower is wired for the lowest speed for all cases and when using boiler heat I don't hear the hum of the compressor on the heat pump. It's much more quiet with just the blower cycling. You really have to pay attention to notice when it comes on.

This is a 24VAC thermostat with battery backup also. You will probably need a DPDT relay with 24VAC coil ($10-15) unless you're pumping water 24/7 throught the HX. If you are it actually makes things simple but is wasteful if you are letting the furnace heat the house when your boiler runs out of wood because the HX will transfer precious gas heat back to the boiler water. If you are not familiar with wiring and electricity have a friend who is help you.

 

[crs7200]

[quote author="huskers" date="1291551101"] This allows me to set the boiler t-stat at 71quote]

Just so I understand, all this 5110 t-stat is doing is turning on your blower fan?? And it is NOT a low voltage t-stat?

If you have it set at 71, what is the differentail?

The one I am using now will turn on around 71 and run to about 75-76. But it is very touchy like I said. Barely touch it and it will change the settings dramatically. Sometimes it seems to have a mind of it's own??

 

[sgrenier35]

Aube makes some nice digital line voltage tstats

 

[rkusek]

This allows me to set the boiler t-stat at 71quote]

Just so I understand, all this 5110 t-stat is doing is turning on your blower fan?? And it is NOT a low voltage t-stat?

If you have it set at 71, what is the differentail?

The one I am using now will turn on around 71 and run to about 75-76. But it is very touchy like I said. Barely touch it and it will change the settings dramatically. Sometimes it seems to have a mind of it's own??

Your existing furnace should already have a blower relay. Probably 24VAC coil that switches 240VAC to power the blower. The output of the 5110 is 24VAC and could tie directly to this existing coil. Your existing furnace probably has a fan only button (terminal G) and this also connects to this coil. In addition, the furnace also has some mechanism that powers the blower relay when it is warm enough to provide heat that also connects to this relay. You didn't say whether your water circulates continously or if it is somehow tied to the line thermostat. That is where the extra relay would come into play. Since you have an OWB maybe it only circulates when it is hot enough. If you are circulating cool boiler water through the HX when the gas furnace is trying to heat the house your gas consumption will be noticeably higher. My electric heat pump works considerably harder when I had my boiler water circ hook up to run 24/7 and the wood had burned out. Describe you system more or post a diagram. There are many variations and ways to accomplish what you are after.

As far as differential, that is somewhat selectable on he menus in the 5110. Mine I guess is zero. When set at 71 degrees, it comes on at 70 and runs for awhile after it hits 71 but never goes to 72. So my home is always at the set temp or maybe 1 degree less and very comfortable. The manual has settings called cycles per hour (CPH). Mine is set for electric furnaces (9 CPH). Low efficiency gas is 5CPH and high (90%+) gas furnaces are 3CPH. Looks like you can set it anywhere from 2 to 12 max CPH (ie. every 5 minutes up to every 30 minutes). I should emphasize this it the max CPH and doesn't mean it always runs that way every hour, only when it cools enough to deem it necessary. I guess maybe on the lower CPH settings it compensates by "building in" differential of 5 degrees like you describe. This is how the gas furnace in our old house worked and I didn't find it very comfortable. Always hot-cold-hot-cold, etc. Here is a PDF file of the 5110 instructions. [url/]http://www.houseneeds.com/shop/manuals/honeywell_th5110dmanual.pdf[/url]
I think this kind of temp control is what your are seeking based on your original post. A big temp differential is not necessary since your OWB probably has a constant temp and you're only cycling a blower on and off like I do when heating with wood.

 

[crs7200]

Huskers
Turning the blower fan on and off is exactly what I need it to do. Nothing else. It doesn't have to turn on any pumps or anything. Just the fan in the oil furnace that blows the hot air thru the house. The reason I ask about the differentail is this. Before turning to wood for heat I used my forced air oil furnace. The old fashioned ( but still commonly used) dial thermostat is what is on the wall. I don't know what kind of diiferentail is built into these things, but I noticed it would run a few degrees above the set point, turn off, and then on again when it reached the set point. I'm sure you know what I am talking about.
That is pretty much all I want the new t-stat to do. I installed a t-stat last year that said it had a =/- of less than degree. As soon as I hooked it up and turned the power on, it turned the blower fan on and off about 25 times in less than a minute Not sure what that t-stat was meant for, but it didn't work for me. I immediatly unhooked it.
Will the 5110 work like the t-stat that is hooked up the oil furnace?

Thanks

 

[rkusek]

The 5110 and maybe that one you already tried has settings for cycles per hour (CPH). The 5110 can be set from 2 to 12 CPH (ie. every 30 minutes to every 5 minutes). I think it makes adjustments for differential based on this setting. For Electric resistance heat is says use 9 (every 7 minutes) CPH, heat pump about the same I believe, old gas furnace 5 CPH (every 12 minutes), high efficiency gas 3 CPH (every 20 minutes), steam heat 1 CPH, etc. Since you will only be cycling a blower the 9CPH setting should be ideal I would think.

 

[rkusek]

You will need 24VAC power at the thermostat. I'm not sure if the old dial type had that.

 

[adkhelo]

CRS,
Don't know if this might help. I have a similar situation here. OWB Circulater pump runs 24/7. Basement oil forced air furnace with a fan limit switch (located just below the plenum), and an old Honeywell round thermostat (24 volt?) in my kitchen. Plumbed the water/air hx into the plenum (of course, due to its location in the plenum above the fan limit switch, it never triggered the fan switch.) First dilemma was to make a 24 volt thermostat turn on the 120v fan only, when the house called for heat. Couldn't figure it out (did it before finding this forum.) What I was trying to avoid was having to fish wires through the wall. Finally broke down and fished a 12/2 w/ground up though the wall and installed a 120v electric baseboard thermostat in the kitchen. I found the junction box inside the furnace, turned off the power, and located the wires going to/from the blower. One fan wire was wire-nutted to the white/neutral wire from the power feed, and the black/hot was wire-nutted to several wires, but noted one from the fan limit switch and the hot line power feed. So, I figured, no problem, just add the corresponding wires from the new thermostat to the same colored wire-nutted gangs. Does this sound like the set-up you already have? I also added a strap-on aqua-stat to return line on the hx and set it for 140F. This way, if the fire in the boiler goes out and the water cools, the fan will not engage and blow cold air. Then if the old Honeywell thermostat, which is set 5F below new thermostat, calls for heat, the oil burner will kick on and serve as backup heat.

Unfortunately, I bought a cheap electric baseboard thermostat and it isn't precise with temperature regulation. I'm, searching for a more precise 120v thermostat to replace this inaccurate one and I hope I'll have less of a temp swing in the house. It ain't perfect, but not hearing the oil burner fire off is sweet!

Hope this helps and always open to new ideas. Let me know if anyone has found a better 120v thermostat.