Household thermostat / SX question
PLJack
Posts: 398
I have a cheap thermostat, the round type, controlling my oil burning furnace.
The performance (accurate) is pretty poor. I got to thinking I could build a better thermostat with one of my SX proto boards.
So, there are two wires coming from the basement to the thermostat. after some experimentation I discover that if you briefly short those two wires that I read 27 volts AC.
After some web browsing I find that this is normal. Its called a 24 volt system but it seems any voltage in the mid to upper 20s is nominal.
I must be searching the web for the wrong terminology because I could not find a schematic of an oil burning thermostat system.
So here are my questions.
1) Those two wires.
Are they the same wire? In other words is that just the Hot leg of the 24Vac system and shorting them is the same as a switch. Is it safe to directly short them?
2)Power.
Is there enough power on that system to feed an SX proto board / numeric LEDs and a relay?
3) Convert to DC.
If there is enough power, I assume there is a simple circuit to convert that to 5-12 volts DC.
4) Measure.
If we get this far, is the DS1620 thermo chip accurate enough to provide comfortable living temperature?
If I could get those questions answered I would be very happy.
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- - - PLJack - - -
Perfection in design is not achieved when there is nothing left to add.
It is achieved when there is nothing left to take away.
The performance (accurate) is pretty poor. I got to thinking I could build a better thermostat with one of my SX proto boards.
So, there are two wires coming from the basement to the thermostat. after some experimentation I discover that if you briefly short those two wires that I read 27 volts AC.
After some web browsing I find that this is normal. Its called a 24 volt system but it seems any voltage in the mid to upper 20s is nominal.
I must be searching the web for the wrong terminology because I could not find a schematic of an oil burning thermostat system.
So here are my questions.
1) Those two wires.
Are they the same wire? In other words is that just the Hot leg of the 24Vac system and shorting them is the same as a switch. Is it safe to directly short them?
2)Power.
Is there enough power on that system to feed an SX proto board / numeric LEDs and a relay?
3) Convert to DC.
If there is enough power, I assume there is a simple circuit to convert that to 5-12 volts DC.
4) Measure.
If we get this far, is the DS1620 thermo chip accurate enough to provide comfortable living temperature?
If I could get those questions answered I would be very happy.
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- - - PLJack - - -
Perfection in design is not achieved when there is nothing left to add.
It is achieved when there is nothing left to take away.
Comments
You short the two wires to call for heat. The problem with using this as a power source is that you don't have power while calling for heat.
My programmable thermostat has 2 AA batteries that last over a year.
You could probably do it with one of those "memory" capacitors that are like 1 Farad.
If you use a relay to short the 24V wires (to call for heat) you will want to get a "latching" relay. That way you don't need to power it the whole time your calling for heat.
Bean.
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"SX-Video·Module" Now available from Parallax for only $28.95
http://www.parallax.com/detail.asp?product_id=30012
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"I hope the goonwave is not vaporware"
·
I thought as much, just wanted to make sure.
How long do you think a SX 48 proto board that flashed the temp every now and again would last if I connected a 12 volt battery to VDC in.
Great idea.
Thanks Bean, great information.
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- - - PLJack - - -
Perfection in design is not achieved when there is nothing left to add.
It is achieved when there is nothing left to take away.
Please be careful as this is potentially dangerous territory.
The more current your circuit draws, the closer you will get to the "trip" point of your heating system. Not all 24 VAC heating systems are the same, however many use a 24 VAC operated valve (for gas); for oil I'm not sure. It would be awful if your design was such that the current your circuit consumed caused the valve to be teetering on the edge of turning on, and then, due to other aspects of your circuitry locking on in the "heat" state.
Also, your tampering could make the unit continuously cycle on and off every few seconds, possibly extinguishing a pilot light or causing some other defect due to the abnormal operation.
Unless you are very knowledgable about these things, which it seems to me you are not by virtue of your need to search for answers, my advice is to leave it alone, an buy yourself a better thermostat.
Cheers,
Peter (pjv)
All that being said, best bet is what PJV said, fuel oil furnaces typicaly have no pilot light to blow out but if something should fail in your circuit, overload etc and lock your furnace in an on condition it would be a potentialy dangerous situation. Digital set back thermostats for furnaces can be had for as little as $20-30 so for economy is probably a better bet, and for safety far more reliable.
Chris
At that time we discussed 'paracitic power supply'. It seems feasible, but awkward. It would be best to recharge a backup battery.
Obviously there are a lot of homes that have just the two wires and many people would like to convert to a better thermostat [noparse][[/noparse]with timer, maybe with a PIR to recognize people are around, and maybe a clock]. In this way, you could really begin to save $$$ and justify a sophisticated system.· It could know it is a Monday and everyone is out or that it is a Saturday and most people are around.
The easiest, but bulkiest is a wall wart and back up battery. That is likely an excellent place to start. From there you can build your basic system to control a mechanical relay located where the thermostate was.· The traditional thermostat has a mercury switch rated up to 5 amps, so a mechanical relay seems optimal as it is insensitive to AC and can adjust to many kinds of systems.
Alternatively and after resolving all that , I wonder if you could disconnect and reroute the wire at the heater and install a good relay [noparse][[/noparse]say 3 amps to be safe] next to the heater. Control of this relay could occur from many different pathways.
Then, you could·have the relay controlled by a X10 system being fed by your BasicStamp.· Or you could have a PINK connected to it.· In that way you could put the thermostat anywhere and even have multiple locations throughout the house.
I also wonder if X10 would work if adapted to a 24volt AC carrier.
If that is the case, you could convert the two wires to a supply for your BasicStamp thermo at the present thermostats location.
Then you could decode the X10 at both ends. From what I understand the 24VAC transformer would block further transmission of the X10 into the 120VAC, but if the 24VAC transformer is near the heater, there should not be any problem.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Post Edited (Kramer) : 1/6/2006 9:20:21 AM GMT
The worst thing would be that your circuit locked the circulator on and the house the very warm, but aside from using alot of oil I don't think anything really bad could happen (famous last words).
Now if you were controlling the burner, then you have another story. Oil and gas furnaces have a mimimum burn time or bad things happen.
I would like to put a "flow meter" on my furnace to meter how much oil I'm using. Any ever do that before ?
Bean.
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"SX-Video·Module" Now available from Parallax for only $28.95
http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module" Now available from Parallax for only·$49.95
http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
"If I was king for just one day...I don't think I could screw things up any worse [noparse];)[/noparse]"
·
PJV, Nice to hear from you again.
I'm off the idea of feeding the SX via 24vAC.
I understand your warning, thanks.
I will be using a 12vDC battery.
You nailed that one, I'm gathering together the facts.
In fact, it just dawned on me that I have a friend that has about 20 years in the oil heat business.
I will consult with him and let you all know what I find.
I agree. It would have to be designed so that a "lock" is not possible. I wonder how the "off the shelf" product handle that.
I think that is how it works as well.
Like I said, I will consult with my friend and report back.
The one question I still have is how long would an SX proto last on one 12VDC battery?
Thanks guys.
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- - - PLJack - - -
Perfection in design is not achieved when there is nothing left to add.
It is achieved when there is nothing left to take away.
I think furnace burners have a rating for the gallons per hour or some such rating. You might be able to come pretty close by finding a way to tell when the burner is firing, for how long, and just do a calculation. A thought...
Jim
··
As you have heard you can safely short the wires.· There are some issues to beware of, also some safeties are....usually...built into the unit.· I'm fairly sure that it's a safety requirement on these appliances here in the·US.· If it a gas fired furnace it will almost certainly have a high temp limit on the heat exchanger somewhere.· If it’s a hot water boiler gas/oil/etc it should have a pressure release at the minimum.· Short cycling the equipment is something your stat will have to worry about.· This causes mainly undue wear to the valves, contactors, motors, etc.· Oil burners are their own beast that I'm not all that familiar with.· I hear they can pack a big bang if the combustion chamber soaks in oil and then ignites.·
·
Your biggest things to watch out for are short-cycling and loss of control.· Here in Michigan it’s not good when the furnace stops in January.· You can eliminate this problem if you’re really worried about it.· Get two normal tstats, one for a high limit and one for low.· Wire the low limit in parallel with your current 2 wires and,·tie the high in series with the whole mess.· If you set these at 40 & 90 they should never affect normal operations but override in a failure.
·
I have metered utilities many times for customers.· Usually the utilities offer a module that will pulse every X units that attaches to theirs.· Sometimes you need a whole new meter, most likely the case in a home I assume.· You may be able to "tap" theirs though, that could get sticky if they see it.· You can find standalone meters just make sure they rated for it.
·
If you want to know more about what I have setup, just ask.
Post Edited (Orion) : 1/7/2006 4:41:19 AM GMT
Oil burner nozzles are stamped (usually right on the flats of the nozzle itself) with the gal/min rating and also the degrees at which the oil 'fan' will spray at.· These two parameters characterize·an oil burner·nozzle.· If you don't know what your nozzle size is, you (or your oil guy)·probably have not been replacing it often enough. You should be replacing your nozzle about every 2500 gal. of oil used for optimum burn performance.
If you put a time logger on the burner itself, you could easily figure out the burn rate by using this number and the outragous number the oil delivery man will bring you after a fill up.· [noparse]:)[/noparse]
My current heating system is an·oil boiler with a wood boiler linked in parallel.· When the wood boiler jacket is warmer than the oil boiler jacket, the oil burner is disabled.· The DHW is also run off this system and will only reheat the water tank when the wood boiler is on.· The entire system (and display that alerts me when the wood boiler temp is close to the oil boiler temp $$$$$!!!!!) is controlled by a SX28.
Nate
Post Edited (Nate) : 1/7/2006 3:40:08 AM GMT
A lot of furnaces have been adapted from other sources wood, coal, and so forth to oil or gas. Many times this was an easy way to move from shoveling fuel to using a thermostat. And in many rural areas, the furnace is designed to go back to the older fuel and shoveled input for the emergency warmth.
I imagine the East Coast [noparse][[/noparse]where Bean is] is far more 'funky' than the West Coast [noparse][[/noparse]where I grew up]. I did have a saw dust furnace in Oregon that I converted to a gas burner after in backfired in my face one frosty morning. Though the saw dust was 'thermonstatically controlled', I doubt that it did very much in the way of accurate control. Mostly conserved fuel a bit so that you would stoke it once or twice a day -- not a taylored comfort zone.
Gas furnaces are quite common in California and they have very unique electrical control for obvious safety reasons. Some have there own piezoelectrical source of power for ignition that linked into the control circuitry. I have a vague remeberance of being unsure if the same device generated power to the thermostat [noparse][[/noparse]I may be thinking of gas water heaters though].
If you don't understand these gas devices and how your controls are really working with these systems, they should not be messed with. The gas company will come to your house at the drop of a hat to assure safety because they know how serious an ignored malfunction can be. Most likely they will tell you disconnect anything they don't understand or don't have certification of its compatiblity with gas systems.
All this points the question of can you really save money by adding digital control to older technologies or do you need to first buy a more up-to-date [noparse][[/noparse]and compatible] furnace?
While I really think heating is a great area for microcontrollers to add sophisticated and taylored usefulness, it is hugely complicated by arcane furnaces.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Post Edited (Kramer) : 1/7/2006 5:00:26 PM GMT
I'm with you there, of course outside of actually controlling a furnace where efficiency is pretty much locked into the furnace design there are always zone heating strategies.... Heated air or water flow control, like a digital set back tied into motion detection or some other means of determing presence. Using something along those lines you could do a temperature setback in unoccupied zones reducing the total amount of heat needed. Its all off the cuff but maybe hacking some X10 type wireless motion detectors, hacked with a photocell circuit so they also indicate presence when the lights are on in that zone? That way you could avoid taking a nap and waking up to a room thats 10 degree's cooler.. Then using an SX or Stamp you could filter appropriately, minimum time before setback with no motion/light detected, the fact that its daylight outside etc, sound could possibly be added as part of the equation. Simple analog circuits with a few components should suffice to simply detect the presence of either condition. A true "set and forget" system, simply pick the desired setback range and walk away, if your asleep, away etc the setback would be automatic.
I've had a lot of recent experience with this since I help the reader build a thermostat in my book. I found the best way to drive the circuit was through the use of external power and here's why. If you build your own thermostat and use power from the 24 volt loop and if for some reason somthing happens and your house burns down your insurance company may not cover the loss. When you start tampering with your heat, hot water, or other system that can cause damage, loss of property, or loss of life, insurance companies get very standoffish very quickly. With the riht design you can drive a thermostat circuit easily for a year and eliminate any interference with the loop that starts and stops the boiler. This approach came about as a result of a conversation I had with the insurance company. As it was pointed out to me, if you design a saw and wack your hand off is it an accident or experiment? If it's the later you will not get a settlement for your loss and the same applies to property. So be careful and if you don't have the cash in the bank to pay off your home loan(s) and rebuild you should always error on the side of safety.
Oliver
Bean.
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"SX-Video·Module" Now available from Parallax for only $28.95
http://www.parallax.com/detail.asp?product_id=30012
"SX-Video OSD module" Now available from Parallax for only·$49.95
http://www.parallax.com/detail.asp?product_id=30015
Product web site: www.sxvm.com
"If I was king for just one day...I don't think I could screw things up any worse [noparse];)[/noparse]"
·
The issues regarding thermostat replacement may be getting a bit too convoluted. This string started with a question of replacing a thermostat that consists of two wires feeding a bi-metal strip and a mercury switch. That's a pretty simple system. As noted, a replacement must include some hysterisis so the furnace doesn't quickly cycle on and off. It might also include a limit switch to avoid extremes of temperature due to system failure. (I had a relay go out on my heat pump, once, so it wouldn't turn off the heat cycle). But this can be built in by programming a range between on and off functions. Hysterisis also exists because it takes some time for the furnace to change the temperature in the house, assuming there is appropriate thermostate placement, ie, not immediately next to a heat duct.
If the system is direct feed with no pressure relief then you will see a pressure drop with a worn nozzle <more flow> and a pressure increase for a clogged nozzle or filter <less flow>.· If your fuel feed system is using a pressure relief or some other type of pressure regulator then its a different story altogether, even if regulated you should be able to look at the bypass or return pressure or flow to tell if your line flow has changed. The ease or difficulty of accomplishing what your looking for will be determined by the magnitude of these changes and the only way to know them for sure would be research, tap in for pressure readings, look at a worn nozzle, a plugged nozzle, and a good one then go from there. Lastly there is the approach of installing a second orifice/jet to create a balanced pressure area and therefore increase the magnitude of pressure change for a non ideal nozzle flow condition.
So, all that being said unless this is something you really want to do, or a product it may be more trouble than its worth. Piezo and wheatstone bridge pressure transducers can detect very small changes in pressure, and some are quite inexpensive.
For what its worth, My 2 cents.
Chris
<edit> The point about insurance is very valid however it would have to be proven that it was a failure in an add on system that caused the claim, and as long as the research existed to show the reliability and safety systems it becomes a hassle more than anything else.
Exhaustive research should be performed in any system that can effect safety and sufficient safety systems installed.
Post Edited (ChrisP) : 1/8/2006 3:52:36 PM GMT
They also retain large legal staffs to investigate, complicate, and delay.
[noparse][[/noparse]It can take 15 years to get through a civil suit.]
While institutions such as the UL [noparse][[/noparse]Underwriter's Laboratory] do provide for excellence in quality, they effectively limit competition and innovation. It is much easier to comply via normal solutions than unusual ones.
When you add to this that insurance companies have huge reserves to invest, we begin to see that they might own the technologies they prefer and want them to prosper even when obsolete in terms of energy efficency.
If you are small innovator, you seem to have to 'sell out' to them to gain entrance or let your product never reach market.
In sum, it is capital investment creating innovation inertia.
But, if you are a home owner I would never suggest you 'void' your insurance for the sake of innovation.
The idea of having two normal thermostats [noparse][[/noparse]one in parallel and one in series] to provide a high and a low override seems to be something that you could get approved with a 'rider' to your insurance policy. Thusly, the automation would be confined to a narrow range and the normal functions of the furnace would be in the control of 'certified UL [noparse][[/noparse]or whatever] devices.
I suspect that distribution of warm [noparse][[/noparse]or cool] air is far easier to gain approval and that with all the fans that have come into production in the computer age, you have a lot that can be done boost effiency in zones of use. Just getting the hot air off the ceiling and back to the floor adds a lot of comfort while reducing waste and one muffin fan in a corner can do that.
Still, if you have to have a 'rider' for each and every appliance you want to integrate into a home automation scheme, it would be best if your uncle or father-in-law is your insurance broker.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
A little more on tapping the utilities meter.· A lot of these units have a visible dial that makes are rev per X units.· My thought was to mount an IR emitter/detector to catch the hand as it·passes by.· Some mechanical meters with this type of dial can be had for cheap that could be hacked for pulse output.
·
Post Edited (Orion) : 1/12/2006 2:28:34 AM GMT
I have to agree with this. Consider this, what happens if that wire develops a short on it's own? You can't tell me because of someting like this, you're going to come home to a blown up house. There HAS to be at least SOME safety precautions built into place for this. In fact, I know there are. I used to re-install thermostats for mobile homes. The reason was because they were some of the lower end class people. And some of the disgruntled tenants, upon leaving, would twist together the leads, making the HVAC unit stay on. Leave balls of foil in the microwave, with the microwave set to 99 minutes, etc. The only time any real damage was done was with a spray can in the microwave.
I wonder if there are themofuses that have the right ranges, ie, above 110F?
Knight.
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Generally under 40 volts is considered low enough that a spark from a short circuit will not ignite a fire. That is why the 24VAC [noparse][[/noparse]24/.707= about 34 actual peak volts].
If your BOE is working WITH the failsafe features AND well under the 40 volts, why doesn't it safely comply for insurance purposes?
What I am trying to say is don't bypass the safety systems that exist. Just find a good low power, low voltage entry for your system.
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Does anyone have help for the DS1620 question?
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Chris Savage
Parallax Tech Support
csavage@parallax.com
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Post Edited (n1iic) : 2/1/2006 11:28:10 PM GMT
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Chris Savage
Parallax Tech Support
csavage@parallax.com