Outdoor temps and tstat adjustments

jgs2nd

Every year it's the same thing.

Here in beautiful Detroit, I have to adjust my programmable tstat +/- 2 degrees based on outdoor temps. During mild winter temps the settings need be a little higher due to the long 'start-up'. When it gets real cold and the boiler is running on shorter cycles the tstat needs to drop 2 degrees.



My question... is there a control out there that can automate this procdure for me?



Thanks in advance.

John

Rich_L

Steam or water?

Are you heating with steam or hot water John? If you are heating with hot water you could add an outdoor reset controller, sometimes called an ODR controller. An ODR controller will automatically adjust the water temp in your system based on the outside temperatures. When it's colder outside you have a greater heat loss from your home and therefore need more heat added. The ODR automatically raises the water temp to offset this heat loss and keep you comfortable. When it's warmer out but you still need heat, your heat loss is less so the ODR reduces the system water temps to again match the needs of your home.

With out this controller the water temps need to be set high enough to take care of your home on the coldest day of the year. This is great on those really cold days but on the milder days can result in temperature overshoot inside.

ODR is a perfect match for radiant floor heat and should be on every radiant system (IMHO). Otherwise your floor heat comes on the floor slowly warms up and satisfies the home. Then it shuts off and you get cool floors. This sets up a warm - cool - warm - etc. floor pattern. NOT what the average home owner wants from his or her high dollar radiant floor system. Plus, when the temps really drop, like they are outside right now in my area, the floors and or radiators are really warm and comfortable.

Rich L

jgs2nd

sorry...

I should have stated that it is a steam system.

It seems like something could be done in the form of adjusting the run time based on outdoor temps. When it's real cold the burner doesn't need to fire as long since the water is already closer to steaming compared to when it sits a while...



anyone?

Jamie Hall

Ah... wait a minute.

The boiler run time is not controlled -- directly -- by the thermostat.  Let us consider what happens in a cycle.  The thermostat senses a drop in temperature below its on value (keep in mind here that the "on" setting for a thermostat is slightly lower than the "off" value -- technically, the dead band) and turns "on".  This sends a signal to the boiler that steam is needed and, if the system pressure is below the cut in value the burner will come on.  The boiler first has to come up to a boil -- how long that takes depends on how long it's been since it last ran (we'll talk about cycle timing later here).  Then the steam begins to go out in the mains and will, eventually, reach the radiators.  Again, how long this takes depends on how cold the pipes are, how good their insulation is, and how good the venting is.  If the pipes are already warm, it will take less time.  Then -- finally! -- the steam reaches the radiation.  Now two things will happen.  So far as the boiler is concerned, it is still seeing very low pressure.  This continues until the radiation is pretty well full of steam.  But at the same time, the space temperature begins to rise as the radiation warms it up.  Now there is a race going on: will the space temperature get up to the thermostat set point before the radiation is full and the steam pressure starts to rise, or vice versa?  If the thermostat is satisfied first, the burner shuts down and waits for the next cycle.  If the pressure rises first, the boiler shuts down on pressure until the pressure drops again (the radiation is still radiating -- no change there) and it will keep doing that until the thermostat is satisfied.



So what we really need to look at is not the boiler run time itself, but the thermostat on time.  I will say at the outset here that a properly adjusted and located thermostat should never have to be changed to keep the space temperature at the thermostat even.  Your experience with having to change the thermostat setting suggests something slightly amiss.  Without knowing which flavour of programmable thermostat you have, let me say that there are several approaches used by different manufacturers to control thermostat cycles.  Each works.  All of them, by one means or another, attempt to correct for the space temperature continuing to rise after the thermostat shuts off (the radiators are still hot, after all).  The simplest approach to this is a straight anticipator -- basically a mechanism which either physically heats the thermostat slightly or which electronically biases the thermostat so that it senses having reached the cut out space temperature before it actually happens.  Once the set point is reached and the thermostat turns off, the anticipator stops, too.  If the anticipator is set right, the space temperature will reach the cut out level and not overshoot.  Too much anticipator, and it will undershoot; too little, and it will overshoot.  Unfortunately, a straight anticipator can only be exactly correct for one heat loss condition for the space.  Some other thermostats also have circuitry which not only senses temperature, but how fast the temperature is changing.  If these also know something about the thermal inertia of the heating system (how much heat is radiated after the boiler is turned off) they can compensate much more closely for varying outdoor conditions, and usually hold the space temperature plus or minus half a degree or so (although they will overshoot, as will the plain vanilla anticipator, on larger changes, such as recovering from a setback, under some conditions).  The usual way of telling the rate sensing type how much thermal inertia is present is part of the cycles per hour setting (which should be 1 for most steam systems).  The cycles per hour setting also sets the minimum time between starts for the system.



All of which is probably more than you wanted to know.



Why you need to lower the thermostat when it's cold out I'm not sure.  But I would check the cycles per hour setting, if it has one, or the anticipator setting, if it doesn't.  I would also make sure that it's location is not unduly influenced by radiators, air leaks or draughts, or outside walls.

jpf321

haha .. thanks Jamie

thanks Jamie .. and all I was going to do was suggest an ODR attached to an aquastat to keep his boiler water "ready to go". ... but this just handles the boiler water and time to make steam portion .. then there is the cold pipes factor, the cold rads problem .. basically the colder it is, the more cycles you fire more frequently, so less cooling occurs.



i see now too that we are really talking about undershoot/overshoot since he is making the manual adjustment on the t-stat .. so that's definitely more t-stat centric.



as a side note, you can see in some of my data .......

   a visualization of the equipment (flue and rad) cooling during long down times...and keeping temp up in more frequent cycling.

jgs2nd

... the strangest part about all this...

Thanks for the responses.

The strangest part of all this is that it seems as if there's a definite temperature at which the different settings are needed. I don't know exactly but it seems like above and below freezing.?

DavidK_2

Do what Jamie suggests

my guess is that your thermostat is not anticipating enough heat. Or since it is programmable, is set for too many cycles per hour (CPH). Make sure your set for 1cph (or sometimes 2). Unless it was set for steam when you installed it, it is probably still set for the default (which is usually hot air.)

Dezso3

Baseboards generating clicking noise

My room is heated with a hot water heating system, utilizing baseboards and copper pipes. As soon as the system was installed, it was making clicking noises. I figured out a way to attach the pipes to the baseboards, and the clicking noise went away temporarily. Just recently, I accidentally knocked a textbook from my desk right on top of the baseboards, and after that, when the heat came on, they started clicking like mad, all over again. It's extremely annoying because I wake up early in the morning when the heat comes on, and I am tired all of the time. What can be done about this?