Can I use parallel thermostats in a 1-zone system?


Old 11-06-10, 02:45 PM
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Originally Posted by rorytony View Post
Fascinating. I understand what you're saying. But empirically, here's what happens.
1. About the gravity comment. First of all, the real observation that although in the cold months there is no problem, but this time of year, when the pump is on, the 9 radiators on the first and second floors are warm to hot, but the two on the third floor are quite cold. the basement there are two large pipes that snake around in parallel, getting smaller and smaller. Across these are eleven pairs of risers, for the 11 radiators. The two going to the third floor have a hot pipe going up and a barely warm pipe coming down. The other 9 have a hot side and an almost hot side. You seem to be saying absent any air, applying pressure to the input side should make ALL eleven pairs circulate equally. Or, if not, then the ones at the end of the string, perhaps, would get short-shrifted? (But that's NOT what's happening, since one of the second floor radiators, which is AFTER the 2 pair going to the third floor, gets nice and toasty.) So you may very well be right -- it could be that air.

2. OK, about the air. When the system is pumping and I open the bleeder valves on the third floor radiators, air does come out (and it's kinda acrid, I might add.) But it stops before any water comes out. It's ONLY when the radiators are warm/hot that I can bleed ALL the air out and have water (pretty black too!) come out.

So, why is that? Should I be focusing on why I can't bleed all the water out? And why do I have any water in there anyway -- is it just leaks, or could it just be dissolved air in the water?

(I know I didn't address your comments about fixing the radiators so I could turn some of them off. But let's save that for another time. In my situation it's really not that big an issue. Except for the third floor in November and March, the house is really quite comfortable, with all the radiators full on, and a single thermostat in the living room. It comes quite close to working very well! (So quitcher *****in' I hear somebody saying!))

Answer to 2: Because the water pressure, and volume, increases with heat. This can be so problematic that that is why they have expansion tanks in the system, in the basement. The expansion tank helps to absorb that rise in pressure. The air in that tank can compress, where water cannot. If the expansion tank gets water logged, the expansion of the water as it heats has nowhere to go, since water cannot be compressed - so what happens is the boiler pressure can rapidly rise when being heated and if it gets to about 30 psi, it can make water come out the PRV valve on the boiler. Think of this analogy with car tires: Why is it that you can have the air pressure in the tires aired to 32 psi, add a whopping 700 pounds in passengers, luggage/etc., or tools to the car!, and yet the air pressure hardly goes up? The answer to this is the same answer as to why the pressure gauge on the boiler does not hardly go up, even though heated water is compressing air in the expansion tank.

Watch your pressure gauge as the water temp rises from relatively cold to hot. It should not go up very much. If it does, we'd have to address that issue.

Does your expansion tank resemble that of a propane tank on a gas barbeque grill, or does it look like a cylinder, several feet long, that may be strapped to the basement ceiling near the boiler?

Water should easily bleed out of those upper radiators when the systyem is not even running and is cold. The water pressure, due to where the psi is set(what your water pressure gauge on the boiler says) is what gets the water up there. Not the circulation pump. You probably have to increase your boiler water psi and bleed the upper radiators.

You said that the water in the 3rd floor supply line is hot. Yet the radiator is not up on the 3rd floor. Did you feel the temp at where the supply line to the radiator enters the radiatior down at the bottom of it, to see if it is at least hotter down there, than where it is up near the top?

WHY is there air in your system? First, I would bring the system back up to where it is supposed to be, pressure wise, then see if the presure drops or something. There are various causes. But I'd first introduce more pressure and get the air that is in there, out, and then see over time how it is.

1. Reason no problem in cold weather is boiler runs longer, giving far-reaching radiators time to heat up before the stat shuts off. You have to either increase stat run time, and/or shut down radiators especially near the stat, to allow the stat to keep running longer. I've had to do that very thing. It works!

There are a few of these old houses I work on that are pretty big, that only have one zone (like you) - 3 levels counting the basement. And since these upper raditors get as hot as the lower ones, we either have to turn the valves nearly off, if they work effectively, or drop the water level down so water does not fill the entire upper radiators. Why? Because usually what happens is the first floor radiators heat up until they shut off the stat 5 feet up an interior wall on the first floor, say to 68 degrees. But alas - the first floor high ceilings(which these old homes usually had/have) gets to about 80! Therefore, the second floor is already being heated in the floors by 80 degree heat! Therefore, lesser heat to the upper radiators are needed, unless of course the upper floors let in a lot of draft through old windows, poor to no insulation or whatever.

Last edited by ecman51; 11-06-10 at 03:31 PM.
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Old 11-06-10, 03:44 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
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Rory, you are getting EXCELLENT advise from Ecman. This is why I asked for pictures and a complete description of your system because dribbling information to us like it is something top secret just doesn't allow us to help you.

Your boiler pressure, WHEN COLD, should be around 15 to 18 psi and when hot it will be higher but in most cases it will be no higher than about 25. The upper pressure is dependent on several factors, not the least being the setting of the boiler safety valve which in most cases is 30 psi but in some rare cases could be as high as 60 psi. To get a more exact measure of what the cold pressure should be measure from the bottom of the boiler to the top of the highest radiator (in feet) and multiply that figure by 0.5 (1/2) or if you'd rather divide by 2. That figure is that absolute minimum pressure you should have on a cold boiler and because boiler pressure gauges are seldom of high accuracy it is standard to add about 4 psi to the calculated figure. Point of fact, the multiplier to use with the feet of elevation is actually 0.433 but 0.5 is easy to do in your head and is sufficiently accurate.

Unless you have the calculated minimum pressure (with the added 4 psi safety margin) you WILL have trouble getting heat to the top floor and you WILL have extreme difficulty in bleeding out any trapped air.

You might have some serious issues with the expansion tank having too much water, especially if you have a "conventional" (non-bladder) type of tank. It is fairly easy to correct expansion tank problems but I need to see pictures of exactly how your tank is connected to the system in order to guide you through the steps.

As for the radiator valves leaking...if they only leak water out of the packing gland it might be fairly easy to fix the problem. I was under the impression that when you wrote leaking that you meant that when turned off the radiator would continue to heat, that the hot water was "leaking" past the closed valve. Post a picture of one of your radiator valves and describe clearly what kind of leak you have and exactly where that leak is. If you can place a piece of tape on the valve where the leak is and then take a picture (or if you can add a caption/arrow) to the picture) it will help to explain your options.

In addition to the link I previously posted on how to include pictures here is the procedure I normally post.

To post pictures you need to first upload the pictures to a photo hosting site such as or and then post the public URLs for the pictures (or album) here. More pictures are always better than fewer. Please have CLEAR pictures and have both close up pictures and ones from a far enough distance that we can see how the various parts are interconnected.

Ricardo, all of this also applies to you.
Old 11-08-10, 01:08 PM
Join Date: Oct 2008
Location: New York
Posts: 117
Thank you Furd,
I had made a decision to go with a parallel t-stat by tapping and running wires from the control board next to the boiler, but 50feet of wire was not enough, I had to go with 100 feet, also I had to buy silicon, wire holders, zipties, new basic thermostat, etc.. and my final bill was slightly over $70 for the parts alone, plus the time I have to spend to run the wire two stories up from basement, and go through roof, back into my living room, etc.. so I decided to with a wireless unit instead. I will start a new thread on this as I would rather keep this topic limited to parallel wiring & related issues. Meanwhile, I will think about fixing the insulation, and going over the routing of my pipes with a plumber next summer, because I know that my exiting piping routing is just non-sense..

Last edited by Ricardo Umbro; 11-08-10 at 01:38 PM.
Old 11-10-10, 03:54 PM
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If you are around, could you be so kind as to update us?
Old 11-13-10, 01:14 AM
Thread Starter
Join Date: Oct 2004
Location: USA
Posts: 36
Ha! Yeah, I'm still here. Sorry for the pause there. I do have some news to report. First of all, I stopped posting because I knew the next thing I wanted to do was take all those pictures and make them available (I have my own domain, so there was no problem hosting them.) But it's a big job -- getting a good view of the way the pipes and risers are positioned in the basement, and a few good examples of the radiator connectors, etc. etc. Then hooking the camera up to a computer, and resizing and editing all the pictures, etc. etc. (And of course, that's really supposed to go into a separate thread anyway...)

Anyway, since I was so busy with my day job, I decided to wait. And then, about 3 days ago, it was annoyingly cold again up here (on the 3rd floor) on a typical 40 degree fall day, so I decided to call the local HVAC folks (I have good relations with two outfits -- one who retrofitted our old oil boiler to natural gas, and another one as well.) So I called the original installers, armed with what I had learned here ("just enough to be dangerous!") and asked it it was possible the tank down there was water logged. So the guy asked me to go down and look at the reading, and you know what? A major problem wasn't that there was too much water in there, but that there wasn't enough! So the guy had me flip a little water turn-on thingy to run water into the system and check the pressure again. And still there wasn't enough. So I just turned the thing on while watching the gauge (which wasn't easy -- why would anybody install a gauge 2 inches away from a big water heater tank, so you couldn't look at it any way but with a small mirror, and then have to hold a flashlight on it, and then read it backwards???)

Yikes! Anyway, I didn't complain (mind you, I'm speaking to the company that did such a thing, but hey, they're helping me out over the phone for free, right? -- I mean, the install was 4 years ago.)

So I get enough water in there, according to the gauge, and you know what -- the problem is solved!! I bled the rest of the air out of the 2 radiators on the third floor, they started heating just as well as the rest of the system (like you guys said they should) and by golly, I went back to my life, and didn't come back here for a few days.

Anyway, all that other stuff about getting our radiators up to snuff is still important to me, and I will post those pictures, I promise. But probably around Thanksgiving when I can relax!!!

Anyway, thank you thank you thank you

Old 11-13-10, 11:26 AM
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Join Date: May 2007
Location: Wisconsin
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It sounds to me like unrelated to the expansion tank, you simply did not have enough water pressure (psi -water column) in your system to get it thorougly to every radiator on the 3rd floor, as we were saying. Thanks for coming back and filling us in.
Old 01-10-11, 06:35 AM
Join Date: Jan 2011
Location: usa
Posts: 1
Parallet Thermoustats for 1 zone.

You can use parallel thermoustats in most heating systems but what will happen is either or both can call for heat but they both must be satisfied or "off" for the call for heat to be called off.
Another way to say this is either thermoustat can turn the heat on but both must be "Off" to turn the heat off.
1. The coldest room will call for heat first.
2. The warmest room will Try to cancle heat when set point is met but the circuit of the other thermoustat is still calling for heat, it won't shut off unit both are calling for "no heat"

One can turn the heat on but both are required to turn it off.
Old 01-10-11, 07:39 AM
Join Date: Oct 2008
Location: New York
Posts: 117
Initially I was going to wire a parallel system, but then I decided to wait for the wireless when I get a chance.

When I decided to run a parallel stat I bought regular 2 lead wires.
Yesterday I opened the back of my existing thermostat and found that it actually has three leads, looks like one is power. But when I look at my control box downstairs, it actually has only two wires going upstairs.

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