Zone heating system causes boiler overtemp

Todd_8

My house has a two zoned heating system. It consists of:

Two Taco 571 zone valves
Single circulator
Weil-McLain HE-5 133,000 BTU boiler
2 Thermostats, one connected to each zone valve

I have been trying to get the zone system working correctly for quite some time. I have had professionals out and ended up doing nothing but spending money and not getting it to work.

My symptoms are as follows:

The system is currently running on a single thermostat. That thermostat is connected to the Zone 1 valve. Zone 2 valve is not currently connected to its thermostat, but the lever is set so that it is always open. Therefore, whenever the Zone 1 thermostat closes, both zones get heat.

When the Zone 1 thermostat opens, I can verify that it opens with an ohm meter on the system at the boiler. The thermostat opens, then 1 minute or so later the zone valve opens and starts the boiler. The boiler lights up and starts to push hot water through the system. I can watch the boiler water temperature rise to about 183F at which point the boiler shuts down. I originally thought that this shutdown was due to the boiler overtemp, but when watching the thermostat with the ohmmeter discovered that the thermostat closed, causing the boiler to shutdown.

So far so good, as far as I know.

When I connect the second zone valve to its thermostat and set the zone valve lever so that its operation is automatic I get correct operation as far as the thermostat and zone valve are concerned. That is, when the Zone thermostat closes, it causes the Zone 2 valve to open and close the relay that controls the boiler operation.

The problem occurs when the second zone calls for heat right after the first zone has already shutoff. Since the boiler has already heated the water to at or near its automatic shutoff, when the second zone calls for heat, the boiler will not actually light up until the water temperature cools to below the automatic shutoff point. I verified that the Zone 2 thermostat is closed and the zone valve is open with my multi-meter. Once the boiler water temperature cools, the boiler lights up. Since the water temperature is already close to the boiler's automatic shutoff point, the boiler shuts down in about 5 minutes or so, even though the thermostat is still closed and the zone valve still open.

This certainly does not appear to be normal operation. The problem appears to me to be obvious. The boiler puts out more heat than can be dissipated by the system. Based upon watching how the water temperature rises I believe that this would be the case even with both zones open if the heating cycle were longer. Since I set my thermostats relatively low (typically around 65F - 68F) it doesn't take long to heat the house to where the thermostat shuts off. If I turn the thermostat setpoint up, my guess is that the heater would shutdown before the thermostat opened.

If my assumption about the cause of this problem is correct, it would seem that I have two options:
1. Increase the heat dissipation;
2. Decrease the heat input rate to something closer to what the existing system can dissipate.

Number 1 doesn't seem viable unless I install more piping. I don't know if Number 2 is even possible.

Can someone provide me with some guidance? Please bear in mind that I am not a heating expert (not even a rookie). I am just a homeowner.

Any help at all will be greatly appreciated.
Todd

Cliff Brady

Question

Does the circulator run on a call for heat from the zone valve end switch even though the boiler isn't firing because it over the highlimit?

Todd_8

I'll have to verify this answer, but I believe that the circulator is controlled directly by the boiler and NOT via the zone valves.

If I understand where you're headed, it might be possible to control the circulater via the zone valve and therefore dissipate some additional heat.

John Mills_3

Sounds normal

You say: "The problem occurs when the second zone calls for heat right after the first zone has already shutoff. Since the boiler has already heated the water to at or near its automatic shutoff, when the second zone calls for heat, the boiler will not actually light up until the water temperature cools to below the automatic shutoff point. I verified that the Zone 2 thermostat is closed and the zone valve is open with my multi-meter. Once the boiler water temperature cools, the boiler lights up. Since the water temperature is already close to the boiler's automatic shutoff point, the boiler shuts down in about 5 minutes or so, even though the thermostat is still closed and the zone valve still open."

Sounds normal to me. When only 1 zone calls, the furnace will cycle on high limit. Nothing can be done with a zoned system. Most boilers I've seen will do that with both zones calling just because their output exceeds the capability of the radiation.

Todd_8

Do nothing?

Is there anything to be concerned about because I am constantly hitting the temperature limit on the boiler? What happens if that limit control fails? Is this a reasonable concern?

Thanks,
Todd

Chuckles

that is how it's supposed to work

When there is a call for heat, both boiler and circulator go on, but the boiler goes off when the water reaches the target temperature, then comes on again when the water cools down a bit. In other words it is supposed to do exactly what you have observed---to go on and off in order to hold temp near 180. The circulator keeps running constantly to move this heat to the radiators.

This is not like a forced-air system, in which the air moves fast and doesn't hold much heat, so the fan and the furnace can go on and off at the same time, decided only by the room thermostat. Hydronic heat is delivered more slowly and so the water will get too hot while your rooms are still too cold.

You say that the burner was going off at 183F but only when the room thermostat stopped calling for heat. But your room thermostat doesn't know that the water temp has reached 180F!

Every boiler or furnace is oversized on a typical day...even if it is correctly sized for the coldest likely day in your area, which is rare, it is still oversized for every other winter day. It is even more oversized if only one small zone needs heat. So it will short cycle. The way to avoid that and to increase efficiency is to get a boiler with a modulating (variable) burner rather than an on-off system.

You are worried about the aquastat failing. There should also be a separate safety limit device. ( I am merely a homeowner, so I hope the professionals here will chip in if I am spreading any misconceptions.)

Larry (from OSHA)

Todd, I have the same boiler

in my home. Just like everyone else has said, it sounds quite normal to me too. The HE boiler uses the high limit aquastat to control the boiler and it is the only temp limiter on the boiler. It has a 10* differential so when it hits the high limit at 180 or so, the burner shuts down and the circulator continues to run until the thermostat is satisfied. The burner will fire up again when the water temp is at 170 or whatever is 10 degrees less than the aquastat is set for. Bottom line is there is likely nothing to be concerned about. If you are new to the house, it would be prudent to have a professional who really knows about hot water heat look at the system and see if it needs cleaning or anything else and be able to explain to you how it works and what is right and what is wrong. As far as the aquastat being reliable, mine has been working flawlessly for the past 12 years (if testimonials mean anything). If you don't have the owners manual it is available from the WM website. Look into buying some books from the books and more section and keep asking questions. The professionals here are very, very knowedgeable. Good luck,

Larry

Steve Eayrs

Todd,

In most places its code to have a second/redundant aquastat, usually stapped on the supply of the boiler, which will shut down the burner in the cae of the first aquastat failing. Would recommend you have this done.

Secondly, Yours may be the way most standard boiler are set up and operate, but you are right it is not the best way to operate it. Its a good place to do indoor/outdoor control, and be able to run the boiler cooler in warmer weather. Would most likely save a fair amount of fuel on your system. Depends on the type of boiler you have as to how cool you can run it.
At the very least you could replace your operating aquastat with what is call a triple aquastat. Has both a high and low setting, w/ an adjustable (5-25 deg. diff.).
Would allow the boiler to run off the lower setting when there is no call for heat, and only run off the high setting when there is a call for heat.

Steve

CMCPHERSON

For all of you who responded about my zone heating problem last week.. thank you thank you thank you!!!!

The problem I described was we have a 3 zone system and one of the zones wasn't getting warm enough (temps in the 50's in the bedrooms)despite replacing the thermostat.

One of the first suggestions I got was that the PSI was too low. I added water to the system until the relief valve popped (popped at 30 and went down to 25 psi). Its been fine ever since!! And if you've been following the temps in MN -- its been colder than ever the last few days. -15F this morning and the rooms were all holding steady at 70 degrees. I'll still get a contractor out to check the pipes and make sure the water is OK and get the air out -- but in the meantime we are toasty!

Thanks again!

Todd_8

Apparent bad assumption and thanks

Based upon what I have read in the responses received, and after doing some thinking about them, it would appear that my concerns are based upon an apparently invalid assumption.

I assumed that when the boiler stopped firing at the same temperature all of the time that it was because it had hit an apparent SAFETY temperature limit. After reading the posts to my question and thinking about it a little more, I believe that assumption to have been incorrect. It now makes perfect sense to me. The boiler's purpose is strictly to keep the water temperature within a temperature range such that the water can provide heat. When the heat output does not equal or exceed heat input, the boiler will not have to remain on the entire time that the thermostats are calling for heat. In fact, if the boiler did have to remain on all the time, that would mean that the house required approximately the same amount of heat as the boiler's max capacity, which would probably not be a good thing.

Thanks VERY much to those that responded and took the time to help me understand this. I now have a much better understanding of how this works and am in a better position to figure out when I need professional help (for the boiler, not me :-) ).

Todd