Boiler TT terminals

[Voyager1947]

I have a fairly new Burnham Boiler which recently started having an intermittent problem. What it does when it has the problem is...it has two zones, taco zone valves, and is wired according to the manual, the internet, youtube, etc, and the fact that this problem is intermittent makes me think the wiring is correct. When either thermostat calls for heat, the circulator turns on, the burner turns on. If the water temp gets above the set limit, the burner turns off, circulator continues, then the burner turns back on after the water temp drops. At times, when the thermostat is satisfied and no longer calls for heat, the burner turns off, but the circulator keeps running. With it running, and the zones closed, the water temp will take forever to drop, which I would expect. In reading everything I can about troubleshooting this thing, one thing that don't make sense to me after doing voltage checks is that if I remove the wires from both T terminals and put my meter across them, it reads 41.5 vac and if I reverse the leads it reads 15.7 vac. I've seen this control thing where the T terminals are called an aquastat, but what is printed on it is Intelligent Oil Boiler Control and has a number of Honeywell L7248L, it just don't seem like it should have voltage on it as there is an external transformer supplying the 24vac to the thermostats. Any ideas would be appreciated.

 

[Tom Sawyer]

I have a fairly new Burnham Boiler which recently started having an intermittent problem. What it does when it has the problem is...it has two zones, taco zone valves, and is wired according to the manual, the internet, youtube, etc, and the fact that this problem is intermittent makes me think the wiring is correct. When either thermostat calls for heat, the circulator turns on, the burner turns on. If the water temp gets above the set limit, the burner turns off, circulator continues, then the burner turns back on after the water temp drops. At times, when the thermostat is satisfied and no longer calls for heat, the burner turns off, but the circulator keeps running. With it running, and the zones closed, the water temp will take forever to drop, which I would expect. In reading everything I can about troubleshooting this thing, one thing that don't make sense to me after doing voltage checks is that if I remove the wires from both T terminals and put my meter across them, it reads 41.5 vac and if I reverse the leads it reads 15.7 vac. I've seen this control thing where the T terminals are called an aquastat, but what is printed on it is Intelligent Oil Boiler Control and has a number of Honeywell L7248L, it just don't seem like it should have voltage on it as there is an external transformer supplying the 24vac to the thermostats. Any ideas would be appreciated.

There is a 24v transformer that feeds the thermostats and zone valves

 

[Sponsor]

 

[Voyager1947]

There is a 24v transformer that feeds the thermostats and zone valves

Tom, not sure what you are saying here. There is an external 24v transformer that feeds the thermostats and zone valves that has nothing to do with the boiler, that is, it is mounted on a floor joist above the boiler. My question is, with this external transformer, and with the wires disconnected from the TT terminals that go back to the zone valves and transformer, should there be voltage on the TT terminals.

 

[Jadnashua]

Sometimes, especially with very high impedance DMM devices, having a voltage has little meaning...it all depends on where and how you measure things. Without a diagram of what's there, and where you are measuring, it's hard to say. With any circuit, you need the 'proper' reference point for your black lead, and ground often isn't it, if you use that. Maybe a little out there as an example, but back when I was fixing HV radar power supplies in the Army, the schematic showed the filaments of the tube had 5vdc on them, but in reality, depending on how you measured it, they were really -4,995 and -5,000VDC, or a 5-volt difference; otherwise, the voltage difference between the filaments and the cathode would have been so high, that if measured to ground, there would have been a constant arc - it wouldn't work! So, your reference point must be proper, and with low-voltage a/c circuits, that is usually the 'common' or return side of the transformer, not ground.

 

[Voyager1947]

Sometimes, especially with very high impedance DMM devices, having a voltage has little meaning...it all depends on where and how you measure things. Without a diagram of what's there, and where you are measuring, it's hard to say. With any circuit, you need the 'proper' reference point for your black lead, and ground often isn't it, if you use that. Maybe a little out there as an example, but back when I was fixing HV radar power supplies in the Army, the schematic showed the filaments of the tube had 5vdc on them, but in reality, depending on how you measured it, they were really -4,995 and -5,000VDC, or a 5-volt difference; otherwise, the voltage difference between the filaments and the cathode would have been so high, that if measured to ground, there would have been a constant arc - it wouldn't work! So, your reference point must be proper, and with low-voltage a/c circuits, that is usually the 'common' or return side of the transformer, not ground.

Thanks Jim. Nowhere can I find a schematic of this L7248L Intelligent Oil Boiler Control, only a block diagram. But what I see is if I remove the wires from the TT terminals that go back up to the zone valves and external transformer, then take my voltmeter and go across the terminals, I see 41.5 with the probes one way, and 15.5 if I reverse them. If I go from either of them to ground, I see 0. If I had a drawing of this L7248L thing to see what the T terminals actually go to inside the circuit board, perhaps it would make sense. Perhaps, and most likely, this has nothing to do with my circulator not shutting off, intermittently.

 

[Houptee]

Here is the manual for that control and it says on pg 10 in table 11 there should be 24vac on the TT terminals, if not replace control.

Also it shows this control unit supplies the 24vac power for the Tstat, not an external transformer.

Do you have the optional outdoor reset module, and are you running a indirect domestic hot water tank or just the heating system?

https://customer.honeywell.com/resources/techlit/TechLitDocuments/68-0000s/68-0281EFS.pdf

 

[DonL]

Getting them readings, I would not trust the meter.

When reversing the leads reading AC the reading should be the same.

Them readings do not sound right to me.

You may just be reading stray radiation in the air, or on the wire.


Good Luck.

 

[Voyager1947]

Here is the manual for that control and it says on pg 10 in table 11 there should be 24vac on the TT terminals, if not replace control.

Also it shows this control unit supplies the 24vac power for the Tstat, not an external transformer.

Do you have the optional outdoor reset module, and are you running a indirect domestic hot water tank or just the heating system?

https://customer.honeywell.com/resources/techlit/TechLitDocuments/68-0000s/68-0281EFS.pdf

Just the heating system, nothing extra. Damn, I've looked at that same document numerous times, I've never seen that. I will try rewiring it, removing the external transformer, and use the TT as the 24v source, man that would be nice. I would have called the contractor who put it in initially, but he didn't seem to know much about what he was doing. He had a relay in it along with the external transformer, I got rid of the relay a few months ago, I could hear it operate when I was sleeping. None of the wiring explanations that I saw on the internet called for a relay.

 

[Houptee]

I think you need one of these units a Taco ZVC 403 zone valve control.
You wire all your Tstats and zone valves to this Taco unit, and then run a wire from XX on this unit to TT on your boiler control.
It has its own power supply for the Tstats and zone valves, the XX terminals are dry contacts that close to trigger the boiler to fire and start the circulator pump.
Search for it online for best price.

http://www.taco-hvac.com/uploads/FileLibrary/102-089.pdf

 

[Jadnashua]

Your contractor may have used the internal 24vac to control an isolation relay - by taking it out and wiring directly with the external transformer, that in itself could fry the electronics, but would have had to see exactly what was done. The older stuff was mostly without electronics...much of the newer stuff has them. Depending on how they build the electronics, sometimes they don't tolerate miswires at all, or if they do, not for long. The noise a relay makes is often (not always) related to the amount of power it is designed to carry on its contacts.

 

[Houptee]

If you didnt already fry the boiler control you have now by removing the relay and connecting two transformers together then the Taco unit I posted will solve your problems.
You wont need that other external transformer or relay anymore.
The Taco ZVC 403 zone valve control has its own transformer and diagnostic LEDs for status and diagnostics.

 

[DonL]

That other relay may have been used for a Needed dry contact pair.

Depends on how it was wired and how you changed it could be a problem.

The dry contacts may have been switching a DC voltage.

That could explain why you got different voltage readings when you swap meter leads.


Good Luck.

 

[Houptee]

Also your existing control has several options that can be enabled or disabled, such as the Pump Cycling Option on pg 7 which runs the circulator for the set time even if Tstat is satisfied and no longer calling for heat.

This pump cycling option is not going to work on your system since you have zone valves that will be closed so the circulator will not be able to move the water thru the system once they close, that option is for zones that each have their own circulator and no zone valves.

The manual for your Honeywell control does not even show an example of a zone valve system, only examples of circulator zoned systems.

This is why you need a Zone valve controller such as the Taco ZVC 403 zone valve control to make the system reliable, easier to wire, and easier to troubleshoot in the future.

 

[DonL]

Something about the words "Taco box" don't sound right.

lol

 

[Houptee]

Something about the words "Taco box" don't sound right.

lol

Yes I agree and edited my posts to list the Taco model number, and no, I do not work for Taco!
Honeywell and Azel make similar zone valve controls that would work also, but I have only used the Taco brand myself.

 

[DonL]

Nothing wrong with a good Taco box. They are great.

But I never would have thought to hook one up to my Boiler, but maybe that would heat them up when connected to the TT terminals.

I ain't right sometimes.


Have Fun.

 

[Tom Sawyer]

The taco controllers are nice, no doubt about it but not necessary. For decades we wired zone valves without them and in reality all it does is consolidate the transformer and associated wiring. You really only need to be aware of the polarity of the transformer when it is in the tt circuit. I can't remember what the original problem was but usually with Taco zone valves it is the amp setting on the thermostat itself not being matched to the Taco zone valve .09 Amps is what the zone valve is looking for. The relay was there either for AC or maybe a fan start I assume.

www.taco-hvac.com
The site contains all the wiring diagrams you would ever need for anything thats out there, residential and commercial.

 

[DonL]

You really only need to be aware of the polarity of the transformer when it is in the tt circuit.

Good Point, The AC phase Does matter.

 

[Voyager1947]

Also your existing control has several options that can be enabled or disabled, such as the Pump Cycling Option on pg 7 which runs the circulator for the set time even if Tstat is satisfied and no longer calling for heat.

This pump cycling option is not going to work on your system since you have zone valves that will be closed so the circulator will not be able to move the water thru the system once they close, that option is for zones that each have their own circulator and no zone valves.

The manual for your Honeywell control does not even show an example of a zone valve system, only examples of circulator zoned systems.

This is why you need a Zone valve controller such as the Taco ZVC 403 zone valve control to make the system reliable, easier to wire, and easier to troubleshoot in the future.

Mine is wired exactly like this...http://vernonwinnelson.com/index.php?option=com_content&task=view&id=35&Itemid=55

It's been working like this since October, there are no error codes on the control, I would think if I would have fried something, I would have a code or things wouldn't work properly. The only anomaly that I have is that the circulator keeps running with no call for heat, and that's only intermittently. I also don't see how I can run zones with just using the TT terminals. From what I can tell, if those terminals are providing 24v, they would go directly back to one thermostat and that's it. I may check out that Taco 403. The old boiler was a Weil McClain that was over 30 years old and to my knowledge, there were no external transformers, relays, anything like that.

 

[Houptee]

Your new boiler Honeywell control is a more sophisticated smart control and aquastat combined into one unit, it supplies its own 24v internally from the TT terminals and performs logic contol of the circulator.

The installer must have put a relay to isolate the two transformers so when either Tstat called a "dry contact" was made with the relay, basiclly to short the TT terminals, hence turning on the boiler.

You said you "removed this relay", what did you put in its place?

Are you sure you have the same type of zone vales as shown in that diagram?