Omnistat RC-2000 Certain outputs passing current when off

[fcwilt]

Hi,

Building new home. HVAC guy checked and said RC-2000s would work.

HVAC guy connected RC-2000s to a WaterFurnace Zone controller and Zone controller "thought" that the RC-2000s were calling for heat/cool even when the RC-2000s were not.

HVAC guy spoke to someone at HAI (I am told) and was informed that he needed to add external relays between the RC-2000s and the Zone control for 3 of the RC-2000 outputs (I wish I wrote down which ones). Something to do with certain outputs of the RC-2000 passing a small amount of current even when "off" and fooling the Zone controller.

Anyone heard of this sort of problem?

Thanks!

 

[beelzerob]

Now you're scaring me...I have an RC2000 and am having a waterfurnace installed in the next week or so. the zone controller is a honeywell though, not waterfurnace.

 

[Digger]

Hi,

Building new home. HVAC guy checked and said RC-2000s would work.

HVAC guy connected RC-2000s to a WaterFurnace Zone controller and Zone controller "thought" that the RC-2000s were calling for heat/cool even when the RC-2000s were not.

HVAC guy spoke to someone at HAI (I am told) and was informed that he needed to add external relays between the RC-2000s and the Zone control for 3 of the RC-2000 outputs (I wish I wrote down which ones). Something to do with certain outputs of the RC-2000 passing a small amount of current even when "off" and fooling the Zone controller.

Anyone heard of this sort of problem?

Thanks!

I have heard of this with other products. I believe your HVAC guy. It sucks but it may be necessary to add the relays as the T-Stat does not have true dry contacts from what HAI apparently told your HVAC guy.

If either you or beezelrob bought the T-Stats from me just PM me and I will make this right since you did not know it could be a concern in your installation when you bought the T-Stat. You can either return the T-Stat for a full refund (they will not be resold as new) or I will provide you with the relays you need. If you bought elsewhere just ask who you bought them from to work with you.

 

[fcwilt]

Inside the RC-2000s there are places on the circuit board marked with K1, K2, etc which is the common designation for a relay.

At those locations there are parts that look like the could be PC mount relays BUT they may be "solid state" equivalents as opposed to true dry contact relays.

IF it is true that the parts are NOT dry contact relays and they have enough "off" state "leakage" current to fool typical HVAC equipment - well, I think that wasn't such a good decision by HAI.

When I get back home I have the equipment to test this and I will make the needed tests.

I hope somebody from HAI will chime in with the facts and the specs of the "relays" used in the RC-2000s.

Thanks.

 

[gatchel]

I t would seem, while inconveinent, that adding relays would be a small price to pay to get the control you desire. Maybe the t-stats work fine on a standard 24vac control but the water furnace doesn't have the same load as a standard 24vac control circuit has. Maybe adding a series resistor of a certain value and wattage would solve the problem. Just a random thought...

 

[RArbour]

Fredrick,
The Omnistat2 does indeed use true 24VAC dry contact relays. The problem exists because when the control signal is off, the signal is left floating instead of being driven to ground (think of the old mercury thermostats). The relay itself is not leaking voltage, but rather a MOV that is utilized for protection. This MOV protects the thermostat from surges. The vast majority of controller boards pull down the control signal with a weak resistance so that it does not see this floating voltage (or is less sensitive). At this time waterfurnace is the only controller that I am aware does not. The decision then became remove the extra surge protection to make the device work with all controller boards, or leave in the extra protection and have the thermostat work with the vast majority of controller boards. Since the water furnace problem can be fixed with a simple external relay, we decided to leave in the protection.

A few weeks ago I heard of a report about a house that had a very bad lightning strike. This strike blew out just about all electronics in the house, except the thermostat. Without the extra protection I do not believe this would be the case. Now if only the HVAC did not get blown out during this same strike, but at least the home owner can see how hot his house became while waiting for HVAC repair.

Thanks,
Ryan

 

[Digger]

Instead of a MOV could you use a unidirectional transorb to minimize the leakage but provide the protection?

 

[fcwilt]

Fredrick,
The Omnistat2 does indeed use true 24VAC dry contact relays. The problem exists because when the control signal is off, the signal is left floating instead of being driven to ground (think of the old mercury thermostats). The relay itself is not leaking voltage, but rather a MOV that is utilized for protection. This MOV protects the thermostat from surges. The vast majority of controller boards pull down the control signal with a weak resistance so that it does not see this floating voltage (or is less sensitive). At this time waterfurnace is the only controller that I am aware does not. The decision then became remove the extra surge protection to make the device work with all controller boards, or leave in the extra protection and have the thermostat work with the vast majority of controller boards. Since the water furnace problem can be fixed with a simple external relay, we decided to leave in the protection.

A few weeks ago I heard of a report about a house that had a very bad lightning strike. This strike blew out just about all electronics in the house, except the thermostat. Without the extra protection I do not believe this would be the case. Now if only the HVAC did not get blown out during this same strike, but at least the home owner can see how hot his house became while waiting for HVAC repair.

Thanks,
Ryan

Thanks for the feedback.

I think the protection is a fine idea.

However I do not understand why the relay could not be a form c relay the makes a connection to ground when "off". A jumper could be used, if desired, to allow selecting floating or grounded when off.

Would grounding when "off" mess up working with other HVAC equipment?

Regards the WaterFurnace: I have not had a chance to do any testing but could the input to the WaterFurnace be pulled to ground with a resistor and yet have the RC-2000, when "on", bring the WaterFurnace input up to the required voltage? Of course it would have to work with a resistor with a value high enough to not cause unacceptable current drain when "on". Just wondering.

Thanks again.

 

[beelzerob]

My HVAC installer is using a honeywell zone controller, so hopefully I'll be spared issues.

 

[cornutt]

Instead of a MOV could you use a unidirectional transorb to minimize the leakage but provide the protection?

Just solder in a 100K resistor between the output and common. Doesn't have to be inside the thermostat.

 

[fcwilt]

Instead of a MOV could you use a unidirectional transorb to minimize the leakage but provide the protection?

Just solder in a 100K resistor between the output and common. Doesn't have to be inside the thermostat.

Is this an educated guess or have you done such an install?

Thanks.

 

[scmluger1]

Has anyone had any success with the new update to allow access to the Infinity system?

 

[cornutt]

Instead of a MOV could you use a unidirectional transorb to minimize the leakage but provide the protection?

Just solder in a 100K resistor between the output and common. Doesn't have to be inside the thermostat.

Is this an educated guess or have you done such an install?

Thanks.

Not this specific thing, but it's very common in electronics, to dissipate small leakage currents. It's usually called a "pull-down".

 

[1970superbee]

We are intergrating a system now and have run into the same prolbem, did anyone try the 100k pull down? does it work or do we need to buy the HAI 29a001 isolation interface.

Thanks

 

[beelzerob]

The system's been in about 2 days now, and so far as I can tell, the RC2000 works fine with the zone controller, which is a Honeywell.