Buderus DHW and Primary loop pump control w/Logamatic

TimG

I've got a Buderus G124 boiler with and older Logamatic 2105 set up with a primary and secondary loop. The secondary loop circulators are controlled by a Taco SR506 exp. The PH circuit on the 2105 switches on both the primary loop pump and powers the SR506 relays through the SR506 ZC terminal. The PS terminal on the 2105 controls the DHW pump.



 The 2105 gives DHW priority over space heating, so when the DHW calls for heat, the PH terminal has no power to run the primary pump.



I found this work around to get the primary loop pump to run with the DHW pump:

<a href="http://www.buderus.us/files/201001202145050.HS2105_WiringSchematics.pdf">http://www.buderus.us/files/201001202145050.HS2105_WiringSchematics.pdf</a>



see page 3:  primsec2.dc2 for the Logmatic 2105 wiring.



Questions:

1)Do I really need the NC relay in this setup? It seems like if the Logamatic is only sending power to either the primary pump or the dhw pump (but not both), then it looks to me like I could run power to the switched side of the 2 NO relays and then to the primary pump. If there was a simultaneous call for space and dhw heating, only one of the NO relays would have power at a time.



2) Is there some kind of control already made up for this application or should I just buy something like a  Tekmar 004 and put it in a junction box?



thanks -Tim

Ironman

2105 + NC Relay

I have to agree with you that I can't (at 11:30pm) see any reason for the extra relay, unless removing it would cause some sort of back feed not seen. I've found that the Germans always have a reason for what they do, though.

TimG

thanks

Thanks Bob,

I've ordered a tekamr 004 and will post back on how it works out. -Tim

TimG

the tekmar worked

The tekmar 004 worked great. The wiring schematic in the box told the tale on how to wire it up. thanks all for your help. -Tim

Ironman

the tekmar worked

Glad to hear it worked out.

ChasMan

Hmmm

What you are trying to avoid here (I think) is overloading the Logamatic. Ie, if you send power to ZC from the Primary circ to get a priority signal then all pumps are powered from the relay and it can not supply that much power. I would wire all three pumps to the  SR506  and since it has an isolation relay in it, there would be no need for an extra relay. I faced the same issues with my boilers controller and I had to add a relay because I didn't have the EXP model with the extra isolated relay in it.

Anyhow, it looks like what you have will work.  Plus having only the current of relay coils on the Logomatic relay could do more harm than good. Are the relays on the Logomatic field replaceable or do you have to swap out the control?