New Combi Boiler, old installation had an add on return line Acquastat?


Old 12-20-16, 09:25 AM
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Join Date: Dec 2016
Location: Canada
Posts: 220
New Combi Boiler, old installation had an add on return line Acquastat?

I have replaced an old Baxi combi boiler with a new Baxi Duotec 40GA. The old unit worked well with an old cast iron radiator system,
but when an addition was put on with two radiant floor loops (and mixing valves, pumps), the boiler short cycled. The installer put a return
line acquastat in place, which solved for the problem, I think the problem was the boiler could not modulate low enough.

With the new Duotec, the boiler can modulate much lower. The current installer did not know what to make of the add on acquastat,
and turned it up very high such that it was effectively not functioning (the boiler set point is 150f, the acquastat was set at 180f). Is this
correct, I can not recall what the original setting was, so I am now guessing. Should it still be set at 20 degrees lower than the boiler set point?

I wish to install an outdoor sensor, which triggers a pre programed curve which alters the boiler set point. I am concerned with a varying boiler
set point the acquastat might compete with the settings. Should I just remove the old acquasat? Or is setting it at a very high level simply make
it a redundant control? What would be the implication if it is set too low?
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Old 12-21-16, 09:23 AM
Join Date: Apr 2014
Location: us
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To understand the broad range of features modern Out Door Reset's have to improve heating systems I would suggest reading the installation manual for one:

The Tekmar 256 for $160 is a best buy. Taco's Model PC700 is a repackaged and branded Tekmar. Both manuals are virtually the same.

Instead of terminals, the Taco has a short 4 wire cable and plug. To use the PC700 you have to buy another different Taco unit to plug it into. The end cost is three times or more the Tekmar price. It is not plug and play, more like plug and pay.

Last edited by doughess; 12-21-16 at 10:53 AM.
Old 12-21-16, 02:15 PM
Join Date: Dec 2016
Location: United States
Posts: 35
The Baxi Duotec Combi boiler has built in modulating and outdoor reset functionality, so no need to buy anything (taco, tekmar, or otherwise).

You should bypass or remove the old aquastat, or set it very high so it never has any effect, as your installer did. The duotec has sensors for both the supply and return water, so it will modulate and shut down automatically based on your programmed target temperatures, without any external controls.

To gain outdoor reset functionality, you just need to add a sensor and hook it up following the installation manual (looks like section 18 according to the manual i found online) and set the design temperatures for warm weather shutdown, coldest/design outdoor temp, and hottest/design boil temp.
Old 12-24-16, 02:22 PM
Thread Starter
Join Date: Dec 2016
Location: Canada
Posts: 220
QAC4 Siemens Outdoor Temp Sensor

Thank you for the responses, confirms my thinking and gives me the confidence to play with all this.

The current system does use a Tekmar switching relay for the three zones controlled by independent thermostats, which in turn is where the return line acquastat has been connected. So for now I would rather just turn it way high and effectively disable it, just was not certain that was without consequence. Will remove once the heating the season is over.

The Baxi DuoTec unit is pre configured to have the outdoor sensor, it just needs to be wired in, very simple to do. The Baxi sensor is a Siemens item QAC34. The installer manual tells how to wire and set the climate curves.

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