Help selecting boiler control w/outdoor reset

Steamback

I'm on a job where I would like to use a boiler control with outdoor reset to modulate temps in 2 individual water tanks. Both tanks need to be the same temperature.

It is obviously easy to do this with 2 single-stage boiler controls, one for each tank, and programmed identically.

But I'd rather use a single control to modulate both tanks...the problem is that the 2-stage controls seem to always operate one stage first, then another stage later. In my case I would essentially need a 2-stage control that operated each stage at the same time, always.

Do I have any choices for a single device boiler control (not 2 boiler controls) with outdoor reset which can modulate 2 heat sources identically, without any staging?

HVACNUT

Can you explain your situation a little more?
As I understand it, there is 1 boiler, serving 2 tanks, and the tanks must maintain the same, but varying temperature through outdoor reset?
What's the purpose of the tanks?

Steamback

Sure and thank you, @HVACNUT. I have actually posted a couple other questions about this job in other places on the forum but will summarize where I'm at now. Forgive the basic diagram...I know a lot is missing but it gets the point across I hope.

I have 2 120 gallon solar pre-heated water tanks each with a 4500w element. Per advice from a previous post (thanks hotrod, you rock), I'm going to use contactors controlled with outdoor reset controls.

The problem is, I realized today that I really should have a sensor on each water tank, rather than just use one control with a sensor on one tank and not the other, to drive both contactors. Safety is the main reason I suppose, since I'm pretty sure our plumbing will be balanced and both tanks will almost always be very close in temperature even if 1 single boiler control is in fact heating both tanks with just 1 sensor on 1 tank.

So I looked at a few 2-stage boiler controls today with 2 relays but if I'm understanding correctly they can only monitor 1 tank water temperature, not 2. And always seem to do as they say, stage between 1 and 2, rather than have a function to always activate both relays together, always...

Ironman

One control can work two contractors.

Look at Tekmar. I believe the 256 is the right model.

Steamback

Thanks, @ironman. Maybe I'm confused, but the Tekmar 256 only shows one heat source sensor. I'm not sure how it could monitor the temp in both tanks with just one sensor...? Not to mention the outputs. Do you think it's a different Tekmar than the 256?

Ironman

If the flow is equal in both tanks, then they should effectively be the same temperature. You don't need to sense both tanks.

I've got solar jobs with multiple tanks piped reverse return so the flow is equal. The solar control only sense one of the tanks.

If safety is the concern, then put a separate high limit on each tank. Safety is not accomplished with an ODR control. In fact, Tekmar specifically states that in their literature.

KVieira85

Use HBX Controls CPU-0550. You can do this application and a lot easier to use than a Tekmar 256 control and less expensive.

http://www.hbxcontrols.com/cpu-0550.php

Steamback

Thanks, @kvieira85. I checked out the install and programming for the CPU-0550 and have a question...which might be a dumb question: I'm assuming in the Dual-Setpoint mode the HBX does not use outdoor reset?

Steamback

Thanks @ironman. Good point. I know that on our solar jobs we also get equal flow between tanks. I won't rule out using one tank for sensing and control just yet...

Steamback

I have an additional question, a general one, about how thermostats handle supply water temperature changes when they don't know the supply temperature has changed?

For this application outlined above, I have decided to control each water tank temperature with an individual outdoor reset control. I'll just be sure they are labeled as "needing to be programmed identically at all times". The boiler controls with outdoor reset will NOT be connected to the house thermostat and will instead be supplied with a "constant heat" signal (necessary I think since these tanks also will supply DHW).

The job is radiant underfloor with wood floor. I plan on using a thermostat with a floor sensor and NOT and outdoor sensor. The thermostat will call the radiant circulators only. I of course will be leaving the thermostat to a constant temperature...no adjustments.

My question is, since most thermostats have some kind of simple learning algorithm about how long to run heat to achieve a desired temp and avoid overshoot and undershoot, how does a modern "smart" thermostat work when it doesn't know the supply water temp is changing according to an outdoor reset curve?

Is this above plan just dumb? Should I really be looking to integrate my thermostat into the reset controls? I'd rather not since I'm really looking for simple but works pretty well or well enough, not necessarily fantastic.

HVACNUT

With ODR, the thermostat is basically just a high limit.
A radiant thermostat, like those offered by Tekmar can be used, or if a "smart" thermostat, it should have an anticipator setting for radiant.

The thermostat(s) won't know about a change in water temp, it will just keep the circuit closed as long as there's a heat call. It might need an adjustment or two to the ODR curve if the zone doesn't reach the set temp. Or reaches it too quickly.

Question. With solar tanks providing both SH and DHW, won't ODR hamper the domestic water temp? Or are you programming a fixed minimum temp for the tanks?

Does this make sense for the application?
Seperate Aquastats for each tank, set to the same temp.
A thermostatic mixing valve for the DHW.
An electronic modulating mixing valve with ODR for the SH.