Modulating valves vs circulator control

Rodney Summers

A buddy of mine has a radiant install. He says that control should be through a modulating valve and controller. The supply house has spec'd my job using circulator control.

In the last conversion I did, which was purely baseboard, I connected the thermostats to an ARM relay box which in turn operated the circulators when heat was called for.

If I understand the modulating valve, it's a three way valve that combines hot boiler water with the radiant return water. The thermostat (maybe a special one?) connects to the actuator which controls the mix of the water to get the desired temperature. The circulator valve is always running.

Way off base here?

Wayco Wayne_2

What is your application?

If you are doing a radiant floor they typically operate at a lower temperature than other heat Emmiters such as baseboard or radiators. A mixing valve may be what you need to dial down the temperature, however.... you must be careful if you are using a cast iron boiler or the like, because it can not tolerate low temperature water coming back to it. In that situation you must then control the return temp and mix it back up in temperature. WW

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Rodney Summers

My app

I've got a 2500 sqft home.
Radiant load 15K.
Other two floor load 50K.
Heating the fully finished basement with baseboard. Mid-level with radiant staple up with plates, upper level baseboard.

I'm installing a Buderus G115. Haven't decided on the fancy optional controller for it or not. Thoughts?

Brad White_9

A Couple of Principles which May Help

As Wayne pointed out, mixing valves are the ideal for temperature control, not only of the space but the water needed to maintain that space. May I expand upon that?

Two basic principles in hydronic heating control.

One is volume control, the throttling of water volume to achieve a variation in output. You want this to be linear. It is NOT linear. You can throttle 50% of your flow and still get 90% of capacity. Characterized ball valves are very linear in terms of "stroke to flow". Best kind of modulating valve IMHO, but even so, the flow to output rate (GPM to BTUH) is still far from linear.

The second principle is temperature modulation control. Modulating the supply water temperature to vary the output of your emitters. This is remarkably linear (hey- a tool you can use!). Your average water temperature is key, halfway between your supply and return water temperatures.

Mixing valves or injection are the best way to achieve this. I prefer modulating valves, 3-way on condensing and 4-way on conventional cast iron boilers.

(The 4-way passes unused hot water back to the boiler which protects it. But it kills efficiency in a condensing boiler, you see.)

The overall operating motive here is the circulator- It should run constantly such that the valve changes can be effective, smooth and steady. This is how you marry the two together.

Pump control alone? Absent variable speed, it is on-off. Wow. Exciting stuff

My $0.02

Brad

Rodney Summers

Logamatic Controller

Seems the Logamatic from Buderus might be a good choice for a consistent system design. In that way I get the reset features and control all in one package.

Pros and cons?

Rodney Summers

Or Taco RMB, Tekmar

Are the options for mixing and setback control provided by Taco or Tekmar a cheaper or better solution than the Logamatic?

Brad White_9

I am not so familiar with the specifics

of each brand or system. I generally defer to Tekmar but many of the on-board systems that come with a boiler do the same things and seamlessly too. Others with more experience on the installation side will have more to say I am sure.

All the names you mentioned are quality products though. Key might be what works best with your system. OEM by the same manufacturer is hard to beat from a warranty/support standpoint.