Using multiple atmospheric boilers

and hydroseparators for full protection

By Paul Rohrs
Radiant Expert

Last month we talked about the hydroseparator, also called a low-loss header, and how it can provide function and practicality in the hydronic market. Let's take last month's information and look at how to incorporate it into hybrid systems with multiple types of heat sources, as well as modcon and atmospheric boiler applications.

We are also going to look at a multiple heat source system with a focus on economy, so that we are heating with the most economical source for that time of day.

There are a lot of hydronic applications that don't feature modcon boilers as their heat source. Multiple atmospheric boilers have been used in the past, the present, and will continue to be installed in future systems. So let's lay out a system with multiple atmospheric boilers and illustrate how to integrate a hydroseparator so that we maintain minimum inlet temperatures. To prevent sustained flue gas condensation, we need a minimum inlet temperature of 138 F.

In addition to preventing these non-condensing boilers from condensing, let's add outdoor-reset and variable-speed injection mixing to this system so that we have a full reset temp option. With this setup we can handle any temperature system, whether it is high-mass radiant floor heat with low temperatures, a high temp panel radiator system or a snowmelt system, to name a few.

You can see in Figure 1 that all three of the atmospheric boilers have their own dedicated boiler pumps pumping into the boiler return. This will allow all three boilers to fire individually, but yet be fully protected, because the S1 sensor on the boiler's return will confirm when we achieve the 138 F boiler minimum. We are also utilizing the S2 sensor to monitor our system/boiler supply temperature, so that we do not create an unsafe temperature condition that will trip a manual reset, high-limit or other safety device.

As the boiler piping enters the hydroseparator, you will follow it on through to see that there is only one tapping on the secondary side that features the P4 variable-speed injection-mixing pump. The P4 pump will not engage or extract heat from the primary side of the hydroseparator until our minimum inlet temp is attained. After that, as the load allows, the injection pump can ramp up and down accordingly until the target temperature is achieved at the S3 sensor.

Multiple atmospheric boilers with a hydroseparator give us flexibility in load matching with variable speed injection mixing, boiler protection and full reset temp. With proper controls and programming, this will be a system that will last a very long time. Controls are vital to making this application function correctly; when it does, you can use proportional mixing or any control with PID logic. This type of piping strategy can be employed for snow melting, space heating and domestic water. This design and piping strategy is not inclined to short cycling when the heat source is properly sized.

Technology has improved at such a fast rate that it is conceivable that 22nd-century advances might be lurking just around the corner. We hear rumblings of hydrogen powered cars, and it is in the realm of possibility that those technologies are going to be developed and able to provide heat. A hydronic system with a hydroseparator will have the potential to make that integration very simple and straightforward.

Let's assemble a hybrid system with four different heat sources. We will pipe for solar, an electric boiler, a gas-fired modcon and a hypothetical 22nd-century heat source. Figure 2 illustrates this multiple source heating system. The load side features panel radiators with TRVs, high-mass infloor heat with variable speed injection mixing and an indirect water heater. The primary side of the hydroseparator got a little "busy," in that we are accommodating several types of heat sources. An electric boiler, a gas-fired modcom, some solar and our new 22nd-century heat source will simply plug-n-play into our hydroseparator's primary loop.

Now we have an advanced hydronic system with hybrid heat sources that offer redundancy and the potential to heat with the most economical source for that time of day. It is true that designing a system like this will be "control dependent," but the advances in controls are as prevalent as the advances in equipment. The piping in Figure 2 shouldn't look that different from systems that we are designing and installing now. With a little bit of planning and foresight, hydronic contractors can be prepared to offer our customers the latest in technology as soon as it becomes available.

Running with scissors, tearing the tags off of mattresses, and playing with matches should be items on your list of things "not to do." Checking out your wholesaler or local supplier for stocked hydroseparators and learning how they can be integrated into the systems you design are things you should be doing. Hydroseparators will allow you to design around atmospheric boilers and multiple sources, and they will allow for future additions. This will keep you on the cutting edge of the hydronics market and provide economic alternatives for your customers.