HPAC Magazine

Multiple Boiler Option

March 1, 2013 | By Mark Norris

When and how to use hybrid systems.

In the February 2013 issue of HPAC’s Modern Hydronics (available online at www.hpacmag.com) we looked at using multiple condensing boiler system design to help eliminate short cycling issues related to large building load turndown. Using more than one boiler in a system can also help keep the system in condensing mode and optimize efficiency when the proper controls are used and configured correctly.

One of the pitfalls identified was initial project cost for more than one boiler. We also saw that there may be at least a portion of the heating season where we need to operate with boiler return water temperatures that keep the flue gas temperatures above the dew point. During those periods condensing boiler(s) are not condensing. When that occurs, boiler efficiency is reduced to that of a good non-condensing boiler. With that in mind, is it worth adding another condensing boiler? Compared to the cost differences between condensing and non-condensing boilers how long will it take to recover those initial dollars when the boiler is not going to condense anyway?

It is questions like these that lead us to another option for multiple boiler systems: the hybrid boiler system. What is meant by hybrid is condensing and non condensing boilers together in the same plant. The non-condensing boiler will cost less than the condensing boiler to purchase. If the second boiler is only going to be needed when the system is operating outside of the condensing range, then why not save some money on the initial costs?


First, let’s look at when to use a hybrid. If the system design or peak load temperature requirements mean that the return water temperature to the boilers is going to exceed 55C, then we could consider it. The higher the design temperature is the more often that condition will occur. As an example, assuming we are using an outdoor reset control setup, a system that requires 80C water with a 12C delta T will spend more time outside of the condensing range than one designed for 70C supply with the same Delta T. This is usually associated with systems incorporating higher temperature heat emitters, such as copper fin convectors, etc.

Next let’s look at how to use a hybrid. There are two considerations to make a hybrid system as efficient as possible. The first is piping. The second is controls.

The piping design of the primary loop is important (yes, we should be looking at a primary secondary piping layout here). We want to be able to send most if not all of the return water into the condensing boiler(s) first. This lets the boiler(s) that can benefit from cooler return temperature use it. The colder the return water temperature coming back to a condensing boiler, the more condensation we can expect. Sending colder water back to the non-condensing boiler on the other hand means we need to protect the boiler from that cold water with some form of low water protection. One way to lessen the need to protect the non-condensing boilers is letting the supply water from the condensing boiler feed the
non-condensing boiler’s return. This elevates the water temperature the non-condensing boiler will see, reducing the amount of concern we have that the non-condensing boiler will be exposed to conditions where it may condense or experience thermal shock potential in lower temperature systems with larger Delta Ts (see Figure 1).  The result is less use of the low temperature boiler protection for the non-condensing boiler. Low temperature protection is an important factor for longevity on non-condensing boilers operating in systems that may return low temperature water, but protecting the boiler has a cost. Sending heated supply water back to the boiler return via a four-way mixing valve, return water mixing valve or shunt pump, means that some of that heated water will not go to the system right away because it is being used to keep the boiler hot. Burner cycles used for the purpose of maintaining heat exchanger temperatures also cost fuel in an effort to protect the appliance. These are very important tasks for the longevity of the heating appliance, but as I said, there is a cost. Note that to simplify drawings we have not shown any piping related to the non-condensing boiler’s return water elevation packages.

The series piping layout in Figure 1 works OK of there are only two boilers. If the system turndown warrants it however, we may have more than one condensing boiler along with the non-condensing boiler, as shown in Figure 2.

The layout in Figure 2 however may result in periods where the second condensing boiler will see return water temperatures that are already preheated by the boiler ahead of it. But if we layout the system with a combination of series and parallel piping as shown in Figure 3, then both condensing boilers will see the lower return water temperature. We can rotate the lead lag operation of the two condensing boilers as we want to without affecting the cascade control’s PI calculations or the temperature of the second boiler’s return water. We also maintain the ability for the non-condensing boiler to see elevated return temperatures as in Figure 1.


There will be some specific control settings to establish. If there are only two boilers, the condensing boiler is always the lead boiler and the non-condensing boiler is always the lag boiler. If we have more than one condensing boiler then we can rotate their lead lag operation if we want but we should always keep the non-condensing boiler as the last boiler. A common supply sensor should be used to let the cascade control know what the combined temperature of all the boilers is. 

Rotating the lead and lag operation of the boilers is mostly a choice made by preference of operation. It won’t generally change the efficiencies of the system as long as the least efficient boiler is always kept as the last boiler. Some people like to have equal wear on all of the boilers, while some think it is better not to have all of the boilers due for maintenance or repairs as a result of even wear all at once.  


There are some considerations for venting that need to be addressed for hybrid systems. While all of the condensing boilers are going to be ULCS636 class II (category IV) positive pressure vented appliances, and may even be approved for common venting in some cases, the non-condensing boiler will not be compatible. Typically a non-condensing boiler will not require ULCS636 vent, or if it does, may require ULCS636 Class I type venting materials (category I or category III). Category I systems require a barometric damper or draft hood to control the overfire draft. Because non-condensing boilers have different venting requirements they cannot be combined with condensing boiler vents. We also need to examine the requirements for combustion air. While many condensing boilers can be direct vented (venting directly to outside and bringing combustion air directly to the appliance from outside), Category I and Category III boilers will most often require combustion air from the mechanical room. Properly-sized mechanical room combustion air would then be required by local code to ensure an adequate supply of fresh air to the burner(s).


If you have decided to go with multiple boilers, you may also want to go hybrid, but look carefully at all of the variables. Once you layout the additional piping, install and set up the controls correctly, and add the necessary venting and combustion air equipment, you may find the costs very similar to going all condensing, even if there will be times when the boilers cannot condense. Be careful with boiler sizing. Do not oversize; remember that short cycling is an efficiency robber.  On the other hand do not u
ndersize because a condensing boiler is not going to achieve its published efficiency when operating in a non-condensing mode and at higher burner modulation points.

One last thought; I have found that there is usually more benefit to hybrid designs for larger systems that have greater Btu loads with larger building turndowns than for smaller
systems.   <>

Mark Norris is academy instructor with Viessmann Manufacturing. www.viessmann.ca



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