Plumbing boilers; now this is a topic of great contention! It is also one where nearly every heating person considers their way to be the absolute best way, and don’t you dare try to tell them anything different. They can take it as a direct affront. In their mind, you are telling them they have been doing things wrong their whole life. They know this isn’t true, because their customers had heat. And so, they will continue to do things as they have been and scoff at anyone who tries to tell them different. 

I was invited to a party one night. This was when I was younger than I am now, which means I was barely out of my teens. Like most other young men of that age, I had some characteristics that frequently got me in trouble. You see, I had been studying hydronics for quite some time, and I had the presumptuous attitude that everyone else in the heating business was dying to know what I learned. And so, unabashed, I was talking about hydronics with almost everyone, whether they wanted to hear it or not. Most, I believe, didn’t, but they were polite about it. Except for this one guy at the party.

This party took place at a cabin on the top of the Blue Mountain, which is part of the Appalachian mountain range. From up there we had a spectacular, bird’s eye view of the Cumberland valley. There were lots of great people there, and of course, I found the only other heating guy there and promptly engaged him in a conversation about boilers.

I asked him, “So how are you piping your boilers?”

He took another swallow of beer, wiped his chin, took on an air of someone in the know and proceeded to give me some details on his method of installation.

I listened carefully, and when he was done talking, I began telling him about the things I had learned and what was the correct way to pipe boilers. As I was talking, I noticed his face getting darker and taking on a surly constitution. Undaunted, I continued until he suddenly interrupted me.

“Are you trying to tell me you know more about boilers than the old plumber that I work for?” he asked. “He’s been doing this his whole life and knows darn near everything there is to know about boilers! He has taught me everything. Are you trying to tell me he is wrong?”

“Well, no,” I said. “It’s just that…”

Not letting me finish my sentence, he leaned in so close I could smell the alcohol on his breath, and in a menacing tone said, “Yes you are. That’s exactly what you are saying!”

I took a step back and fumbled for words. He looked like he was ready to take a swing at me.

After a brief but awkward moment of silence, he sneered and said, “Oil heat is the best heat.” After delivering that powerful punch line, he took another swallow of beer, wiped the dribbles off his chin and strutted away.

This experience was quite a letdown for me, but it taught me a lesson. Not everyone wants to hear about hydronics, particularly at a party. And when trying to share knowledge about piping boilers, a great deal of finesse is required for most people. And most importantly, don’t try to teach someone who doesn’t want to be taught.

Throughout the years, I have also learned there is no “one way” of piping boilers that is always the right way. How one should pipe a boiler has a lot to do with what kind of emitter system they are connected to. So, here is one method that has given me marvelous results.

Cast iron boilers with old radiators

There are still many of these old cast iron radiator systems out there today. Some have been in place for close to a 100 years and are still keeping occupants toasty warm, but it seems the radiators last longer than the boilers, and so we frequently find ourselves doing a boiler replacement in such a system. Some people elect to go with a new high-efficiency condensing boiler, however, the majority of replacements are still cast iron boilers.

One of the first things that comes to mind when installing a cast iron boiler in a radiator system is “boiler protection.”

What is boiler protection you ask?

Well, it works this way: a cast iron radiator system usually holds a lot of water, and just look at all the big old steel pipes. All that water, steel and cast iron equates to a lot of mass! All that mass must be heated up before the radiators begin to produce heat.

So, when the thermostat first turns on the boiler and pump, cold water will enter the boiler return for quite some time before the system heats up. This makes the boiler block cold and causes the flue gasses to condense both inside the boiler and in the chimney. This is not a good thing because these boilers and their chimney vents are not designed to handle the condensate. The condensate is acidic, with a PH of 3-4, and will cause the cast iron boiler block to rust away. It will also destroy the boiler vent and eventually the chimney flue.

How do we keep that from happening?

I’m glad you asked.

There are several different ways to solve that problem, and they are not all equal in the level of effectiveness and resulting boiler efficiency.

For this discussion, let’s assume you did a heat load on the building and sized the boiler according to the actual load, rather than sizing it to the radiation or merely replacing it with the same size as was there before. This is, after all, proven to be the best way.

The first method, and the least effective for this type of system, is as follows.

The boiler aquastat will typically have a ZC-ZR terminal along with a C1 and C2 terminal. These terminals can be used to control the system pump or pumps. When wired properly, the terminals will not energize the pumps until the boiler heats up and reaches the low limit setting on the aquastat. Once it does, the pump will turn on, send all the heated water out into the system and replace it with cold water coming back from the system. The boiler quickly drops in temp, and the pump is turned back off. Meanwhile the boiler, full of heated water that was sent out into the system, starts doing what hot water does when it is mixed with colder water; it tries to find the highest spot in the system.

If you have a two-story house, you will typically find this heated water going to the upstairs first. This is because hot water is lighter than denser, cold water, and gravity forces it to do that. This process repeats itself until the whole system is heated up and the thermostat is satisfied. It’s not a very efficient way to run a boiler.

The second method takes a step in the right direction. It incorporates all the procedures described in the first method, but adds one more function — a bypass loop with a manual balance valve. What this bypass does is allow some of the return water to bypass the boiler and go directly into the boiler supply pipe. That slows down the flow through the boiler and allows the pumps to stay on rather than turn on and off as described in the first method.

The bypass pipe must be the same size as the boiler supply and return pipe. To set the flow in the bypass, one should start with the valve fully open. Then all zones should be turned on. Once the boiler reaches the low limit and turns on the pumps, begin slowly closing the bypass, while keeping an eye on the boiler temperature. The valve should be closed as far as possible while still allowing the boiler to remain above the low limit, and by extension, the pumps to stay energized. This will result in less boiler cycling and allow the emitter system to heat up more evenly.

This does not, however, add complete boiler protection. The return water temp will still be the same temp as what is returning from the system. It will merely be at a reduced flow rate, which can put an unwary heating contractor in a predicament. The reduced flow rate through the boiler will also reduce the boiler’s total BTU/hr output once the system is heated up. Likewise, the radiation will have a reduced output as well due to the lower water temp caused by mixing the return water into the boiler supply. This is usually not a problem on these systems, as many of them have oversized radiation as compared to the actual building heat loss. When this is the case, the radiators can adequately heat the space with a lower water temp. If the radiation is properly sized though and requires the higher water temp, the system may not deliver the required heat on the coldest days of the year.

The third method uses the same principals as the second method, but the bypass is positioned differently. This method provides better protection to the boiler, as it mixes hot water from the boiler supply with the return water coming back from the system, thus raising the actual return water temp going into the boiler.

But let’s take a look at what is happening in the system. First of all, we have slowed down the flow rate going to the system due to the bypass. This means reduced heat output to the system, but, more importantly, we have created a large Delta-T between the supply and return water temps. This does not usually work well with these radiator systems. The slow moving hot water quickly finds its way to the highest radiators and turns them into robust heat emitters while the lowest radiators sit patiently waiting on their turn for heat. And it will come, but not before the tenants on the second floor start hatching a plan to come downstairs and rip the thermostat off the wall.

In a nutshell, this results in uneven heating.

Moving on.

Let’s take a new approach and look at primary secondary piping as our fourth method. With this method we decouple the boiler loop from the system loop and add a pump for each. This provides full flow through both the boiler and the system and allows for a mixing point at the closely spaced tees that connect the boiler loop to the system loop.

This is a great approach for a zoned system most of the time. It allows individual zones to turn on and off without affecting the flow rate through the boiler. For example, let’s say one zone turns on. At this point the system flow rate should be lower than the boiler flow rate. When this happens, the boiler supply water will enter the supply tee and split directions.

Some heated water will go out to the system (equal to the system flow rate), and some will take the opposite direction toward the return tee. At this point, it will mix with the return water from that zone and raise the temperature of the water returning to the boiler. As you can see, this is doing great! Providing boiler protection and maintaining full boiler output.

But what happens when all zones call for heat at the same time?

Let’s look into that. If everything is properly sized and balanced, the total system flow rate should equal the flow rate in the boiler loop. At this point, all the boiler supply water is entering the supply tee and heading out to the system. Likewise, all the system return water is headed back to the boiler return. In this scenario, we have no boiler protection whatsoever. It is as if the closely spaced tees were not even there and the boiler was direct piped.

You might say, “Well that will never happen. All the zones never send a call for heat at the same time.”

I might agree with you if today’s trends coincided with the lifestyles of yesteryear. Lately, it seems everyone is insistent on having programmable thermostats installed. And you know what they do with them don’t you? I do too. So, twice a day, all the zones send a call for heat at exactly the same time.

So what do we do, you ask? How can we make things better? How can we provide even heat to the radiators, decouple the system flow rates from the boiler flow rates, allow full boiler and system BTU outputs and provide boiler return protection all at the same time?

The answer, as is usually the case, comes from an unexpected place. And it is created by something you wouldn’t expect — high energy costs. In recent years, we have seen the price of oil and LP gas go through the roof on several occasions. This has spawned an acute interest in renewable energies. One of these systems that runs on renewable energies are Biomass Systems. These systems typically lack fine control of the boiler’s heat output, and therefore need a large storage tank to store the heated water until the system needs it. Heating such a large volume of water with a non-condensing boiler spawned the invention of a mixing device designed to raise the boiler’s return water with the boilers supply water.

In this, the fifth and my preferred method, we have both a bypass installed and primary secondary piping via closely spaced tees. In this application, you will notice there is no manual balance valve installed on the bypass. The bypass is controlled by boiler protection valve (thermostatic mixing valve). This valve is controlled by a thermostatic element designed to maintain an outlet temperature not lower than the specified value. It does this by controlling the flow of the bypass and the flow from the return tee of the closely spaced tees. It is able to completely close off either inlet port.

So let’s start up a cold system. The boiler pump and burner turn on; the system return port to the boiler protection valve (BPV) is fully closed; and the bypass is fully open. All the flow from the boiler is just making a loop through the bypass and right back into the boiler. Meanwhile the system pump is running and all the water is zipping right through the closely spaced tees with no heat being added. As the boiler heats up and the return water temp reaches 130 F, the BPV begins to slowly close the bypass and open the system return port, while keeping the boiler return at 130 F. As this happens, some hot boiler supply water begins entering the supply tee, where it is mixed with the system water. This slowly adds heat to the system water and brings the temperature of all the radiators up evenly and consistently.

This process continues until the system return temperature equals 130 F, at which point the bypass will be shut off completely, or until the call for heat is gone.

This method of piping provides quite a few benefits. The boiler can be set up to operate as a cold start, meaning the low limit can be disabled. On a call for heat, the boiler reaches its peak efficiency operating point within minutes. This point is when the block is as cold as possible without causing flue gas condensation.

On a single zone system, it will run at this operating condition for most of the heating season. Only once the outdoor temperature gets cold enough to require a higher water temperature, will the boiler start running at increased water temps and slightly lower efficiency. On a single zone system, this method outperforms one that operates the boiler on an outdoor reset curve. The boiler turns on when there is a heat demand and doesn’t turn back off until the demand is met.

It also provides a very even and consistent heat across all the radiators.

This equals happy customers!