In my last column, we discussed thermostatic radiator valves. I think we can safely call TRVs old technology. After all, they’ve been around for 75 years. Cast-iron radiators certainly are another example.
Not that it makes them any less relevant. Here in the East, I still run into them everywhere.
But what happens when you take old technology like this and attempt to marry it with new technology like mod-con boilers and variable speed smart pumps?
Well, it can end disastrously, if the right precautions aren’t taken. However, using proper techniques can yield exceptional results.
This Giant Old House
As I mentioned at the end of my last column, I got hired to update and modernize the heating system in this giant old house. The house had a behemoth of an old gas boiler sitting in the basement. Between it and the pipes, it occupied a fair portion of the basement all by itself.
The system originally operated on gravity flow, but at some point, many years ago, someone slapped some pumps on it. The pumps were huge, and the way they clung to the side of the boiler, it would have seemed as though they could have tipped the boiler over. I had to suppress the desire to place a support under the motor end of the pump the moment I laid eyes on it.
Connected to the boiler were all these giant old steel pipes. They meandered around the basement, ever sloping upward from about chin height to forehead height. Perfectly positioned to turn the unsuspecting individual into a stargazer.
Those big old pumps must have sent water rushing through those steel mains with a voracity similar to the Niagara Rapids. This system guzzled a lot of gas and wasn’t friendly to the electricity bill either.
The new owner set out to change all that. He wanted to keep the old cast-iron radiators in the rooms, but wanted the boiler and those giant steel pipes gone.
A professional piano player and teacher by trade, he was also quite handy with tools. He was in the process of remodeling the whole house by himself and restoring it to its original beauty. Everything from moving walls to restoring the layout, to researching and sourcing the original trim details.
I was very impressed with his capabilities and was comfortable designing his new heating system. His plan was to install everything himself except for the gas piping, boiler venting and combustion setup. All he needed was a set of plans and some guidance, and he was good to go.
For the system we decided to go with a Lochinvar WHN boiler. All the radiators would have homerun PAP piping coming back to distribution manifolds that offer flow balancing and visual flow meters.
Each radiator was fitted with a TRV to allow for room-by-room temperature control. For the system pump we selected the Grundfos Alpha and set it to the Auto Adapt setting. We also have and indirect water heater connected. See Picture A for the system layout.
Before we could size all the new equipment and pipes, however, there was bunch of math to be done. As is always the case, a room-by-room heat load calculation needs to be performed first. This is the base, or roots if you will, for all proceeding calculations and equipment sizing.
On a job like this, it is convenient to lay everything out in a spreadsheet like the one shown in Picture B.
It keeps everything orderly and allows you to set up the calculations and their results in the correct order. It is also easily filed away for future reference.
The next task was to figure out the output of every radiator.
First you have to identify the radiator and then measure the dimensions and count the sections. Dan Holohan wrote a book called “EDR (Every Darn Radiator).” This book is very helpful to identify the radiators, and it gives their “equivalent direct radiation” outputs. That’s how they used to measure the outputs of these radiators in the old days.
Once you have the EDR for each radiator, you have to convert it to our current method of measurement, which is BTU/hr.
Once again, I consult Dan’s book. It has some handy calculations to convert the radiators EDR to BTU/hr. at various different water temperatures. Typically, you will start the calculations using 170 F average water temperature. This implies a system operating at a 20 F Delta T, which is a good place to start.
After jotting down all that information in your spreadsheet, you can now compare the radiator outputs to the heat load requirements of the room they are in. If they all have higher outputs than the room loads, you can decrease the average water temperature until you reach the point where the radiator outputs match the room heat loads.
On this job, I had to stay at an average water temperature of 170 F. In some rooms, I also had to add more radiators to meet the heat load requirements. In other rooms, however, there were radiators that were grossly oversized at that average water temperature.
To combat that, I reduced the flow rate for those radiators, which by extension, reduces the average water temperature and increases the Delta T. There were several radiators that ended up with an average water temp of 150 F and a 60 F Delta T.
That is where the radiant distribution manifold comes in handy. It allows you to quickly and easily set the correct flow going to each of these radiators and balance the system.
After the radiators are all balanced and flow rates established, you can now size your piping and figure out the head loss for the whole system. This is necessary to get the correct size pump and know what the correct settings for that pump will be. There are many sources from different manufacturers to help with pipe sizing, flow rates and their corresponding pressure drops.
Back to the system at hand.
The boiler is setup to run an Outdoor Reset curve and supply the system with 180 F water on the coldest day, following a linear curve to 85 F when the outside temp reaches 68 F. Also, at 68 F, the Warm Weather Shut Down activates and prevents the boiler from turning on for space heating, while still permitting it to turn on to heat up the indirect water heater.
There is no thermostat in the house and the TT terminals are jumped on the boiler, creating a continuous call for space heating, until it warms up outside and the WWSD feature turns off the boiler.
The system uses continuous circulation and variable speed pumping with Auto Adapt technology. Also, a TRV on each radiator modulates the flow rate based on indoor air temperature.
One of the first declarations everyone makes about this setup is, “You’re going to kill the boiler! It will get warm in the house and all the TRVs will close. When that happens, the boiler will short cycle itself to death!”
That is a plausible concern, but it doesn’t happen in a system like this. The reason it doesn’t is because of the interaction of three distinctly separate technologies.
• Auto Adapt
To understand how they all interact and make the system function, we first have to examine each technology individually.
ODR, is pretty simple and I’m sure most, if not all of you, know how it works.
ODR varies the temperature of the supply water going into the system based on the outdoor air temperature. It does this by modulating the burner to add more or less heat. The burner modulation rate depends on the required water temperature and the flow rate of water going through the boiler.
Auto Adapt is much more complex and can take a while to wrap one’s head around. It took me a while.
Auto Adapt is split into three parts:
• System Analyzer
• Curve Selector
• Proportional Pressure Control
Auto Adapt’s first task is to analyze the heating system in which the circulator is placed. The System Analyzer does this. The analysis is aimed to tell if the circulator pressure is too high, too low or OK.
Auto Adapt’s second task is to use the knowledge obtained from the System Analyzer to select a proper proportional-pressure-curve for the circulator. The “Curve Selector” does this. Finally, the circulator is controlled according to the selected proportional pressure curve.
In order for the analyzer to measure the system pressure requirements, it measures and records the total hydraulic conductivity. The total hydraulic conductivity, symbolically expressed as K, is a measure of the total system pressure drop, or resistance to flow, from the discharge of the pump, through the entire system and back to the inlet of the pump.
In a system with TRVs, the K value tends to change in response to the heat load requirements in the rooms. As the TRVs modulate, the pump responds, continually optimizing the flow.
This is how the System Analyzer works: It plots the system K value over a period of time.
Take a look at the chart in Picture C. In this chart, the pump pressure is too high. That can be seen by looking at the system K value. Thermal saturation is occurring with the TRVs nearly closed, resulting in poorer temperature control and possible velocity noise. In this case, the curve selector will select a new, and lower position, for the proportional pressure curve.
Next, take a look at the chart in Picture D. In this chart, thermal saturation is occurring with the TRVs nearly open. This, once again, hampers the TRVs ability to precisely control room temperature. It can also result in one room heating up faster than another room in the house, if the branch circuits are not perfectly balanced.
In this case, the pump pressure is too low and the curve selector will revise the proportional pressure curve upward.
Finally, let’s look at the chart in Picture E. In this chart, thermal saturation is occurring in the average center between the highest system K value and the lowest system K value. This means the TRVs are modulating around halfway between their open and closed positions.
In this position, they maintain the greatest authority over flow and the room set point, while also reducing the risk of any velocity noise.
In a nutshell, Auto Adapt strives to keep the TRVs close to the center of their stroke to give them better authority over room temperature.
This also means the pump will be moving the least amount of water necessary to heat the house. In so doing, it will also use the least amount of energy.
On to the TRVs. In my last column, we took a look inside the TRVs and examined how they work.
We also learned that TRVs are typically operating within the last millimeter of their operating stroke, meaning they are nearly closed most of the heating season, and only open further if someone changes the room set point or the system is operating near design conditions where full flow is required to the radiators.
I would like to point out here, this is how TRVs function when used in a system that delivers a constant set point water temperature all season.
TRVs behave a bit differently, however, when paired with ODR. When you have a system using ODR tech, the water temperature is being modulated proportionally to the outdoor air temperature in an attempt to match the boiler’s input to the system demands. For this to work properly, system flow rates have to be kept close to the design flow rates so the radiators produce enough heat at a lower average water temperature.
This means the TRVs have to be further toward the open position during normal operation. This may be hard to fathom at first. Especially when you open the TRV manual and see the dial numbers and their corresponding set point temperatures.
But take a look back inside the TRV. When you rotate the knob on the TRV head to set the temperature, all you are doing is changing the position of the valve disk in relation to the ambient temperature that the TRV is sensing.
The solution is simple. In a system using a properly set ODR curve, the TRV adjustment knob should be set to a higher number to allow for proper flow. The TRV will still work the same, it will just do so at a higher flow rate.
When you put these three technologies together, the system works beautifully. It is very simplistic, yet the underlying technologies and their interactions, create a system performance level that can only be rivaled by the most sophisticated and expensive electronic controls.
Let’s follow through with system operation.
First, the boiler is getting a heat demand any time the WWSD is not active. In turn, it activates the system pump and fires the burner, modulating it, to produce a target supply water temperature, that has been calculated by the ODR.
Take a look at Picture F. Note the orange line along which the supply water temperature is being calculated. It is important to realize, that while ODR is attempting to match the building’s heat load, it is really only getting you in the ballpark.
Environmental conditions affect the heat load in ways that ODR cannot accommodate. Some of these are internal gains, windy conditions and solar gain. To combat that, we need something to tell the heating system that we need more or less heat then is calculated by the ODR.
Enter the indoor feedback loop. With some electronic controls, you will have a temperature sensor inside the building providing room temperature feedback. This feedback is used to recalculate the target supply water temperature, allowing it to deviate from the ODR-calculated supply water temperature. This works well, but it can cause the boiler to operate at a higher temperature than is absolutely necessary.
In our case, the system flow rates are providing the indoor feedback to the boiler and the supply water temperature stays pegged on the ODR-calculated target.
It works like this: The boiler is attempting to match the heat load of the structure. The TRVs sense the indoor air temperature and modulate towards their open or closed position to maintain room temp. Auto Adapt senses the change in the system K value and selects a new position on the proportional pressure curve too return the TRVs to their center position.
In so doing, it changes the system flow rate to meet the real time demands. The boiler senses the change in system flow rate and in turn, changes the modulation rate to maintain the target supply water temp.
It is indoor feedback with no wires. Beautiful!
What if the room temperature gets satisfied, and the TRVs are all closed, then what?
That can’t happen. The TRVs are set to a higher number to account for ODR and won’t be closed until the room is well above set point. Plus, we have Auto Adapt technology continuously monitoring the system K-value and adjusting flow rates to keep the TRVs close to their center positions.
What happens when the boiler reaches its minimum firing rate? Won’t it short cycle?
If the boiler is properly sized, absolutely not.
At this point the boiler will operate on a preset differential of the supply water temperature. The boiler cycles will be buffered by the entirety of the mass in the whole heating system and also the current heat load of the structure.
In my opinion this is more efficient than using a buffer tank. With a buffer tank, it is almost unavoidable to prevent the boiler from over-accelerating when it needs to recharge the tank.
What if the system flow rate drops below the boiler minimum flow rate?
This is a valid concern and one that must be approached carefully. First of all, you must ensure the boiler is properly sized for the system. You also want to select a boiler that is able to fire at a lower modulation rate and immediately start modulating downward from there.
You also have to be careful how you set your ODR curve. You want to set the bottom end of your curve as low as you can. If you find the system not providing enough heat during warmer outdoor temperatures, you can revise the bottom of your curve upward a few degrees at a time until you reach the correct setting.
Remember, if the supply water temperature is too high, the result will be a lower system flow rate. If it is too low, the result will be a higher system flow rate.
If you set it up this way, the boiler will never suffer from low flow rates.
In this system, we found 85 F to be the appropriate temperature for the low end of the curve. This will vary from system to system and different types of emitters can affect it dramatically.
This system has been purring along for a few years now without any problems. The homeowner is happy, and he should be! He did a better job installing this system than the majority of professional installations that I see. I guess professionally playing and teaching piano gives a person more focus on the details. l
Harvey Ramer is the owner of Ramer Mechanical (RM) LLC. RM specializes in radiant heating and hydronic heating systems. The company also provides other mechanical services to the residential and light commercial market. Ramer also provides heating system design services and consultation across the country. Contact him at firstname.lastname@example.org.