Frequently Asked Questions Concerning Hot Water Boilers and Hydronic Systems
When I see an AFUE rating on a boiler, does that mean I will be operating at that efficiency anytime the boiler is operating?
No, the boiler rating is dependent on the AFUE testing procedures, which include, but are not limited to, proper flow, 120F inlet water temperature, and 140F outlet water temperature. As the water temperature exiting the boiler is higher than 140F, the vent temperature increases, and the efficiency decreases. The higher the water temperature, the lower the efficiency. On the higher efficiency products with an AFUE rating of 95%, again tested at 120F inlet and 140F outlet water temperatures, operating at 180F outlet water temperature will be around 86% to 87% efficient. The higher the efficiency and the lower the water volume, the greater the change in efficiency as water temperature increases. For instance, a cast-iron boiler will experience significantly less efficiency decrease with an increase in water temperature.
Does proper sizing of the boiler really save more money?
When equipment is oversized, it causes the product to short-cycle, which affects the efficiency and standby losses. It also affects the lives of all the mechanicals as they cycle on and off too often. When a motor or control starts, it receives an inrush of power, which can cause solid-state components to fail. For more information on proper sizing, see the Heat Loss index on the left.
Can't I measure the radiation to size the new boiler?
Measuring the radiation is almost a guarantee that the boiler will be grossly oversized. Here is an example. A home has 157,231 BTUs of radiation. This is likely a combination of cast-iron radiators, copper tube baseboard, and radiant floor heat. The heat loss is 68,783. If you install a boiler according to the radiation, the boiler would be more than twice the size it needs to be. When you install the boiler according to the heat loss, and choose a 70,000 BTU boiler. The fuel consumption would decrease, and, of course, the comfort level would increase dramatically.
I was told the size of the amount of radiation because you have to have enough boiler to heat the radiation, or the house will be cold
No, let's assume you installed new windows, weather-stripped the home, and added insulation in the attic. Won't this change the home's heat loss? Of course it is, or why waste your money? This suggests using a smaller boiler. But what about the radiation? Are you going to remove some of the radiation? Of course, you will reduce the flow and/or water temperature. If the amount of radiation is 30,000 btu's, and now that zone is 20,000 btu's. You change the flow from 3 gallons per minute (gpm) to 2 gallons per minute (gpm). Depending on the type of radiation, you may also need to reduce the water temperature. This is how you maximize your fuel savings.
Can the boiler piping affect the operating cost of the boiler?
Yes, it can. This is called near-boiler piping. Several variables can affect the operation of the boiler. If a cast iron boiler is installed and boiler bypass is required but not installed, the boiler could run excessively. We must be concerned with protecting cast iron and steel boilers against flue gas condensation. high efficiency boilers can operate well below condensing temperatures. See boiler protection
Is there a big difference between cast iron boiler manufacturers?
There can be differences between the casting design. Most cast iron boilers are of the pin-type design; however, some manufacturers also produce three-pass cast iron boilers. The three-pass boilers are more efficient because the flue gases remain in the boiler for a longer period of time. Some cast iron boilers may have internal baffles, which make them slightly more efficient by controlling the path water flows. Many European boilers are being sold today, and American boilers are just as efficient. Just compare apples to apples. If it is a three-pass European boiler, it cannot be compared to a pin-type American-made boiler. Three-pass boilers made abroad and in the USA are tenths of a point apart. For instance, the Burnham MPO oil boiler has a higher AFUE rating than almost all other cast iron boilers on the market. They also have built-in boiler protection against flue gas condensation.
Compare efficiencies and warranties to see what manufacturers offer.
Do all cast iron boiler installations require bypass protection?
No, there are rules to follow. The basic rule is to use boiler protection if any of the following conditions occur. If none of the following exists, the installation would not require a boiler bypass 1. When there is any amount of radiant floor heating or floor warming in the system.
2. When the system is an extensive water volume system. This would be a system with cast iron radiation.
3. When the amount of radiation in the system exceeds the DOE output of the boiler. This is typically the case for most jobs where the boiler is sized based on a heat loss calculation. The exception to this rule would be if it were a low water volume system (copper tube baseboard), which would be zoned, and no one zone has more output than the boiler DOE Output.
4. There is a zone of snow melt on the system
5. Anywhere low return water temperatures are expected for an extended period of time. (Below 120f, some boilers can return as low as 100f)What is the difference between a boiler bypass and a system bypass?
The difference is the flow in the bypass. A system bypass means that a percentage of the water leaving the boiler will go through the bypass and return directly to the boiler. A boiler bypasses a percentage of the water, which then bypasses the boiler and returns directly to the system. The name of the bypass is what is bypassed. With a smaller boiler than the connected load, I would use a boiler bypass, which maintains a higher flow rate in the system and reduces the flow through the boiler. Smaller boilers require less flow. This results in increased heat output from radiation and alters the system's water temperature in response to changes in outdoor temperature. With large water volume systems, the system temperature will rise more quickly with a boiler bypass than with a system bypass, as the boiler operates at a higher temperature. A system bypass will reduce the flow in the system, which slightly reduces the heat output from the radiation. It will also reduce the air elimination process if the flow is less than 2 ft per second. I would pipe the system as primary/secondary if I used a system bypass, so the flow in the system is not affected. This is an example of system bypass and boiler bypass. For more information see Cast-iron boiler protection.
Can a boiler bypass save fuel?
Yes it can. When a boiler bypass is installed in a large water volume system, the reduced flow through the boiler occurs. Although all the water in the system needs to be heated, the entire system does not have to be raised to 180F. With reduced flow through the boiler, the boiler can exceed the condensing point of 140°F. It will not be uncommon for the system temperature to be much lower than the boiler temperature. You may see the boiler reaching 160F - 180F, but the system water may only be 120°F or 140°F. The advantage of the boiler bypass is that the system temperature increases as the outside temperature decreases and decreases as the day warms up. With a large water volume system (typically cast-iron radiators), the system will operate at significantly lower water temperatures. See required water temperature for cast iron systems.
How do you adjust the bypass valve?
This is normally done after the system water temperature gets above 140f. If there is a valve on the boiler bypass, it is always wide open. Some people do not even install it. Do not partially close it, as a lot of water needs to pass through it. There could be as much as three times the water going through the bypass as going through the boiler. The best way to adjust the valve on the boiler is to install thermometers on the supply and return pipes near the boiler or use temperature probes strapped onto the pipe and insulated.
- I usually adjust the delta T through the boiler as follows
- Low water volume systems whether zoned or not 20f to 25f rise
- Large water volume systems such as cast-iron radiators and radiant heat 30f - 40f rise through the boiler
Some say the circulator should be on the supply side of the boiler, and others say it does not matter. Does it make a difference?
How much pressure should my hot water heating system have?
The pressure varies according to the height of the heating system. The system pressure is calculated on the vertical distance from the lowest piping and radiation to the top of the highest radiation. Use the formula H/2.31 + 4psi. After you calculate the vertical height of the system, divide the distance by 2.31. This will indicate the amount of pressure required to elevate the water to the highest point in the system. We need to have a positive pressure of 4 psi at all times at the high point of the system. This means whether the circulator is running or not. We add 4 psi. for example, 21 ft vertical height of the system.
What is outdoor reset, and should I have it installed on my boiler?
An outdoor reset control (ODR or ODR) will sense outdoor temperatures and adjust the system water temperature accordingly. When a heat loss is done, it is done at a specific outside design temperature. This is the coldest average temperature in your area. As you know, it is not always that cold. As the temperature outside is warmer than the design temperature, the heat loss is reduced. The ODR will sense the outdoor temperature and adjust the system water temperature to try to match the heat loss every time the boiler runs. This would need to be added to steel and cast-iron boilers and is typically included with newer modulating/condensing (mod/con) boilers. If you have a mod/con boiler installed, verify that the contractor installed the outdoor reset sensor and wired it to the boiler. I have seen situations where mod/con boilers were installed, and the ODR feature was not connected. As much hype as modulation is given, the true savings are boiler sizing and ODR
I believe every boiler should have an outdoor reset control. The fuel savings will typically cover the cost of the power in the first two seasons. The increase in comfort is worth it alone. The comfort level will increase dramatically. It will be challenging to determine if the heat is on or off, as the temperature in the home will remain more constant and undergo more gradual changes. Most people do not reset the thermostat settings with ODR, as this will increase the recovery time. If you still want to turn the temperature down at night, I would suggest thermostats with intelligen recovery. Intelligen recovery means the thermostat will learn how long it will take to bring the house temperature back up and start the process when needed. This may take a week or so. If you manually change the temperature during this process, it will take longer. Find temperatures you are comfortable with and leave them alone.
Is there an advantage to multiple boiler applications?
Absolutely, this depends on the application for heat and or indirect hot water heating. There are situations where it makes more sense than others. See multiple boilers for more information.