Hot water heating systems FAQ

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 not limited to proper flow and 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 goes down. 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 lower water volume the more the efficiency changes as water temperature increases. For instance, a cast iron boiler will have much less efficiency decrease with water temperature increase.

>Here is an efficiency chart for high efficiency boilers at different water temperatures.

Does proper sizing of the boiler really save more money?

When equipment is over sized it causes the product to short cycle which affects the efficiency and stand by losses. It also affects the life of all the mechanicals as they cycle on and off too often. When a motor or control starts it gets an in-rush of power which can start breaking down solid state parts. For more information on proper sizing see the Heat Loss index on the left.

Can't I just measure the radiation to size the new boiler?

Measuring the radiation is almost a guarantee the boiler will be grossly oversized. Here is an example. A home has 157,231 btu's of radiation. This is possibly a mix 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 go down and of course the comfort level would increase dramatically.

I was told size to 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. Isn't this going to change the heat loss of the home? Of course it is or why waste your money. This says use a smaller boiler. But what about the radiation? Are you going to remove some of the radiation? Of course, not, 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 gpm to 2 gpm. Dependent on radiation type you may also 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. There are variables which 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 cast iron and steel boiler protection 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 pin type boilers but some manufacturers use three pass cast iron boilers. The three pass boilers are more efficient as the flue gasses are in the boiler longer. Some cast iron boilers may have internal baffles which make them a bit more efficient due to controlling the path the water flows. There are a lot of European boilers being sold today and the American boilers are just as efficient. Just compare apples to apples. If it is a three pass European boiler you cannot compare it 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 rules as use boiler protection if one of the following occurs. 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 a large 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 would be most jobs where the boiler was sized with a heat loss. 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 a percentage of the water leaving the boiler will go through the bypass and return right back to the boiler. A boiler bypass a percentage of the water will bypass the boiler and go directly back 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 will keep the flow rate in the system higher and lower flow in the boiler. Smaller boilers require less flow. This gives more heat output from the radiation and changes the system water temperature as outdoor temperature changes. With large water volume systems the system temperature will rise quicker with a boiler bypass than a system bypass as the boiler runs at a hotter 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. Although all the water in the system needs to be heated the entire system does not have to be raised to 180f degrees. With reduced flow through the boiler the boiler can get above the condensing point of 140f. It will not be uncommon for the system temperature to be much lower than the boiler temperature. You may see the boiler getting 160f - 180f but the system water may only be 120f or 140f. The advantage of the boiler bypass is the system temperature gets warmer as it gets colder outside and gets cooler as the day warms up. With large water volume system (usually cast iron radiation) the system will operate at much 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 wide open all the time. Some people do not even install it. Do not partially close it, as there has to be a lot of water passing 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 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?

Yes it does but just putting the circulator on the supply side is not truly accurate. The circulator on the supply pumping away from the expansion tank connection (point of no pressure change) is a more accurate statement. The change came about with the smaller circulator know as wet rotor circulators. The old three-piece circulators had a relatively flat pump curve. See comparison. The older circulators moved more water but had lower ft of head limits. The newer wet rotor circulators have steeper curves. The steeper curves allow better air elimination. Air elimination is crucial to boiler life. When we have air in the system the air is 21% oxygen. The oxygen will cause corrosion or erosion depending on the metals of the boiler. The better the air elimination the longer the boiler life.

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 tell us how much pressure you need to get the water to the high point of 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.

  • 21 / 2.31 = 9 psi
  • 9 psi + 4 psi = 13 psi required
  • The minimum pressure we ever want in a hot water system is 12 psi

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 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 have to be added to steel and cast-iron boilers and usually comes with the newer modulating/condensing (mod/con) boilers. If you have a mod/con boiler installed verify 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 is boiler sizing and ODR.

I believe every boiler should have an outdoor reset control. The fuel savings will normally pay for the control in the first season or two. The increase in comfort is worth it alone. The comfort level will increase dramatically. It will be hard to tell if the heat is on or off as the temperature in the home will remain more constant and more gradual changes. Most people do not reset the thermostat settings with ODR as the recovery time will increase. 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 it 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.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.