Fig. 1 Boiler Bypass
Fig. 2 System Bypass
The piping configurations in modern hot water systems may seem a lot different than those of the past. Indeed, I have heard people say that the newer hot water systems are more difficult to diagnose and service. However, if we stand back and look at the entire system, and not be intimidated by the new piping strategies we find in basements, we will see they're not as different or difficult as we might have been led to believe.
Reheat Principles Still Apply
Many of today's primary/secondary systems share some similarities with the old monoflo systems. We must remember that heat is always produced in the boiler and transported to the system via the piping. How this is done is very similar to ways of the past: heated water must move out of the boiler, into the system, and return back to the boiler to be reheated.
The important thing to remember when looking at system problems is, the problem may not reside where the symptom is found.
Whether the piping strategy is a simple manifold system, primary/secondary, monoflo, or reverse return, many problems begin in the boiler room and show up elsewhere. Some key concerns when troubleshooting a heating system include:
- Is the boiler sized correctly and producing enough hot water?
- Do we have the proper near boiler piping?
- Is the boiler Delta-T and flow within manufactures specs?
- Is the circulator sized properly?
- Is our Delta-T of the system acceptable?
Determine GPM and Delta-T at the Boiler
With the acceptance and popularity of the new condensing boilers, the flow through the boiler is very critical. Most manufacturers will supply you with a minimum and a maximum flow through the boiler and/or minimum and maximum Delta-T. This is a very simple calculation with non-modulating boilers. If you measure the return water temperature and an outlet supply temperature, you can determine flow in GPM through the boiler.
Delta-T is the difference between supply and return temperatures. Just subtract the inlet (return) temperature from the outlet (supply) temperature.
Example: 140 outlet 100 inlet = 40F Delta-T.
Determining GPM is a little harder. First determine the gross output rating (DOE output) of the boiler. Once you know the Delta-T and DOE output, you can calculate the GPM flow through the boiler as follows:
This calculation is easier when you're working with a cast iron or steel boiler, which do not modulate. The DOE output is on the boiler rating plate. The unit won't work as designed if the flow rates aren't within the manufacturer's specifications. If the flow is too fast, the water passing through the boiler won't pick up enough Btus (heat). When this happens, you'll be working with a lower Delta-T. This could create condensate within the boiler, because it is taking too long to warm up. Condensation in a steel or cast iron boiler is the boiler's worst enemy, because if condensation mixes with the acids of combustion by-products, the condensate will include sulfuric, hydrochloric and carbonic acids, which will destroy the cast iron or steel.
The idea is to dry out the flue passes before the boiler shuts down. The flue passes is the path the flue gasses take to exit the boiler. This is the area of the boiler that sweats while flue gas condensation is taking place.
The easiest way to do dry out the flue passes is to control the flow through the boiler. The average water temperature within the boiler must heat up to about 140F. We use this temperature as a reference point as the condensing temperature changes with fuels. Cast iron and steel boilers have been around a long time without many problems, and today's boilers are designed with less water volume. With proper near boiler piping, condensation issues will not be a problem.
Low Water Flow = Boiler Stress Points
Water at too low a flow will cause stress points in the boiler, or start flashing into steam, which will damage heat exchangers in very low water volume boilers. Low flow is not normally a problem with cast iron products. However, it can be more of a problem with products other than cast iron or steel, due to very low water volume and improper care to minimum flow standards set by the manufacturers. In primary/secondary piping systems, proper flow through the boiler can be controlled with correct circulator sizing. Sizing the circulator for the recommended manufacturers flow is a must. Even with primary/secondary piping, non-condensing boilers may require boiler protection. Contrary to some beliefs, primary/secondary alone does not offer full boiler protection.
Manifold piping and boiler primary loops normally require a minimum of bypass piping for boiler protection. The use of electronic or mechanical protection will provide the best protection with changing system conditions. Bypass piping has worked for many years to control the average boiler water temperature or flow rates through the boiler. Flue gas condensation is a result of return water that is too cold, or cool return water at a high flow rate.
There are two types of bypasses and each one handles the water differently. The two types of bypasses are boiler bypass (Fig. 1) and system bypass (Fig. 2). Their names describe exactly what they do. The boiler bypass bypasses system water around the boiler, and back to the system, reducing the flow through the boiler.
The boiler bypass is normally used on residential cast iron and steel boilers piped in manifold systems or primary/secondary where low return temperatures could be encountered. To have the boiler operate within specs, the flow through the boiler must be maintained at a certain flow rate. This rate is normally between 20 and 40 degrees. The new cast iron boilers have much less water in the boiler, and the volume of cold water being returned from the system could be very detrimental if all the return water was to go through the boiler.
A system bypass is normally used on commercial and smaller residential boilers, which have a minimum flow requirement such as copper tube, stainless steel and aluminum blocks. The system bypass will allow a determined flow through the boiler by taking some hot supply water and returning it back to the return which will increase the flow in the boiler. When doing either type of bypass piping, the bypass pipe should be the same size as the manifold piping and include a set of valves: one valve in the bypass, and the second located so it can control flow through the boiler or system depending on the bypass chosen.
Importance of Near Boiler Piping
Providing we have the proper near boiler piping, flow, and water temperature leaving the boiler, we should heat the home adequately. If the heat upstairs is not adequate, we must assume it is a system issue.
Examine the obvious. The list of possible problems change, as the types of systems change.
First, is the boiler too small? And second, does the boiler shut off on limit while the thermostat is calling? If so, the boiler is producing more Btu's than the radiation can dissipate. If the boiler is short cycling, check boiler size, anticipator setting and the following possible causes.
1. BASEBOARD SYSTEMS
- Check for dust or pet hair build-up on the bottom side of the baseboard elements
- Check to see if new carpeting or other floor coverings are reducing the airflow under the baseboard. The volume of air exiting the top and bottom of the baseboard should be equal.
- Check for a high number of fins bent over on the element.
- Are the air dampers wide open?
- Is the front cover in its proper location, and snapped on the top and bottom to control the airflow properly?
- Is the element properly installed? The element usually has two sides open and two sides closed. The open sides must face up and down. If you're looking down into the baseboard, you should be able to see through the element if it's properly installed.
- Check for air in the system.
2. RADIANT FLOOR
- Does the actual water temperature meet design specifications?
- Does the flow meet the design specifications?
- Has the floor covering changed?
- Is there proper GPM flow?
- Check for air in the system.
3. CAST IRON RADIATION
- Determine water temperature. Peform a heat loss calculation, determine sq.ft. radiation, divide heat loss by sq.ft. radiation to find required Btu's per sq ft. Use manufacturers' information to determine proper water temperature required for the amount of radiation.
- Check for proper water temperature. Existing cast iron radiation systems normally will not need to operate at high water temperatures, but there is a minimum temperature at which it will heat properly.
- Is there proper water flow? How long does the water take to return back to the boiler?
- What is the Delta-T at required water temperature?
You can use the same formula (boiler flow) to determine flow in system. The formula for radiations works best at design water temperature as output changes with water temperature.
You could also install flow meters or buy test equipment for this purpose.
Ron Beck is a training manager for Burnham Hydronics, Lancaster, PA, with 25 years of experience in troubleshooting piping, steam, and electrical systems. He can be reached at [email protected]