Single zone boiler bypass

CTD01

I searched and cant seem to find the answer to what I'm looking for. I'm installing a new Yorker APU 110. I have a 1300 SQ ft ranch with aprox 100 feet of cast iron baseboard with a single zone. Book says to put a bypass in before the circ. I read some people do that and some put it after the circ. Circ is on the supply pumping away. What is best? Also should I put just valves or get an automatic bypass valve? Also should I put a gauge on the return? I have the taco 007e circ. Thanks 

EdTheHeaterMan

There are several reasons to use a bypass on the near boiler piping. Zoning is one of them. Since you are not zoning, then there is one reason that you might be interested in the bypass. Condensation of flue gas. This is when the return water is too cold for too long causing condensation to form on the fire side of the heat exchanger. The cooler return water will cause the flue gas temperature to reach the due point (the temperature at which the water vapor in the flue gas condenses) and leaves corrosive water droplets on the metal surface of the heat exchanger. This water mixes with some of the other byproducts of combustion to form carbonic acid and sulfuric acid. This will cause premature metal failure.

If this is the case, then you want to pipe the bypass this way.


Here is the text form a popular boiler manufacturer' I/O manual describing the reason for this
"Large Water Volume Systems - The piping shown in Figure 8.3 will minimize the amount of time that the boiler operates
with return temperatures below 120°F on these systems. A bypass is installed as shown to divert some supply water
directly into the return water. The bypass pipe should be the same size as the supply. The two throttling valves shown are
adjusted so that the return temperature rises above 120°F during the first few minutes of operation. A three-way valve can
be substituted for the two throttling valves shown."

EDIT: Figure 3-3 on the New Yorker manual shows the circulator pumps after the bypass because they are illustrating zoning using pumps. It would be impractical and more expensive to bypass each zone circulator after the pump location. So a compromise is reached using one single full size bypass before the pumps. Slightly less effective but it gets the job done.

Since you are not zoning, then the diagram above is "best practice"

CTD01

Yea that is the reason I want the bypass. I'm worried with the baseboard I have it will cool too much and condense. Another question if I put bypass in after the circ does it matter the distance away from the circ?

STEVEusaPA

100 feet of cast iron is a lot for one zone, unless it was perfectly installed to get the right temperatures in the right places.
Many go with a simple ball valve between the supply & return near the boiler, then throttle it to protect the boiler. It's a manual process done basically by feel.
A 3 way or even a 4 way mixing valve would be better. Caleffi, Esbe & Taco make them.

CTD01

STEVEusaPA said:

100 feet of cast iron is a lot for one zone, unless it was perfectly installed to get the right temperatures in the right places. Many go with a simple ball valve between the supply & return near the boiler, then throttle it to protect the boiler. It's a manual process done basically by feel. A 3 way or even a 4 way mixing valve would be better. Caleffi, Esbe & Taco make them.

The heat always worked fine just my boiler failed and I am putting in a new one. Does distance after circ matter with the bypass. An ideal spot would be about 9-10 feet after the circ for my setup 

EdTheHeaterMan

CTD01 said:

Yea that is the reason I want the bypass. I'm worried with the baseboard I have it will cool too much and condense. Another question if I put bypass in after the circ does it matter the distance away from the circ?

Not really. But don't get carried away. You should be able to do the piping with a 20 ft length of copper from supply to return. And keep it simple. three way and four way valves with temperature sensors are more accurate, but you only need that if there are several zones that can dump large amounts of cold return water back to the boiler at different intervals while another Zone just completed a call for heat. You don't have that problem

10 feet is fine

CTD01

Tha k you guys for the help!

CTD01

Thank you guys for the help! Should I keep the size of the bypass the size of the return? It's 1"

CTD01

EdTheHeaterMan said:

There are several reasons to use a bypass on the near boiler piping. Zoning is one of them. Since you are not zoning, then there is one reason that you might be interested in the bypass. Condensation of flue gas. This is when the return water is too cold for too long causing condensation to form on the fire side of the heat exchanger. The cooler return water will cause the flue gas temperature to reach the due point (the temperature at which the water vapor in the flue gas condenses) and leaves corrosive water droplets on the metal surface of the heat exchanger. This water mixes with some of the other byproducts of combustion to form carbonic acid and sulfuric acid. This will cause premature metal failure. If this is the case, then you want to pipe the bypass this way. Here is the text form a popular boiler manufacturer' I/O manual describing the reason for this "Large Water Volume Systems - The piping shown in Figure 8.3 will minimize the amount of time that the boiler operates with return temperatures below 120°F on these systems. A bypass is installed as shown to divert some supply water directly into the return water. The bypass pipe should be the same size as the supply. The two throttling valves shown are adjusted so that the return temperature rises above 120°F during the first few minutes of operation. A three-way valve can be substituted for the two throttling valves shown." EDIT: Figure 3-3 on the New Yorker manual shows the circulator pumps after the bypass because they are illustrating zoning using pumps. It would be impractical and more expensive to bypass each zone circulator after the pump location. So a compromise is reached using one single full size bypass before the pumps. Slightly less effective but it gets the job done. Since you are not zoning, then the diagram above is "best practice"

One more question. Is the isolation valve redundant in this pic. Could I just shut off the bypass throttling valve? 

EdTheHeaterMan

CTD01 said:

Thank you guys for the help! Should I keep the size of the bypass the size of the return? It's 1"

What size is the supply? It says "Full Size" in all the instructions manuals I ever read. So if the supply is also 1", then yes.

EDIT: I looked at the boiler spec sheet. You selected the correct boiler for the job. 1" supply and 1"return is perfect for you with a 1" bypass and full port 1" throttle valves

If you only have a 1" return shared piping from all the radiators then you don't need a very big boiler. AHRI NET rating does not need to be much more that 80,000 or 85,000 BTUh. That might be a 120,000 BTUh input boiler depending on the AFUE of the new boiler. So you selected a boiler that is not too big for your system. Now did you do the load calculation to see if you can use a smaller nozzle like perhaps a .65 GPH

1 Inch pipes can not move much more heat energy than that, under normal design parameters of say 8GPM or less with a 20° temperature difference, between the supply and return water.

CTD01

It has 1 1/4 feed 1" return. Ahri net is 83k. This house had a 131k

EdTheHeaterMan

If you are using that boiler for your DHW, be sure to include a mixing valve. My favorite was the AM100 series with union connections from Honeywell, but Caleffi and Taco also make good ones.

CTD01

Ok. I'm plumbing in a domestic coil to have as a back up. I just put a heat pump hot water heater in. Cost me about $8 a month.

EdTheHeaterMan

There is a "best practice" that I use when installing 2 water heaters. The primary water heater should be piped direct to the hot water faucets while the unused water heater is used as a preheater. In the season when the one water heater is more economical than the other, you can swap the primary and secondary by changing the position of 3 valves.
Just substitute the tankless coil for one of the tanks in the diagram.


Just make sure the mixing valve is the last stop for the hot water before it leaves the boiler room/

CTD01

That's the plan! Thank you for all your help!

CTD01

EdTheHeaterMan said:

Now did you do the load calculation to see if you can use a smaller nozzle like perhaps a .65 GPH 1 Inch pipes can not move much more heat energy than that, under normal design parameters of say 8GPM or less with a 20° temperature difference, between the supply and return water.

I did not do the load calculation yet but I will. I wanted to get it all piped in first.