Boiler Pump Size

dineshj

Hi. I currently have a Laars JVS 75K input hot water boiler. Output is about 63K BTUS. It heats the cast iron radiators. There's lot of water mass in the system, about 45 gallons.
On a cold start, the system takes about 3 hours to heat up in 35 degrees weather. I leave the thermostat steady at 72 in winter. I have solar gain through the windows during the day, so the system usually only does cold starts.

The system has been condensing even with a full-size bypass. The return water temp for the first 1.5 hours is below 130.

My thinking is that the boiler pump is too fast. The system has the Taco 007 pump.

Can someone help me size the one my system would need? The longest run of a pipe from the boiler to the last radiator is about 45 ft. The first 20 ft are 7.5 inch OD pipe and the rest is about 5 inch OD. Same for the return pipe. These are old thick cast iron pipes.

Thank you.

Jamie Hall

Seven and a half inch outside diameter? EEK. Are you sure?

dineshj

IMG_3913.jpg

Yes sir.

EdTheHeaterMan

That is circumference. the diameter is about 2" ID which is normal for an old gravity hot water system in an average size home.

So basically you measured 7.4612" (about 7 1/2") circumference to establish that the outside diameter of that pipe is about 2.375". That is the specification for 2" nominal iron/steel pipe size

LRCCBJ

The system has been condensing even with a full-size bypass. The return water temp for the first 1.5 hours is below 130.

The details of this installation need to be investigated. A proper bypass with a suitable ESBE valve will limit the return water to the boiler to the specified temperature……………usually 140F. If the boiler is below 140F, NOTHING will flow to the system until the boiler reaches 140F.

I suspect you do not have a proper valve on the bypass or it is piped incorrectly.

Of course, Primary/Secondary piping would cure this issue immediately but I suspect you'd rather not bother with it.

dineshj

@LRCCBJ Attached is the bypass piping.

The supply side valve (the one after the bypass) is half closed and the bypass valve is fully opened. This is forcing the water back to the boiler.

IMG_3914.jpg

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1809108#Comment_1809108

As I figured.

That setup can, in no way, manage the return flow to prevent it from going below 135F. You have a dynamic system whereby the valve needs to be fully closed at the beginning of the cycle and transitioning to about 1/2 of the way open when all the rads are fully heated (about 90 minutes later).

You have a small boiler with giant heat sinks (CI rads). You need proper protection of the boiler with a valve that will vary its opening.

This is the valve you need instead of the ball valve:

https://www.caleffi.com/sites/default/files/media/external-file/NA10292.pdf

dineshj

Yeah I have heard of that. Doesn't that cause pump cavitation at the startup when all the heated water is going back to the boiler?

So is the current pump sufficient with the water volume and length of the pipe I have?

Thank you for the suggestion.

EBEBRATT-Ed

Your boiler output is 63,000 so 6.3 gallon/min is what you need. (10,000 btu/gpm circulated) this should give you about a 20 degree td between supply and return. Taco 007 should be fine

But I agree you need some type of automatic 3 way valve to manage the bypass. There are different types and different controls. With a condensing boiler I would keep the boiler return about 90 deg a standard boiler should be 130 minimum. But you heat emitters have a say in the water temperature design as well.

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1809122#Comment_1809122

The bypass valve will not cause pump cavitation.

The current pump is more than adequate.