Pump sizing with bypass

imaddicted2u

The boiler is a Newmac NBR-2000. It's not making domestic hot water. It is wet base and the construction is steel. I bought it used. As the model implies it was made in 2000. Circulator is a Bell and Gossett NRF-22. The boiler was connected in parallel with a coal fired boiler that was gravity feed. It has since been removed. There are two pairs of 2 1/2 headers with 1" and 3/4" takeoffs feeding the cast iron radiators. One set of headers is 12 feet long the other pair is 24 feet long. It is acting as a single zone.
Originally there was a Honeywell L8124L triple aquastat doing the temperature control. The boiler is fired to about 98k btu/hr. There is around 550 sq. ft. of radiation and the calculated heat loss is 55k btu/hr at 0f. All last winter it was set to the boiler manufacturer spec of 160f low and 180f high 20 diff. The book calls for a minimum return temperature of 135f, it never gets there. I recently changed the Honeywell for a Hydrolevel 3250+ controller. I expect the boiler has been subjected to low return temperature its whole life.
When I took out the coal boiler, I installed a bypass in hopes of raising the return temperature into the boiler. The circulator is on the supply side and the bypass is tee'd off the system side of the pump to the return. I can open open the bypass so that the boiler return never goes below 115f but when I do I don't get any heat at the far end of the house and lose heat in areas randomly, any opening of the recirc impacts the heat to the far end of the house. I tried for a 20f temperature rise but that was even worse.
According to the table in this link by @Steamhead my pump is the right one for the job. I don't think the table allows for any bypass flow.
https://heatinghelp.com/systems-help-center/sizing-circulators-for-old-gravity-hot-water-heating-systems/
I wonder if changing the pump for a higher flow rate model would help me get the return temperature up and allow more flow to the radiators with the bypass open. Would I have enough hot water in the boiler to accommodate the higher total flow?
Here are some pics of what I found inside the boiler after I opened it to clean it. It's been 13 months since the last cleaning. It looked similar when I last opened it. I found some sulfur on the leading edge of the tubes. The pics make it look worse than it was. There was no soot to speak of. The steel still looks in good shape for a 22 year old boiler.






The boiler:


Some piping pics:




I put an hour meter across the burner contacts so I can measure the burner on time and calculate the oil used.
Outside temperature was 46f for both of the trials I did.
At 160/180/10 and recirc set to allow 115f minimum return temp. The call for heat was 52 minutes. Oil used was 0.74 gallons. Boiler temp was 164 and return was 129 when the thermostat was satisfied. There was a 1f overshoot. Much of the time was spent with the circulator off reheating the boiler water and a part of the house was not getting heat due to the open circulator.
From cold start, recirc closed and using the 3250+ condensation protection. The circulator doesn't come on unless the boiler temperature is above 125f and shuts off at 115f for a few times then the pump runs continuously until the call for heat ends. The call for heat was 24 minutes. Oil used was 0.41 gallons. Boiler temp was 117 and return was 86 when the thermostat was satisfied. While my wallet likes the result but I'm afraid to leave it like that so I put it back to 160/180/10 with recirc closed so the whole house gets heat. The boiler is still going to see low return return temp but the whole house will be warm.
I did some hacking, added burner on and circulator on LEDS, the hour meter and I took the thermister out of a wireless temp sensor, added some wire to it. The thermister is taped to the return line a foot or so after the bypass and covered with insulation. I have a security camera pointed at them and a short video clip is recorded every time the LEDS turn on and off. Then I record the data in a spreadsheet.

One of the video clips, circulator going off on low boiler temp.
https://i.imgur.com/dbvSTzQ.mp4
An aside...Last winter I tried a Grundfos UPS 15-58FC, check valve removed. It's claimed to be equivalent to NRF-22 at high speed. It's not. I lost heat at the far end of house and ended up changing back to the original NRF-22 circulator. So it seems I'm on the edge of required flow at the best of times.

mattmia2

Are there orifice plates in the radiators?

A thermostatic bypass might be what you need instead of a static bypass.

imaddicted2u

Can I tell if there are orfices without taking things apart? I hate to be taking things apart on this old system. This is what the valves look like.

I thought about thermostatic valve but if the pump is too small, wouldn't I still lose heat at the far end of the house when it opened?

imaddicted2u

If the bypass is cracked any more than this...heat at the far end of the house is lost.

hot_rod

imaddicted2u said:

If the bypass is cracked any more than this...heat at the far end of the house is lost.

Not a great position to operate a ball valve, 50% is about as far as you want to regulate them.

Below that it has a bad passage way as you flow across the two sharp edges of the ball inside. Noise, turbulence, and possible cavitation at higher flow rates

imaddicted2u

hot_rod said:

imaddicted2u said:

If the bypass is cracked any more than this...heat at the far end of the house is lost.

Not a great position to operate a ball valve, 50% is about as far as you want to regulate them.

Below that it has a bad passage way as you flow across the two sharp edges of the ball inside. Noise, turbulence, and possible cavitation at higher flow rates

You are correct about ball valves for control. I'd really like to be able to open it further so I can raise the return temperature to avoid condensing of flue gas but when I open it any further I lose heat to the far end of the house. All rads that come off the end of the 24 foot header stay cold.

Kickstand55

Insulation, Insulation, Insulation for starters. Lots of radiation via pipe size there and if the basement is cold, even worse.
Next. Re-pipe the system to a primary secondary with temperature sensing circulators that speed up and slow down on water temp. rise and fall. Said this way for a better understanding for the lay person.
If it's a cold start boiler, you'll need a high firing rate to heat quickly. If it's down fired, it takes much longer.
Be sure to insulate ALL the pipes and floor area in the basement. Check for drafts too and seal
them.

hot_rod

Basically your radiators are over powering your boiler. If it runs 52 minutes and the return is below 130, you will not pump your way out of this. Regardless or what method you use to assure return protection thermostatic, variable speed pumping, on off circulation, the
result will be the same. It will limit the heat out to the system to prioritize the boilers return temperature.
So you need to just sneak up on the load, while keeping the boiler running healthy.

Any idea how the radiation matches up with the boiler actual output.

Getting all the heat to the radiators with pipe insulation will help some. Lowering the load in the rooms would be a big help.

EBEBRATT-Ed

The radiators control the temperature of the water not the boiler. Any unused rooms you can cut back on heat? Or reduce heat loss as @hot_rod said

hot_rod

Here is a visual of hydraulic equilibrium 
Notice how the heat emitters dictate the return temperature at the boiler

pedmec

You need to balance the flow. Changing circulators just changes the pressure differential between radiator supply and return. Try throttling the flow thru the closest radiators so your adding resistant to those radiators. This will force the water else to other radiators.