Help with circulator size

Edouard Langlois

My son recently purchased an older home this Spring which has a hot water boiler heating system with a circulator feeding six radiators on two floors (three on each floor) The home is 1100 square feet split evenly on two floors. When running the boiler this Spring it seemed to take an very long time for the radiators to heat up even to minimal temperatures. They were all bled and assumed full. Also the boiler exhaust seemed very hot and indicated to me that we are loosing significant energy up the chimney. The home has new siding and windows along with good attic insulation.

The piping feeds to the radiators appear to be roughly 2" with separate feed and return line loops. The circulator is a small Taco unit. I read your article on sizing circulators on old systems and was wondering if the circulator was sized incorrectly.

The boiler is about a 1980 vintage and is natural gas fired and I believe is about 100 MBH input.

Considering changing out the circulator to get more heat out of the boiler to the radiators.

Any help would be appreciated.

Bill de Jong

If your circulator is a 007 (or higher model #) , your problem is probably not your circulator. I would look at (or have someone look at)the piping arrangement, Air elimination devices,etc. The heat from your boiler stack is a normal occurrence. You could partially alleviate that with a new condensing boiler, buts sounds like you need to get the space heat on first.

Steamhead (in transit)

Is this a little rowhouse

or "townhouse" as they're called in places other than Baltimore? With a house that small having 2-inch hot-water heating mains, this is certainly a system that used to circulate by gravity. They don't make gravity boilers anymore, but on this type of system you only need a circulator to move the water thru the new boiler. The rest of the system will circulate by itself. That boiler probably has a Taco 007 circulator on it, it seems to be the standard one supplied with packaged boilers.

Many of these little 2-story rowhouses that I see with converted gravity systems have less than 320 square feet EDR of radiation. Never mind the house's heat loss or the boiler size, these don't matter when sizing circs to old gravity systems- just check the amount of connected radiation to see if your son's falls in this range. Don't forget to add a bit for rooms that may have had radiators removed (most often in bathrooms or kitchens, remodelers don't seem to know what else to do with radiators- shame on them).

Also, if you have an infrared thermometer, you can check to see if there is any temperature difference (delta-T in engineerese) between the supply and return mains when the system is running. At minimum this delta-T should be 15 degrees or so. 20 is better. 30 might be too much.

If there is little or no delta-T when the system is running and all rad supply valves are open and all rads bled, the circ is too big. With 320 square feet EDR or less I'd use a Bell & Gossett NRF-9F/LW circ. This will fit onto the Taco 007's mountings perfectly, and move the water at a rate that will approximate the maximum flow that would occur under gravity circulation with an old-style boiler. If there's more than 320 square feet EDR but less than 560 or so, I'd use a Grundfos UP-15-42-F. This unit's flanges are positioned at right angles to the Taco but it should still fit.

On the other hand, if the delta-T is much more than expected, the Taco 007 your son now has might have quit working. To confirm this, check the thermometer on the boiler- if it rises quickly to 180 or so but the rads are still lukewarm, there's no pumped flow going on. In that case you'd get some gravity flow but not enough to properly heat the rads. The cure here is to replace the circ if it's getting power when the boiler comes on, again with a properly-sized one. If it's not getting power, the relay is probably bad.

BTW, that boiler is probably oversized. This can be confirmed by doing a heat-loss calculation on the house, but that's a job for another day.

If you're at all leery about this, the best bet is to hire a pro. Try the Find a Professional page of this site. If your son really does have a Baltimore rowhouse, contact me!