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Can I add an additional hydronic radiator?

Bucky Member Posts: 47
Hi gang, it's been a while! I have an oil boiler that feeds four different zones in my house: Radiant slab/underfloor in two room; an "aux" heating coil in my mini-split system (first floor only, overlaps both other heating systems, but was a trade-off that was somewhat necessary); an indirect DHW tank (on "Priority"); and the traditional radiator-based system that services the rest of the house with the exception of the two rooms with radiant floor. It's all plumbed primary/secondary with each zone being its own set of primary/secondary pipes (thanks "Pumping Away!") Psshew!

Anyhow, it's a very old (1790's) house with a few newer additions. The radiator part is the old part of the house. The third floor, which is now the master suite, started as two small single bedrooms, each with their own ~2640BTU radiator. At some point, a dormer was added that converted the space to a single bedroom and master bath and small "walk-in" (I use that term loosely!) closet. The bath and closet are on both sides- i.e. not within the dormer space. BUT these two spaces maintain the only two radiators on the floor.

SO, needless to say, the bedroom (dormer) part up there can get pretty chilly, while the bath and closet are very toasty. So my latest project is to think how to add an additional radiator in the bedroom. and I'm not sure how anal I need to be with my calculations and sizing based on a ~2640BTU add to this system.

The other radiator components to this system are five ~5K BTU full-sized rads, and about 48" of 8" steel baseboard (haven't figured out the load for that yet- guessing around 2K- but I can't believe in 18 years I just now realized that someone relieved this uni of its convection qualities by vertically tiling the bottom 4" of the entire run!!!)

All BTU ratings are based on a 155° average temp (boiler is set to 160°, guessing 10 degree delta T, but will investigate further). Radiator ratings are based off the charts found here https://coalpail.com/coal-forum/viewtopic.php?f=71&t=3401 and here http://photobucket.com/gallery/http://s1043.photobucket.com/user/TheAgent24/media/ToHeating/sqft.jpg.html .

So all in all about 36K BTU of radiators. Does it require much more analysis to add another 2-3K of radiator?

And I'm planning to loop it off of one of the existing 2640K BTU radiators on the third floor. Both are fed with 3/4 in pipes. One is basically directly above the basement "manifold" area, so no more than 20' of pipe to get to that rad. In that scenario, the added rad would be probably 6-8' off of the first rad (and I could also easily downsize the existing rad, if absolutely necessary). The other, which is of course the more convenient side to place the radiator, has a fairly long run: It splits off the end of the 'trunk' in the basement (about 40'), then up to 3 on the rear side of the house (another 20'), then under the floor to the radiator at the front of the room (another 15'). Of course that one is the longest run in the house!! In that 2nd scenario, the run to the added rad would only be about 3' (thru to the other side of the wall).

So my inflection points here:

1- Should I worry too much about another ~3K being added to the system?
2- Should I worry about a single 3/4" pipe feeding an additional rad?
3- Should I worry too much about pump head in the 2nd scenario where I'd be teeing off of the longest run? (Prior to re-working the system with primary/secondary, that existing rad got very little flow, but now it really gets hot). Not really sure how to account for friction loss in a standard cast-iron rad.

Thanks all!!


  • Gordy
    Gordy Member Posts: 9,546
    No to 1,2, and 3 depends, but I think it would be fine.

    3/4 inch copper will carry about 45 k.

    The system pump only needs to overcome the head of the highest head run.
  • Bucky
    Bucky Member Posts: 47
    Thanks Gordy! Making me feel better about my aspirations now...
    EBEBRATT-Ed Member Posts: 15,635
    3/4 pipe ot tubing will supply 4500 btu/hr with a 20 degree td