Pipe Sizing to CI rads and BTU's

Brad White_194

1) The best way to find out the potential output? Several sources, among them Dan's "EDR" book. There are also on-line sources which are more general but may be close enough for what you are doing. I assume that your building heat loss is accurately calculated, correct?

As for the variable output, it is usually based on the average water temperature [(Supply + Return)/2].

An easy guide is that each EDR (square foot) of radiation in a 70 degree room emits 190 BTU's per hour (BTUH) with 190 degree water.

(I mention this because the average temperature and output are the same 190 number, kind of handy...)

At 170 degree average water, the output drops to 150 BTUH... from this you can see that the output drops two BTUH for every degree drop in average water temperature. Continue this downward and your radiation output drops to 90 BTUH per SF at 140 average water temperature.

Now, for pipe size, don't get too hung up on branch flow to any one radiator. 1/2" or 5/8" PEX or PEX-AL-PEX will handle 15-20,000 BTUH at a 20 degree drop and twice that with a 40 degree drop. I did our house with 5/8" PAP even using one such run to risers serving two radiators. Plenty of heat.

Flow is over-rated as a factor and is quite forgiving if you fall a little short.

A given radiator, say 10,000 BTUH with 1.0 GPM flowing through it will still give 9,000 BTUH with half the flow. The the average water temperature if initially 150 degrees (160 in/140 out) drops to only 140 degrees (160 in/120 out).

So, if your flow is a tad short, you will not likely notice.

Conversely, if you are short on heat, doubling the flow will give you barely another 10 percent output.

Jason Horner_3

Pipe sizing, CI rads & BTU's

Need a little help:

Have a 30-year old atmospheric copper fin tube boiler 250K/200k in/out that needs replacing. Currently the supply is set at 160F and the return comes back at about 140F but even then the boiler short cycles and the house is plenty warm even on the coldest days with the wind howling. Am looking to use a Buderus GB142 or Viessmann Vitodens 200 (with appropriate water chemistry) as a replacement, with supply/return temps as low as I can make them go.

House is 1915 vintage 3-story with CI rads. Distribution piping in the basement is iron dropped down from the ceiling at least a foot and I want that ceiling height back. Want to replace the iron pipe with pex or copper placed inside the ceiling joist cavities. Supply/returns on the rads range from 1"-2" O.D.

All the rads are on one circuit (no primary/secondary piping) and the whole thing is circulated by one Taco 007. There are 12 rads on the upper floors tee'd off from five supply nipples on the manifold at the boiler. The basement is currently heated only by radiant losses from the piping, but this will be changed too.

I'm in the process of significantly upgrading the insulation in the house and expect the final Btu load to be in the 150k or a bit less range depending on how much insulation I can get into the walls in some tough to insulate areas.

So my questions are this:

a) Best source to find out the BTU output of the various size/style rads installed in the house, and a way calculate the BTU output based on various supply temps to the rads?

2) Where can I find heat carrying capacity of 1/2", 3/4", 1" Pex and copper at different GPM flow rates and temperatures so I can understand if I can deliver enough BTU's to each rad with new smaller diameter piping?

I want to zone the house using manifolds when I replace the distribution piping (ie. 1st floor, 2nd floor, 3rd, floor, basement), or use individual TRV's on each rad, but preferably zone with thermostatically controlled actuators. I'm hoping that I can use 1/2" or 3/4" Pex or Kitec to do the job.

Thanks.