Rethinking the BTU Rating of My Hot Water Baseboards

I used to (completely naively and totally mistakenly as it turns out) consider that my hot water baseboards were delivering something on the order of 550 BTU's per foot into my home. But that was before I began monitoring my supply and return temperatures, whereby I finally came (kicking and screaming at none other than my own self) to the realization that the average temperature of the water traveling through my various zone loops must in fact be midway between the monitored supply and return temperatures.

Now (after monitoring supply and returns and actually thinking it through instead of guessing) I believe that on average my baseboards are honestly delivering much closer to only somewhere between 350 and 400 BTU's per foot. Call it 375 BTU's per foot of baseboard. Fully 1/3 less than I had previously believed.

This actually brings both the total footage and the BTU delivery potential of my baseboards closely in line with my homes roughly 46,000 BTU heat loss calculation, meaning that whomever calculated them for the home initially did so correctly, and it was dummy me who (for years no less) had way over-rated their BTU's. Until now....

lsayre wrote:Now (after monitoring supply and returns and actually thinking it through instead of guessing) I believe that on average my baseboards are honestly delivering much closer to only somewhere between 350 and 400 BTU's per foot. Call it 375 BTU's per foot of baseboard. Fully 1/3 less than I had previously believed.

So you arrived at the BTU output of the baseboards by looking at temp difference between send and return water? uhhh, what about water volume? How did you measure that? Wouldn't precise volume need to be known to come up with BTU out put of the baseboards?

Yes, you should use an average temperature for baseboard sizing rather than the boiler high limit setting. For one thing, the boiler operates within a 10 degree high limit differential (usually), and the radiation is usually sized with a 20 degree delta T in mind. 170 degrees is a common figure...see where that lands in the output chart for your baseboards.

The old IBR books have a nice way of guiding you through the sizing process. Basically you determine the heat loss of the house, room by room. The room with the most heat loss heat per foot of exterior wall length can be used to determine the average temperature required from the boiler. e.g. if a room has a heat loss of 5,000 btus/hr and 10 feet of exterior wall, you should be able to mount 10 feet of baseboard and heat the room with an average water temperature of 170 degrees. If that same room had a heat loss of 6,000 btu's/hr, you would either need to use 190 degree water or add baseboard to an interior wall.

Heating guys like to use lots of baseboard, because then the system can be run at a lower temperature and theoretically have lower fuel consumption. It also leaves some headroom for unusually cold weather.

Lightning wrote:So you arrived at the BTU output of the baseboards by looking at temp difference between send and return water? uhhh, what about water volume? How did you measure that? Wouldn't precise volume need to be known to come up with BTU out put of the baseboards?

The baseboards only care about temperature to meet their rating. You need sufficient volume at design temperature to meet those needs. The rating itself is a measure of volume at a specific temperature.

Lightning wrote:So you arrived at the BTU output of the baseboards by looking at temp difference between send and return water? uhhh, what about water volume? How did you measure that? Wouldn't precise volume need to be known to come up with BTU out put of the baseboards?

Per SlantFin the total difference in rated BTU output per foot of their baseboards for 1 GPM of flow vs. 4 GPM of flow is only 20 BTU's at the nominal temperatures which my boiler is delivering through the baseboards. Thus an accurate determination of actual flow would not change my conclusion by very much. But based upon my head loss calculation and my pumps curves for its various speeds I can get fairly close to knowing the actual velocity of delivery, which varies as zone valves open and close, with this variation moderated (neutralized to some degree) by the presence of my DPBV.

Since I'm not hearing any "velocity noise" in my zones, I'm not likely exceeding 4 GPM of flow through any of them (thanks to the DPBV). And even at low speed and with all 4 loops having open zone valves (at which juncture my DBPV should be closed or at least nearly so), my overall flow should be exceeding 1 GPM in each loop.

My actual circulator flow into 8.4 feet of head is 5.8 GPM at low speed and 8.9 GPM at high speed. When one zone valve is open the DPBV is also (theoretically) wide open. The DPBV (which is dialed in to the pressure setting equivalent of 8.4 feet of head) then closes progressively as each additional zone valve opens, until it is (theoretically at least) closed when all 4 zone valves are open.

franco b wrote:The baseboards only care about temperature to meet their rating. You need sufficient volume at design temperature to meet those needs. The rating itself is a measure of volume at a specific temperature.

OK I understand that now.. But I'm still confused. If they already have a BTU rating, and they are given the right volume flow at the right temperature to output the rated BTU's, then why is it being questioned?

lsayre wrote:I believe that on average my baseboards are honestly delivering much closer to only somewhere between 350 and 400 BTU's per foot. Call it 375 BTU's per foot of baseboard. Fully 1/3 less than I had previously believed.

Sorry for the hydronics 101 questions, I look forward to installing a boiler at some point in the future.

For about 2 GPM of flow (give or take) 550 BTU's per foot of baseboard assumes an average water temperature of about 175 degrees. 375 BTU's per foot is closer to an average water temperature of about 148 degrees.

The following data is for the SlantFin Fine/Line 30 hot water baseboard. Most others are fairly close to these figures.

For 1 GPM of flow
-------------------------
140 degrees = 320 BTUH per foot
150 degrees = 380 BTUH per foot
160 degrees = 450 BTUH per foot
170 degrees = 510 BTUH per foot
180 degrees = 580 BTUH per foot

For 4 GPM of flow
--------------------------
140 degrees = 340 BTUH per foot
150 degrees = 400 BTUH per foot
160 degrees = 480 BTUH per foot
170 degrees = 540 BTUH per foot
180 degrees = 610 BTUH per foot

I did however completely forget that my surface mounted digital barbecue thermometer probes on the supply and return pipes both seem to consistently read about 5 degrees low, so correcting for that I'm probably closer to delivering around 420 BTUH per foot for much of my typical run cycle. Only near fear fan cut off do I break above 500 BTUH, hitting perhaps 510 to 540 at actual fan cut-off.

If I was designing (or retrofitting) a home around this hot water baseboard (or one similar to it as to its BTU ratings) I would conservatively assume 400 BTUH per foot of fintube to be on the safe side. Assuming 550 BTUH per foot (as I might have before thinking about this) would potentially be a big mistake, leaving one with insufficient footage of baseboards.

And here is how my silly mistake of "assuming" 550 BTU per foot of baseboard began. I have about 140 feet of hot water baseboards spanning 4 zones. Originally (due to some documentation I received when I purchased the home, and also to what is written upon the covering of my electrical resistance boiler, I had always assumed it was rated at 22.5 KWH, which equals 76,770 BTUH.

76,770 BTUH / 140 feet = 548.36 BTUH per foot of baseboards. (Thus my presumption of 550 BTUH per foot)

Subsequently (during its recent repair) I shockingly discovered that my resistance boiler is actually only 13.5 KWH, which equals 46,062 BTUH.

History has shown me that 46,000 BTUH is sufficient to keep the house warm even on the coldest of days, so I presume at present that my homes heat loss is actually near (or perhaps a tad below) this figure.

46,062 BTUH / 140 Feet = 329 BTUH per foot of baseboards. As long as I'm delivering this many BTU's to each foot of the baseboards my house should stay warm. It seems that I'm delivering at least 420 (and on up to 510-540), so life is good.

Last night I watched 3 zones open and knock my boilers temp way down to 145 degrees via the supply side "barbecue" receiver reading, but the digital barbecue thermometer was making it appear far worse than it actually was because the supply side of the barbecue thermometer was reading 10 degrees lower than my boilers temp/pressure gauge that probes the interior of the boiler proper (which was reading 155 degrees at its lowest point of descent). But by the time the temp came back up to boiler (fan) cut-off level the supply (outlet) side of the digital barbecue probe was only reading about 5 degrees lower than the gauge in the boiler tapping. Apparently surface mounted temperature monitoring with these type of transmitting barbecue thermometers has some seriously questionable issues with accuracy. Either that or the combination temperature and pressure gauge inside my boiler tapping is wacked out.

By inference I presume that the barbecue probe on the return (inlet) side also reads 10 degrees low at lower temperatures and 5 degrees low at higher temperatures. But perhaps its output is sending a worse deviation from the truth on the return side, as temps there are naturally lower than the supply side at all times, so the return temp error may be perhaps as much as 15-20 degrees to the low side at the lowest of my observed return temps for all I know. This observation makes trying to determine what is actually happening via the use of these surface mounted barbecue thermometers highly suspect.

In my quest for monitoring temperatures, I have found using an ir temp gun on non- reflective surfaces are by far the most accurate. Ie slightly rusted black iron fittings. I've tried externally using bbq temp probes on copper, pex, and iron and I found if you wrapped the pipe in insulation and sandwiched the probe you will get a closer reading. The ir gun varies with surface reflectivity. So I ended up adding a tee with a well type probe.

I have also found the temperature to vary significantly from the gauge on top of the AA vessel. I believe this is due to the location of the well, which is basically on top of the burn pot. And since I don't have my return ports tee'd it doesn't get immediate flow during circulation. But when there is no or low circulation, it sits on top of the burn pot, so its temperature varies from well and ir readings.

My ir gun marches the well probe if I hit the slightly rusted black iron pipe fitting just before it. I think the best way to do it externally would be to permanently and tightly wrap piping in Matt black construction paper in points that you wanted to know monitor and use an ir gun. Or for long term put a well type probe.

Larry, what exactly are you trying to get out of this exercise? Seems like the primary goal of keeping the house warm on the coldest day of the year gas been met...mission accomplished.

As for the variance between the gauges, Mike brought up some good points. Surface mounted thermometers aren't very accurate, and the temperature of the boiler water is not the same throughout the boiler when under load. My EFM is the same way, the gauge on the front reads lower than the aquastat in the rear, and the industrial grade Mercury thermometer in the supply riser usually reads highest by a few degrees.

How do you have the BBQ probes mounted? They need to be secured tightly against the pipe and wrapped with insulation. Otherwise the opposite side of the probe facing away from the pipe looses a few degrees to the room air.

Lightning wrote:How do you have the BBQ probes mounted? They need to be secured tightly against the pipe and wrapped with insulation. Otherwise the opposite side of the probe facing away from the pipe looses a few degrees to the room air.

I don't have them well insulated. All I did was place them against the pipe, wrap a paper towel around both the probes and the pipe, and then duct tape them down to the pipe. The paper towel barrier keeps the duct tape from making any direct contact with the pipe so I can easily remove the probes without leaving sticky duct tape residue behind anywhere. I didn't originally intend to keep them in place as long as I have.

Rob R. wrote:Larry, what exactly are you trying to get out of this exercise? Seems like the primary goal of keeping the house warm on the coldest day of the year gas been met...mission accomplished.

As for the variance between the gauges, Mike brought up some good points. Surface mounted thermometers aren't very accurate, and the temperature of the boiler water is not the same throughout the boiler when under load. My EFM is the same way, the gauge on the front reads lower than the aquastat in the rear, and the industrial grade Mercury thermometer in the supply riser usually reads highest by a few degrees.

The house is warm. I'm pretty close to terminating this experiment and removing the barbecue temp probes. Life was much more simple without them.

As for reflective surfaces and the IR type non-contact thermometers, just spray a spot of flat black spray paint where needed and you're good to go.