Baseboard Output At Low Temps

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Baseboard output is not linear with temperature. If you have a linear reset curve, and the temperature is right for the coldest weather, you will not get much heat out of the baseboard in warmer weather.

As an example, let's say you need 150F water when it is -10F outdoors (a 1.0 slope). A linear interpolation will tell you that you should need 90F water when it is 50F outside. Wrong...you will need warmer water, maybe around 110F.

The baseboard curve is discussed in documents you can find on Tekmar's web page. Tekmar controllers use "characterized heating curves" instead of linear curves.

I don't know if the WM will do nonlinear curves, but if not, you'll have to raise the room-temperature end of the WM's reset curve. Instead of 70F water at 70F outdoor temperature, you will need to try maybe 90F water at 70F outdoor temperature.

If you make the curve steeper as suggested in the previous post, that will work too but you won't condense on colder days because the water will be too hot then. Better to raise the room-temperature end of the curve; the water will then be too hot in warmer weather, but that's no big deal if you have a thermostat turning the boiler off. (Of course, I am assuming that the WM will allow you to shift the low-temp end of the curve. The WM firmware was created by clueless morons---18F water temperature boost every 10 minutes???- so maybe it doesn't allow this.)

Steve Rockwell_2

Do they do any good at 84*?

After changing the parameters on my new Ultra 155 to match the conditions here in Grand Rapids, the boiler setpoint today is 84*, on a day with outdoor temps of 52*.

I have a mix of baseboard and cast iron radiators, though mostly baseboard.

Even when the boiler is operating, the baseboards seem to be room temp, not the slightest bit warm. Are they doing anything to keep the house warm at this low temperature? The Slant Fin heatloss program only goes down to water temps of 140*, as does their website's baseboard specification sheet. Does anyone have any info on the output of baseboard at temps below 100*?

Obviously I will let the system run this way for the day and see how it does, but being the impatient type, I would love to hear some opinions on the subject.

Thanks.

KCA_2

low temp BB

I've used baseboard on an injection system before.. It did work well but I had to rum the system on about a 1.5 or 2 heating curve.

The baseboard manufacturer had BTU output down to about 140deg and I had to interpalate (sp) the lower temps.

The BTU output though is not only a factor of temp but of GPM.

:-) Ken

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Steve Rockwell_2

Still On Track

Despite the fact that the supply side of the system is essentially luke warm to the touch, and that the system calculates a setpoint in the upper 80's for outdoor temps in the low 50's, the house continues to be dead on in matching the 68* thermostat setting, after running all day. With the heat turned off, the house would have dropped several degrees in this time period, so I know it's not "coasting." Go figure.

I think you're right about bumping the temps at the low end of the range Chuckles - that's what I assumed I would have to do to keep the house warm if the settings I put in this didn't do it. But the Ultra is making it work as is.

As for the boost feature, at least the "morons" allow me to defeat that feature! I am not a fan of that concept either, so I did disable it. I thought having short run times defeated the concept of a condenser in the first place, since most say that long runs mean better efficiency.

So far, the Ultra is one slick machine.

Mike T., Swampeast MO

If the water exiting the baseboard is cooler than the supply, then it is giving off heat.

Fin baseboard is linear in a way... Look at the "typical" output table as published by Burnham and you'll see that a 20° increase in supply temperature always results in an increase of 130 btu/hr. If you interpolate from that data, you'll find that at 90° it has an output of NEGATIVE 15 btu/hr. Obviously that's impossible.

Fin baseboard is not however linear if you look at output as a function of output vs (emitter temp - air temp). For example: At 180° supply and 65° incoming air you get 4.8 btu/hr per degree of temperature difference; at 140° supply and 65° incoming air you get 3.9 btu/hr per degree of temperature difference.

Note that the standard output tables for standing iron have a similar problem. The calculated output "runs out" well before it does in reality. Also, output per degree of temperature differences changes, but not to the same degree as fin baseboard.

Your boiler is doing just what it's designed to do--match output to loss as closely as possible.

The Vitodens just installed in my house has temperatures quite similar to yours, even though mine is a gravity conversion system with standing iron. With outside temp in the low 50s, measured supply temp runs right around 83°.