ODR curve question

Brad Barbeau

I'm interested in opinions about how to set the curve properly for my system.  It's been working just fine and we're very happy (more here: <a href="http://www.heatinghelp.com/forum-thread/135093/My-new-Ultra">http://www.heatinghelp.com/forum-thread/135093/My-new-Ultra</a>), I just want to be sure that I've set this properly. 

I've been tweaking the Ultra's settings trying to get it to run as long as possible, I think I've finally got that working right. 

The Ultra doesn't seem to give a great deal of conrol over the curve directly but I have set this: At 70F outside the water temp is 70F (seems reasonable enough to me).  At the other end, -10F outside, water temp is 135F.  I had designed the rads for 160 actually but when I had it set at 160 it cycled quite a lot.  There have been a few cold days when it has approached -10 but the things have worked just fine at those temps. 

My other question is: my delta t is pretty small most of the time, about 5 degrees.  I thought 20 degrees was normal.  Is that a problem or does it effect efficiency?  I have the pumps on low speed too. 

Thanks

Jean-David Beyer

How I did mine...

I am not a heating professional, but I do have an Ultra 3 boiler with outdoor reset (and indirect hot water heater). I have a radiant heat zone and a baseboard zone, and I run each one with its own reset curve. I will describe only the baseboard zone, because it is likely to be more similar to your radiators than my radiant slab is.



First of all, I had 14 feet of Slant/Fin installed in each of the two rooms in that zone, which is so that I could use lower temperature water and still get enough heat. I wanted lower temperatures so I could get more time in condensing mode.  Each room has about 3200 BTU/hr heat loss at 0F outside, but design day here is 14F. Now the default settings for that are 130F at 70F, and 180F at 0F. I have calculated that 135F would be enough when it is 0F outside. I set my thermostat to 69F; I do two degree setback there at night, when I am not in that zone.



I did not go with the defaults, but set the warm end to 70F at 70F outside and 135F at 14F outside. This was not ideal, so once a day, I would set one end of the reset curve depending on whether it was "warm" out or "cold" out. The upstairs zone was easier because it responds quicker than the slab zone. When all was said and done, I could get long periods of operation, but the boiler cycled pretty quickly (2 minutes on, 2 minutes off -- I am guessing, but something like that). The temperature drop ended up about 1F on warm days. I raised the minimum temperature to 110F so as to increase the temperature drop, and that helped raise the temperature drop, but it is still only a couple of degrees when it is warm out. I let the temperature stay at 110F until it gets down to 52F outside. I increased the limits from +|- 5F to +7|-8F and this reduced rapid cycling somewhat more. I then reduced the maximum firing rate for this zone from 94% to 55%, and now it typically cycles 5 minutes on, 10 minutes off except on very warm days. The boiler is definitely oversized when heating only the one zone.



Now to get it to run longer, I flattened the curve by making it hit 135F at lower and lower outside temperatures, but just able to keep the rooms warm enough on the coldest days. So now it does not hit 135F until it gets down to 6F outside. So the heat can run several hours to recover from 2F setback on cold days.



I did the same thing with the radiant zone. That can run 12 to 18 hours a day when it is very cold out, but less when it is warm out. Winter seems to be nearly over here in New Jersey, and it has run only 4 hours downstairs so far today, and is unlikely to run anymore today (58F outside right now).



My delta Ts are quite low. I have Taco 007-IFC circulators, and the house is small. I calculated the flow rate for the upstairs zone at about 2.5 gpm. I made wild guesses as to the tubing in the slab downstairs, and got something like 3 gpm, but I would not trust that number. In the baseboard zone, the temperature drop on warm days can be as low as 1F and on very cold days it can be almost 10. The downstairs zone is very difficult to measure because it takes about 24 hours for the slab to stabilize. Before it does, the return temperature reads low until the slab warms up, and that is many many hours. My guess is that it is perhaps 5F, but that is not a reliable number either, not because that is not what the thermometer says, but because it is constantly increasing. But while that is going on, the outdoor temperature changes. It really takes about 24 hours to make a measurement, and unless the outdoor temperature does not change in the meantime, I would not believe it.



I have decided not to worry about the low delta T values. I believe they are low, but I do not get rushing noises indicating flow over 4 feet per second, so it should not be hurting anything. If I replaced the 007s with 005s, it would slow the water down a little, probably increase delta T. But those motors draw about the same electricity, so it would not reduce costs much. An ECM circulator might help some, but since the system is zoned with circulators, I do not think it is worth the bother. Also, with lower flow rates, I would get less even heat distribution, at least in theory. I do not know how slow it would have to be to be a problem.

Brad Barbeau

Excellent

 It's helpful to know that you've used a similar process to me.  The defaults were not even close but had me wondering if I my temps were too low.

Thanks!

Jean-David Beyer

Temps too low...

In my non-professional opinion, you want to run at the lowest possible temperatures you can, consistent with getting enough heat at all times. This willlpermit your circulators to run for longer times, giving more even temperatures in the house, and less expansion and contraction noises. The disadvantage is that recovery from setbacks takes longer. But my main zone is a huge concrete slab, and setbacks there are not really practical. In my baseboard zone, it takes a couple of hours to recover from a 2F setback, so the temperatures there vary from 67F to 69F with the thermostat set at 69F. For me, that is acceptable. If it were not, I would use only a 1F setback there. If that thermostat would not do setbacks, I would not bother to replace it with one that did.



I violate this slightly when it is very warm out. This is because, even though I have the smallest mod|con in Weil McLain's product line, it is somewhat too big for my house. Consequently, it will not modulate down far enough.



For my downstairs zone, when it is in the 60s outside, I have it supply 75F water even though lower temperature water would be enough. Actually, I do not need 75F water until it is 52F outside. I run it hotter so the boiler does not cycle so fast. Raising the minimum temperature allows the heat to leave the water and into the house a bit quicker.



Similarly, for my upstairs zone, I supply 110F water until it gets down to 52F outside. Here too, the boiler cycles too fast and I had to do several things to make it cycle slower, or stay in the modulation range. Otherwise, I could let it get down to about 80F. The load up there is at the very worst about 6500 BTU/hour and the boiler will go down only to 16,000 BTU/hour. So if the upstairs is the only zone calling for heat, it will not modulate low enough and goes into on-off mode. This results in satisfying the thermostat sooner (not actually desired). I also reduced the maximum firing rate when heating the upstairs zone from 94% (default) to 55%. This is because the PID servo in the control board is underdamped and runs too quickly up to the maximum firing rate. By the time it gets to slow down the firing, the boiler temperature gets up to the upper limit and shuts down. Lowering the firing rate allows the servo to keep up with what the boiler is doing. I also increased the spacing of the limits. It was either that, or putting in an extra tank and circulator, and I did not want to do that.

Brad Barbeau

Run times

I've got mine so that the boiler runs probably 23 hours a day, longer on cold days and less when warmer.  The solar gains are large, so often the boiler shuts off in the afternoon which is fine by me. 

I think mine is sized okay, the low end is about 30,000 BTU and I guess if it runs at that much of the time (it's only hit the modulation off limit once or twice that I've seen) it's okay.  I'll be adding at least another 50,000 BTU in another part of the house to it this summer, I'll be interested to see how it handles that. 

At any rate, I'm very happy with it - it's quiet and seemingly effecient.  Only regret is that I should have gone with valves and an ECM pump instead of the regular pumps.  Something to do in the future!  In the meantime, my wife thinks I spend too much time watching it though!