New Vitodens 100 - dialing in the reset curve

csnyder

We have a 100-year-old house with big cast-iron radiators. We recently had the original gravity system converted to pump-driven, replacing the old boiler with a new Viessman Vitodens 100 with the combi DHW add-on. The heating system is on a single zone, with a Viessman low loss header.



We have the outdoor sensor installed to give us outdoor reset, with the indoor thermostat (basic $30 universal stat) serving as a high temperature limit switch for the living space. So far, we've been working to balance the system by adjusting the valves at the individual radiators (as part of the project, I had all of the radiators moved out to be powder-coated, and the contractor installed new valves when the radiators were reinstalled). Everything is working a treat, but I'm having some difficulty dialing in the reset curve to run continuously while maintaining a stable temperature at all outdoor temperatures.



The first problem is that the room thermostat is in the living room, which is the warmest room in the house; the windows are among the best-sealed in the house, there are four radiators, and one of the radiators is just a few feet from the stat. To compensate, I've completely shut off two of the radiators (including the one close to the stat), and dialed another way back, so that the smallest one (in a bay window) is the only one at full flow. This has helped to balance the rest of the house tremendously.



The other issue is trying to find the appropriate reset curve. When the temperature was around 40 degrees outside, I had to bump the curve up to 3.5 or so to get the system to provide enough heat. However, when the outdoor temp went down to the mid-20's, I had to dial the curve down to 2.5 to prevent it from tripping the room stat and short-cycling.



Looking at the boiler manual, I think I understand the problem: The shape of the curve is too steep for my situation. Since the house is dramatically over-radiatored (it's my understanding that these old systems were originally designed to heat the building with the windows open for ventilation), we don't need a high water temperature to heat the house. However, we still need <em>some</em> heat when the outdoor temp climbs above freezing, which the built-in curve does not provide below 3 or so. It seems to me that the curve adjustment is better at coping with variations in building thermal loss rather than variations in system design.



I've seen the Como-OT mentioned on here as a good add-on to allow more fine-grained control over the Vitodens 100, and I'm considering purchasing one and ditching the dumb stat. I'd install the new unit in the dining room to get a truer read of the indoor temp than it'd get in the living room. I understand that the Como-OT overrides the curve settings on the boiler - does this mean that it actually substitutes its own curve, or does it simply dial-in the setting of the boiler's own curve? If the latter, it'd still be an improvement, but perhaps still not perfect for my situation.



Also, would the Como-OT be able to use the reading from the outdoor temp sensor connected to the boiler, or would I have to rewire the sensor to connect it directly to the Como-OT?



Thanks in advance for reading this, for your response, and for all of the other useful information on this site. My contractor is knowledgeable about these systems, but this site has enabled me to learn more beyond his expertise.

CMadatMe

Heat Loss

First, was a heat loss done? Second was the square foot of edr of the rads calculated then converted to btu/hr based on different water temps? If not, needs to be done. How would you know where to start the curve without it



As for the Como-OT. The outdoor sensor currently on the boiler is removed and hook up to the Como-OT. The Como is the in turn hooked up to terminals 1 and 2 on the same block the sensor is currently hooked to.



With the Como you will be able to change both the slope and shift of the reset curve. It will also provide indoor temp feedback back to the boiler if you utilize the feature.



Suggest your read and get that heat loss and rad measurement done.





http://www.kwe-tech.com/index.php?section=Products&subs=Controls&page=Como-OT.html

csnyder

Rad EDR: yes; Heat loss: no

Thanks for your suggestions.



The contractor did calculate the radiator surface area, but he did not do a heat loss calc (heretical, I know) - he claimed that measuring the radiators was enough. He has seemed very knowledgeable about both old gravity systems (he pointed out details - such as a smaller diameter pipe for a rad in a small bathroom to constrict the flow) as well as the new systems, and has visited Viessman's German facilities for training. However, I have come to suspect that he's prone to bluffing as well.



So far, I've been dialing in the curve by trial and error, and I've quickly discovered that the slope in the built-in curve in the Vitodens 100 is not appropriate for our situation - I've needed to adjust the curve setting down as the outdoor temperature has dropped.



After posting yesterday, I went back and read the Como-OT manual again (I'd already read it a couple of times). The manual does seem to suggest that the outdoor sensor can be connected to either the boiler or Como-OT; I guess I'll start with leaving it connected to the boiler, and see if it works - easy enough to change it if needed. I also was reminded of the settings that the Como-OT has for slope, as you also confirmed.



Thanks again for your help. This is my first winter with radiator heat (last house had a "cold 70" forced air system), and I'm enjoying the chance to play with this new toy. I am pleased that both Viessman and my contractor have made it easy for me to understand and tweak the system myself.

CMadatMe

Wouldn't Recommend

You leave it hooked to the boiler. You really want it hooked to the Como.

csnyder

Reason?

What's the reason for your recommendation? Assuming the Como is able to get the outdoor temp either way (as the manual seems to imply), wouldn't everything operate the same? Am I missing something?

csnyder

How smart is the Como?

Got the Como-OT installed - it's working great so far. I left the outdoor sensor hooked up to the boiler, and the Como has no problem getting the reading from the boiler over OpenTherm. It isn't dialed in yet - with the stat set at 68, yesterday morning the indoor temp overshot way up to 75, while this morning it didn't reach 68 until late morning; an adjustment of the curve slope and indoor sensor influence made the difference.



The Como manual is fairly detailed, but one thing I'm not clear on is how much "intelligence" it contains - apart from the documented automatic adjustments (such as the heating slope adaptation mode), does the Como "learn" about the heating system over time, or does it solely use the manual settings?



Basically, should I keep tweaking the Como's settings, or should I let it figure things out on its own for a couple of weeks?