Best Tstat and boiler settings to use on a mod con?

BobAlu

I have a question about the best way to set up my system and perhaps there are too many settings to make and they work against each other. A little about what I have here...a Vitodens 200 with an indirect HW heater and 2 heating zones. I use two programmable thermostats for the heating zones and I wonder what is the best way to set them? On the Vitodens I have the normal room temp set at 72 degrees (my wife likes comfort over economy!) The thermostats are set at 70 and drop back to 65 at night. Would I be better leaving the stats at one temperature and let the Vitodens do it's thing and determine how it needs to run? The thermostats have their own memory settings and they "remember" how long it takes to reach temp in the morning, so they set what time they need to call for heat. Would I be better off not using any type of night set back on the thermostats and just control everything with the Vitodens? I think I've reached a point of all these devices having their own computer settings and then they fight each other to control the system. I know Viessmann makes a thermostat, but I have two heating zones and using two of these t stats just complicates things! Any recommendations? What would be the best way to set up what I have here? I'm happy with my present setup, just wondering what the best settings would be. National Grid visited me a few times saying that I should be using more gas than I do to heat my home! I guess they think I'm cheating them! They push to get efficient systems and when I do, they complain that I'm not using the "normal" amount of gas! Go figure!

icesailor

Settings:

Its my understand and experience that set back thermostats and outdoor re-set thermostats don't go well together. Ste-back thermostats really just manipulate the outside temperature by making the outside temperature to falsely go up and down. Reset boiler controls only sense the actual outside temperature.

When the thermostat turns down, it is like raising the outside temperature. When the thermostat comes back up, it is like lowering the outside temperature. If the outdoor sensor  senses 30 degrees outside, and you have been running the thermostat at 60 degrees of set back, and you move it up to 70 degrees, it is like it is 20 degrees outside but the sensor doesn't know it. This excludes the re-set ratio.

If you set your ratios so that you maintain the lower temperature, 60 degrees, the system will be hotter and then able to bring the rooms up more effectively.

In my opinion. (worth nothing)

Jean-David Beyer

Setbacks, reset, and mod-con boilers.

I have a mod-con boiler with outdoor reset. The controller allows me to have an indirect hot water heater on one thermostat input, and two heating zones, each with their own thermostat inputs. Each thermostat input has a unique priority and its own reset curve.



My highest priority input runs the indirect, and gives out 175F water no matter what the outside is doing.

My second priority runs my main heating zone, a radiant heated slab at grade.

My lowest priority is to heat my upstairs heating zone, which is small and heated by oversized baseboard.



There is, practically speaking, no setback and no reset for the indirect.



The radiant slab zone does not work well with setback, but I have adjusted the reset to provide barely enough heat to maintain the desired inside temperature. (Right now, it is 52F outside and it is putting 75F water into the slab; or it would be, but the thermostat is not calling for heat.) In very cold weather, the reset is so close that the boiler will run 18 hours straight sometimes. The thermometer thinks the desired temperature has been achieved, but the hysteresis in the thermostat has not quite been overcome. I.e., my reset curve is almost perfect at the cold end. At the high end, I cannot quite manage that because the boiler will not modulate down far enough and so it goes on and off.



In the baseboard zone, I use 4F setback at night. Here, too, the reset curve is pretty close and in the morning it would not recover in an acceptably short time (say 2 hours). The fancy setback thermometer is too dumb to start the recovery soon enough. It is supposed to learn from experience, and it sort-of does, but it seems unwilling to start more than an hour sooner than the time I set it to be recovered. My boiler has a "boost" feature. I have enabled it this year for the baseboard zone. What it does is when it receives a call for heat, it starts a timer and if the thermostat is not satisfied in the time specified (I picked 2 hours), it raises the supply temperature 10F over what the reset curve is requesting. If after another two hours, if the thermostat is still not satisfied, it raises the supply another 10F, etc., until the thermostat is satisfied. In any case it will not raise it any higher than the maximum temperature specified for that reset curve. If at any time the thermostat is satisfied, the controller goes back to the temperature specified by the reset curve. It seems to be working well, but it has not gotten really cold so far this year: the record so far is 30.6F according to my cheap Radio Shack indoor-outdoor thermometer.



I do not know anything about Veissman boilers, but aside from the features and ease of programming them, they may be pretty much the same. I am not discussing quality here as I have no basis for comparison (I am a homeowner, not a heating specialist).



So as to recommendations.



1.) If you have a radiant slab, you are almost certainly better off using no setback; it would take me 8 hours or more to recover even partially from that, and 24 hours for everything to stabilize.



2.) If you have fast recovery heat emitters, such as baseboard that is large enough, you could probably use quite a bit of setback. I am getitng away with 4F setback, but I may have to reduce it to 3F when it gets really cold. I am not sure the savings are enough to justify the bother. I would normally be putting 110F water into my baseboard this morning but after two hours, the controller boosted it to 120F. That is all very nice, but with the higher temperature, I get less condensing. Do I save more by the setback at night than I lose by condensing less during recovery? I do not know, but guess it is very close. I certainly would not pay extra for that boost feature, but it comes with the boiler, so I figured, after two winters, that I might as well try it.



My suggestion is to adjust your setback curve(s) as I did, to make the curve just barely higher than the heat loss. This took me a couple of years to get it really really close. I tried just calculating it first, and that worked, but it showed my heat loss calculations were not accurate enough to come up with really good settings. The reason it took so long to get the settings is that I could not adjust the outdoor temperatures to get what I needed at each end of the curve, so I had to wait until nature got around to it. And I really needed to switch back and forth for each end of the curve several times. It was quite interesting doing that, but If I had to pay for a service call from my heating contractor about a dozen times, that would be really impractical. My installing contractor did not really know anything about reset and did not want to bother hooking it up, saying it did not make much difference. He used factory defaults for everything and that would have been  way way off.

R Mannino

Me Personally

I set the "dumb" thermostat at a single temperature setting and let the boiler control do the work (setback). In the case of the Vitotronic it requires a little programming in the control, which I'd need the manual for myself. Sure it can be a little challenging, but that is the power that the control has (knowledge is power, right?).



How are those gas bills treating you Bob? Sounds like pretty good.

CMadatMe

Let the Vitodens

Do its thing. Set the thermostat at the temp you want and leave it alone. If you want night time or even daytime set back make sure you program the boiler the same as the thermostat. You can run daytime and nighttime set points on the boiler. If you do set back I would not go deeper then 3 degrees.



Using Vitotrols will give the boiler indoor temp feedback. Now the boiler is looking at outdoor temp, indoor temp and LLH temp. The boiler will adjust LLH temp to ensure proper water temp heading out to the zones as solar gain or another source of indoor heat influence indoor temp.



Use the boiler control that you paid for to provide the comfort. A thermostat is nothing more then a switch to tell something to come on or go off.