viessmann help

noah99

i have a viessmann vitodens 200 boiler for my basement and garage floor heat. I am trying to set the proper slope/shift for my heating needs. In Saskatchewan it is not uncommon to get -20 to -30 C temps in the winter . i have 5 individual thermostatic zones in my basement(all set for 20 C) as well as 1(9 C) in the garage. originally it was set for a slope of .6 and a shift of 0. it seemed to be fine as the thermostats would call for heat the individual rooms would warm up until satisfied then shut off. recently i had someone come out to look at another issue with my boiler and he changed the settings to a slope of .3 and a shift of +3. his reasoning is that all the zones should be continuously calling for heat. the temps on all the thermostats do stay around 20C however i now almost always have 1 or 2 zones open calling for heat constantly to maintain this temp for the different rooms.he said this is the proper setup on  how these boilers were designed to be used to just generate enough heat to maintain the room temp and not overshoot it and this will make the system more efficient(is this right).i cant understand how a boiler constantly trying to maintain a lower water temp to just satisfy the thermostats would be more efficient. any input in helping with this setup would be appreciated.

Thanks

CMadatMe

Heat Loss

What is the heat loss, size of boiler and what are the emitters? Sounds like he is trying to run costant circulation. One problem, works great with Vitorols provding indoor feedback and youn have emitters such as panel rads with thermostatics but no good with fine tube baseboard or hydro. Is this a WB2A or WB2B?

noah99

viessmann help

the boiler is a WB2 8-32 124000 BTU heats a 2500sq ft ICF basement and a 1000sq ft garage. the home also has a forced air furnace. the system is a glycol based radiant floor heat (pex lines). i just want to know a recommended starting point to set up the slope/shift for this type of zoned system based on our colder climate in Canada and i can try to tweak it from there

Paul Pollets

Curve

Start the curve at 1.0 for radiant heat embedded within the slab. If the tubing was installed at proper centers for the heat loss, I've found that I can go as low as .6 for highly insulated homes, to 1.2 for those not so tight. The goal is to see how low you can go on the curve, and still be comfortable at design temps.German technology is somewhat at odds with how we do it in N. America. The Vitodens is made to run without zone valves or thermostats. Stopping the flow of water in a system that is supposed to be on constant circulation causes room temp drop. It's less expensive to run the RFH at 65 degree setpoint, rather than bouncing the temps up and down. If a room needs to be zoned, the radiant loops should have telestats for zoning.

SpeyFitter

Your installer knows what he is talking about.

In a perfect world, a well designed hydronic heating system would be sized to provide exactly enough heat to counter the heat loss to maintain a set temperature as per your requirements in each zone as outdoor temperatures move up and down thanks to weather responsive controls (outdoor reset).  Your zones/loops would be balanced as such to distribute the boilers heat evenly and in theory you would not need thermostats. Your boiler would put out just enough heat to maintain the temperature, and as such your condensing boiler would see maximum efficiency since it'd be putting out the lowest water temperatures possible for maximum condensation of flue gas and recovery of latent heat transferred to the water.

Not only is it an efficiency measure, but it also helps things last longer. Your stainless steel heat exchanger will last longer if you keep it at a set temperature utilizing minimal burner input instead of expanding and contracting all the time through short cycling from high fire pulse loads from an improperly set up system. Your gas valve(s) and ignition components will generally last longer and suffer less wear and tear.  Pressure regulators on the gas train will tend to not wear and tear as much as the soft seal will not be overworn by the constant closing and opening of it trying to experience positive shut off by the knife edge seat.  All those little things add up to efficiency and cost savings in the long haul.

scott markle_2

Good advice

I don't generaly tweak the shift adjustment, but agree fully with the logic of running water only as hot as it needs to be. As far as cycling is concerned I will add that on small systems with very low curves I have had some problems. The original wb2a was very keen on keeping a narrow setpoint, and I saw a lot of cycling (very short burns) when targets were bellow 90 f.



The down side of your configuration is setback recovery, with no indoor feedback to shift the curve you may have to wait a long time for a zone to come up to temp. Since it nearly impossible to balance a system perfectly and solar and internal gains are constantly changing, my view is that one of the best way to incorporate indoor feedback is to do flow modulation via TRV's. Thermostats with autonomy over reset are great (tekmar, vitotrol, etc.) but I think TRV's are the best, although the setback recovery issue is still a problem for the instant gratification people we often must peddle too.

noah99

viessmann help

so what would you normally set the slope as a starting point for this type of application? (right now set at slope .3 shift +3)(i was thinking at putting it to a slope of .5  with no shift or incresing the slope to .4 with a +3 shift) the water leaving the boiler into the system is around 80f what should bet a target temp  here? i do notice alot of short cycling with the boiler

thanks

Paul Pollets

Slope

,3 is way too low. A slope between .7 to 1.2 will work for your home, depending upon application. I always start at 1 and adjust downward, as required.

noah99

shift change

have my boiler now set at slope .6 and runs fine, anybody ever play with the shift and what is the purpose of it. does anyone ever use it?