Baseboard zone sizing with a ModCon

Mosherd1

Trying to make sure I understand this properly. If I have a ModCon 110,000 boiler with an 8 to 1 turn down that means the minimum firing rate is just over 13,000. If I have a zone with 23 ft of HWBB that should keep the boiler from short cycling. 13,000 btu/580 btus per foot of bb = 23ft @ 180 degree water, but if the boiler is setup with ODR does that mean when the boiler is only making 120 water the boiler is going to cycle on reaching setpoint because 23 ft times 210 btus per foot only will dissipate 4830 btus if this is the only zone calling for heat?

Ironman

The thing that needs to be understood is that burner modulation is to compensate for varying heat loss of the structure, not over-sizing.

But to answer your question, yes, the boiler will short cycle more with ODR.
 

I’d recommend that you combine smaller zones or install a buffer tank.
  

Big Ed_4

Why a ModCon? Space, efficiency or cool factor ?

Mosherd1

@Ironman thank you. The house needs approximately 85,000 btus ( I know this because it has a 5 element electric boiler currently—5 elements x 5000 watts per element = 25,000 25,000 watts x 3.41 btus per watts = 85,250 btus and the house does not keep temp when one element goes bad). The smallest boiler Triangle Tube makes with an 8-1 turndown is 110,000 btus. I’d like to go with a TT as I have a good working relationship with with my salesman from the supply house. (I’m an electrician, but have connections with the other trades).  That 85,000 btu need is split between 2 zones originally, but there is a family room addition that is heated with a wood stove/electric space heaters. They would like to add a propane boiler and keep the electric one for when the ModCon acts up, and then add hydronic heat in the family room.  I figured it will need a buffer tank, but just wanted to verify. 

Mosherd1

@Big Ed_4 a ModCon for convenience.  There is no chimney in the house, and they don’t want to run a metal flue up the side of the house.  Sidewall venting with PVC will be a much simpler install.  

hot_rod

How common is it for that one, smallest zone to be the only one running? Is it a high load zone?

Mosherd1

@hot_rod very common, they aren’t there very much during the day.  They leave the two other zones pretty cool day and night and when they are home they spend most of the time in that room. I think once they switch from electric to LP they will keep the house somewhat warmer than with the electric boiler, especially with using ODR they won’t have a fast response if/when they turn the heat up in the rest of the house 

Mosherd1

@hot_rod I haven’t done the heatloss calc yet, room is approximately 12 x 20. Three of the walls are exterior with 3 4x4 single pane windows and two exterior doors.  I would guess in the 6000-8000 btu loss range

hot_rod

We have come full circle with zoning hydronic systems. Maybe 40 years ago zoning started to become more common place, more manufacturers building product, industry awareness to comfort options with zoned systems. It was a nice upsell, the concept worked nicely as the boilers were high mass, high water content back then.

The writing in this guide may look familiar

The concept worked well as high mass slab radiant systems also offered "self buffering"

The challenge now is the modern boilers, with a few exceptions, both mod con and cast iron have lost the mass and water content. The use of ODR puts another wrinkle in the package.

Tank type boilers like HTPs work great with micro zoned systems. They started with their Voyager, then Phoenix, now the Pioneer 55 gallon tank type heater. I suspect that Viessmann high mass 300 is a good option. Good that these also modulate!


Or build in a buffer. Consider the time and $$ to add and pipe a buffer based against one of the high mass options.

I'm still holding out for a 35K modcon, 10:1

Rich_49

Here is the unit Hot Rod referenced . They are quite good for not cycling . https://www.htproducts.com/pioneer.html