MOD-CON and one pipe hydronic

Roland_12

G'Mornin All, I have a sneaky feeling this question has been asked at least once before. When I was shopping for a heating contractor I was originally intending to install a Mod-Con boiler. This was pre-Wall so I had limited information. ALL the heating contractors refused to install anything much newer that a standing pilot/atmospheric unit. It seems they were deathly afraid of the new technology and the inevitable (they said) call backs for no heat and the ensuing heafty repair bills. Now I know better(I think). My question is; does a Mod-Con make sense when mated to a one-pipe (diverter Tee) heating system driving fin-tube convectors? It seems that due to the high temperature requirement of fin-tube (180*F) the boiler would not be in condensing mode all that much. Also in this configuration, what is the average efficiency one could expect? Certainly not the advertised 90%+ when properly attached to a properly installed radiant system. Thanks to all for your terrific help in the past, Roland

kpc_12

if the system....

is designed for 180F ...that means it is designde to do this @ design temps 0F or lower usu...if it is not 0F out then you can bring those water temps back down...therey condensing more....afue still should be in the 90% range.

Derheatmeister

Even sometimes the heat emitters are Over sized ,so again it will go into the Condensing Range faster(Also the 90% Eff is on the Combustion Side , but the Additional savings come from the boiler Working on Outdoor reset). You can also Change the Baseboard for Panel rads and towel Racks. nice... HM.

Uni R_2

Why not just do the math?

Go to SlantFin's web site and download their free heatloss tool. Do the heatloss calculations at both outdoor design temp (the local building inspector should know this number) and also calculate the heatloss for an average winter temperature. HeatLoad@DD and HeatLoadAvg


Then add up all of your baseboard.


HeatLoad@DD / feet of baseboards = BTU per foot (worst case)


HeatLoadAvg / feet of baseboards = BTU per foot (typical)


If the typical supply temp is 140°, you should definitely be able to take advantage of a modcon under nearly all conditions. What you want is the ability to keep return temps under 130°. The heat loss calculation will probably be 30% too high which will mean that you'll be lower than these numbers. The one downside to your application is that the diverter tees require a lot of flow and this tends to keep the ΔT between the supply and the return temps fairly close. Diverter tee piping doesn't help, but it shouldn't be a road block. A modcon with ODR will make your fintubes extremely comfortable.

I have the same arrangement as you. I'm still condensing at design. If you aren't condensing enough, make the shell of the house better insulated and tighter.

ALH_4

Condensing and Supply Temp

Contrary to conventional wisdom, a mod-con can condense at temperatures above 140°F. The attached chart shows the efficiency of the Vitodens at different supply temperatures and burner modulations.

If the heat load is low, the burner modulation can be low even if the fluid temperature is high. The 140°F limit for prevention of flue gas condensation is an old rule of thumb for non-condensing boilers that no longer applies when we are talking about mod/cons.

Diverter tee systems may not be ideal when using a mod/con, but as Uni has experienced first hand they can still be more efficient than a conventional boiler. There are very few applications where a mod/con boiler is not appropriate.