Mixed CIBB and Fin-Tube

peterwit

I live in a circa 1950's 1400sqft cape in southeast New Hampshire. Moved in about a year ago. My 30+ year old WM oil boiler seems to have a section leak - my boiler guy was afraid to brush it this year for fear of making it worse. So I'm planning to replace it in the spring. But I want to make some other changes, as well.

The main floor consists of 100' of cast iron baseboard. I believe it's WM Snug Baseboard. The two upstairs bedrooms each have fin-tube convectioners. All the emitters are fed from a single diverter-tee loop. There are no zone valves, and there is no boiler bypass or secondary loop of any sort - its just the 1 1/4" loop right into and out of the boiler. One concern I have is for boiler protection - typically when the circulator kicks on, the boiler temperature drops all the way down to 100 degrees and stays there for a long time. It looks like someone added a zone relay and connected the circulator directly to it, so that the low-limit on the aquastat does not inhibit the circulator.

When I replace the boiler, I want to do primary-secondary with some sort of mixing valve for the CIBB, especially since I have an 8 month old crawling around now and sometimes the radiators get pretty hot. I'm thinking about a Taco I-series with outdoor reset. I also want to put the upstairs fin-tube on its own zone.

Here's the questions I'm trying to answer for myself:

1. Is an outdoor reset mixing valve worth the price premium over a plain old thermostatic mixing valve?
2. Is the boiler protection feature built-in to the Taco valve a robust feature? Or should I be looking at something like a Caleffi ThermoProtec in a bypass leg?
3. Should the fin-tube be fed from the same tempered loop as the CIBB, or should I feed it off of the primary loop on its own?

I'd appreciate any insights. I'm not looking to redesign the whole system, but I do anticipate replacing all of the near-boiler components, including the plain steel expansion tank.

kcopp

Typically cast iron baseboard and fin tube should not go on the same zone. The fin tube heats up faster and cools faster... Where its the same zone" its not the same floor.
How does the house heat now? Is it even? Are you comfortable?
100' of CIBB in a 1400 sq ft cape is an awful lot... then the fin tube on top of that.

How does the existing boiler look now?
Does it show signs of condensing from the low temp water? Is is the 66/68 series or the Gold? They installed a lot of 66/68 in SE NH. I have pulled a lot of in the last few years.

How do you get your hot water? Im presuming its not from a tankless coil...

Dave T_2

A large CIBB loop like yours will works great if you put it on an injection loop. It will heat more evenly and you can control the injection rate.

If you have a cold start, low water content boiler my concern would be to minimize short cycling. Outdoor reset is very effective with excess CIBB.

No matter what, the finned tube should be on a seperate zone

peterwit

Kcopp:
The house doesnt heat super evenly, but I had attributed that to the placement of the thermostat in the kitchen, which seems to stay pretty cool compared to the rest of the house. We have it set to 65 because it was getting too warm in the bedroom with the door shut. 

The boiler model looks like P-368-WT. Is that a 68 with 3 sections? It is definitely not in good shape. After a setback, i get some dripping from the “back right” corner of the jacket. I had a look under the top of the jacket and it was wet on top. What would specific signs be of condensing?

hot water was fed from a tankless coil but I installed a heat pump tank this fall because it was miserable. 

Dave T_2: 
what are the differences between an injection loop and a thermostatic valve? That would require a variable speed pump with a temperature sensor, right?