Unique Situation for con-mod vs. CI boiler advice

yeilon

Hi everyone, I have read a lot of the conversations (on the Wall and elsewhere) about con-mod/ vs. CI boilers for residential use. I believe I now better understand these systems' constraints and benefits. I would appreciate it if you could make a recommendation relative to our unique situation.

We live on Long Island, NY. The house is about 3,400 SF (this is including a basement of 1,000 SF) with 4 heating zones. We have a hydronic system operated by an old H B Smith CI gas boiler installed in 1980. It is way oversized at an input of 150K BTU/Hr and output of 120K BTU/Hr (80% efficiency by design).

I have attached a heat loss calculation based on the Slant/Fit app at 15F which is the “Heating 99% Dry Bulb” in our area. Also included the heating elements type and heat emission at each zone at water temperatures of 180F and 150F. I was conservative with my insulation assumptions.
I have also crossed checked it based on the gas usage and get a reasonable correlation. On average we use about 1,400 Therms annually for heating (1,200 Therms) and DHW.

Our situation is unique since, as you can see, zones 2 & 3 (that were renovated in the past few years) heat emitters have much higher capacity than the heat loss, even at 150F, while zone 1 requires 180F to marginally deliver. Zone 4 is not heated since the heat emission from the boiler is sufficient to keep it comfortable. We are thinking to incorporate a thermostatic mixing valve to reduce the water temperature supplied to zones 2 & 3 for a longer boiler cycle.

We are considering either a con-mod or a CI boiler (both with outdoor reset) with either an indirect water tank or a solo tankless DHW heater.
We are 2 adults and a teenager with an average or less water usage and not interested in a combi unit.
We would appreciate your wise advice.

STEVEusaPA

Can’t open your attachment on iPad for some reason. What was your final number, 49k?

yeilon

Very close. Please try this JPG file.

EBEBRATT-Ed

No doubt you have read all the posts so there is no point in me listing the pros & cons od CI versus mod con boilers.

It's not a question of which is better or worse it's about the best choice for you.

One criteria is how long will you keep the house? Are you willing to replace the boiler again?

Mod cons have been around about 20 years now so we are starting to get a track record. About 15 years seems to be the maximum life and if they are still running after 15 years parts become unavailable.

A cast Iron boiler can last longer ....or not. Even that has changed but on hot water if properly applied they should last at least 20 years with 30-40 years (yours is 40 years old) not uncommon. And parts will always be available unless the boiler sections fail or leak.

You did a good job on your "homework" now to find the right contractor which is tough in your area

The big advantage to a mod con on your job is the ability to modulate down to match the load when only one zone is calling. You will probably end up heating zone 4 somewhat as the boiler will give off less heat. Skip the tankless water heater and use an indirect for DHW

Jon_blaney

I would investigate adding and reducing emitters, maybe a buffer tank.

EBEBRATT-Ed

Multiple posts @Erin Holohan Haskell

Erin Holohan Haskell

EBEBRATT-Ed said:

Multiple posts @Erin Holohan Haskell

I've merged the duplicate posts. Thanks!

lkstdl

Is it feasible to add more emitters to Zone 1? Heated floor, some couple panel radiators, etc. Try to get your supply water temperature requirement down to 130-150F on design day, so it is in line with the other zones.

Benefits:
- No need to add a thermostatic mixing valve since everything is one temperature (less complexity)
- Return water will be at condensing temperatures almost year round (better efficiency)

I assume you've already picked any low hanging fruit as far as reducing heat loss? For example Thermal shades, interior 'storm' windows (from the excellent BuildItSolar site).

Luke

clammy

Stick to a properly sized cast iron boiler ,less service longer life span the annual maintance will eat any fuel savings . Have a decent indirect tank installed using a priority relay and a seperate circulator pump and be sure to have a mixing valve installed . If using a outdoor reset possibly have your boiler installed using a thermic valve to protect your boiler from flue gas condensation ,also have your existing chimney inspected and lined . Even though a good stainless steel liner will cost its lifespan will be about 30 years a mod con would possible be replaced 2 times. Stick w the cast ,I will only install a mod con if the home owner is dead on having one or if the existing system will function at low water temps and allow the unit to condensate at least close to design day other wise it cast iron .on a side note either way don’t forget water condition weather it hard and high in tds most low mass mod con and cast boiler have perimeters for water hardiness and tds that should not be over looked as time goes on this will be defentely be a issue as even ci boiler are starting to approach low mass w there water content . Mod con are nice but always require aside from maintance proper install and fore though like lowest modulation to smallest heat zone btu requirements possible buffer tanks and all the good stuff that comes w a mod con but often forget or over looked by most installers .In closing think of all the maintance you have had done to your old boiler if any and remember w a mod con if your lucky u will only see and get a bill once a year for maintance unless you have to pay a monthly service fee for emergency service or some type of service agreement which will cost more then what you have ever spent on your existing system . Peace and good luck clammy

yeilon

Thank you all for your valuable input.

We are thinking to replace the boiler when it stops functioning. We are willing to pay the premium for the oversized boiler for now.
However, we do have to replace the water heater. Although the indirect tank is the most economical option we are concerned with overloading the old boiler that has been working only during the heating season and would work year-round with an indirect tank. What is your opinion and can you recommend well-insulated traditional and indirect water heaters.

jinbtown

if you have a basement, get a heat pump water heater, after rebates it's the only way to go.

My vote is for modcon because you have zoning. Friend with a similar house here in NH has a Buderus that's eating itself alive with 1.5-3 minute run times (horrible for efficiency and for lifespan) and he's at a loss in terms of how to get it fixed. Yeah, it needs a 60 gallon buffer tank piped in, better controls, zone combining, etc, but by the time you do all that you're left with less comfort and have spent $$$$$$ to get it right. Like someone else mentioned adding emitters in your underserved zone would be my first thought. Get those supply temps down, good for efficiency no matter what route you choose.