A New Boiler? So what type I say!

Cyclist77

So I retrofitted our home 13 years ago for radiant and it is great! My boiler is a coal stoker that has performed flawlessly. But coal is getting very expensive and we now have NG available.

So my thought is a CI chimney vented boiler or a condensing boiler. Yep,I understand the efficiency difference but the cost difference up front is significant. Also am I correct that a condensing boiler requires more maintenance?

Advice would be great!

Thanks

Tom

LRCCBJ

It's simple:

Do you want CI efficiency of approx. 75% with the benefit of minimal maintenance over the life of the boiler (25 years expected) or do you want the efficiency of a mod-con with approx. 87% efficiency with the task of annual maintenance and the high cost of replacement parts throughout it's 15 year lifespan? The mod-con will also suffer from the lack of competent individuals to service it………..especially when it gets older and goes out of production.

The efficiency of the mod-con be be higher if you have CI radiation that allows lower supply temperatures.

The odds of coming out ahead with the mod-con increase with the size of the building. A small building (1500 square feet or less) will never come out ahead with the mod-con. It uses too little gas.

GroundUp

Yes, condensing will require more maintenance. The upfront cost can't be a whole lot different, if you can reuse your existing chimney? Efficiency can be up to 30% higher with low temp radiant and a proper outdoor reset curve, so depending on the actual BTU load, it may very well be worth going the condensing route if the numbers add up

ethicalpaul

cast iron boilers are 75%?

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1829447#Comment_1829447

We're talking about actual efficiency…………not the number on the side of the boiler for combustion efficiency! Surely YOU are not deceived……………….!!

As @GroundUp noted, a mod-con can be 30% more efficient under the proper circumstances. That's only possible if the actual efficiency of CI is well below the placard!! The value of 75% is probably generous!! They lose a tremendous amount of energy to the basement and most of it is LOST!

leonz

For the money you would spend installing a New Horizons 490 gallon fully insulated rectangular hot water storage tank as a buffer tank for the coal stoker and piping it to feed the hot water to the bottom of the tank and taking the hot water off the top of the tank to circulate it would be more cost effective.

Steamhead

Well, a mod-con cannot exhaust through a chimney if there are any other appliances using it.

Parts can be hard to get, there is no standardization across brands, and they may not be available at all after a while. I've been in the position of having to tell a customer the part they need is two weeks away. It's not pretty.

Maintenance is more complicated and costly.

And they just don't last as long.

Now, with a cast-iron boiler, 9 times out of 10 if something goes wrong you can fix it right then and there off a well-stocked service truck. And they don't go wrong nearly as often.

What's in my house? Cast-iron, and that's my recommendation.

EzzyT

Is your entire house radiant heat and low temp then a condensing boiler makes sense and yes will require more maintenance. I’ve done many cost analysis between cast iron and condensing boilers on low temp systems keep it simple and go with condensing.
With cast iron you’ll need Thermatic valve to prevent the boiler from thermal shock and then you’ll need a mixing valve for the radiant. Most likely will need a chimney liner going from coal to gas. Might need it for the condensing boiler too if the chimney is the best way to vent the boiler. There’s several polypropylene companies that make chimney liner kits which cost less then stainless steel liner and easier to install. Definitely plumb the air intake for the boiler from the outside don’t take it from inside of the house. I would highly recommend a hydro separator or buffer tank over closely spaced tees.

Cyclist77

Thank you fellows for your replies! More information for my wintertime research!

Current chimney is a clay tile lined flue. I understand that a condensing boiler would require a new liner or can be vented with the appropriate plastic pipe through a sidewall.

One other question. "If" a mod-con" what is your opioof having a combi unit with DHW? All my eggs in basket?

hot_rod

my experience shows you can go several years without a complete mod con tear down and cleaning. Ive gone 5 years on several of mine. It depends in how well it is commissioned and the air it breathes

I have owned about 10 mod cons of all brands and have not had a single expensive part replacement needed?

Gasket kits and consumables. Again this is related to how well they are piped and tuned

If you can leverage low temperature operation, mist applications can at least part of the year, go with a reputable brand mon con.

Cyclist77

Now for a curve ball!

What would be the disadvantage of using a tank type gas fired water heater for a radiant system ?

LRCCBJ

There is no disadvantage other than fuel.

You must use a heat exchanger to separate the DHW from the CH for safety reasons. Otherwise it's an excellent idea.

I use one in one building for that exact purpose. The building has CI rads and can barely manage with 140F SWT. @ 10F design.

Be advised, however, that the efficiency of the HWH is approx. 68% when a condensing boiler for the same application is typically 90%. You'll absolutely notice the additional fuel cost.

Steamhead

https://forum.heatinghelp.com/discussion/comment/1829477#Comment_1829477

This would be a Code violation in many areas, since a water heater is not ASME certified as a heating boiler. Yes, I know some WHs have extra tappings for connecting a heating loop, but Code is Code.

The other problem is since a WH maintains a constant temp, the standby losses are greater.

newguy101

Commenting as a homeowner, not an expert. I had a mod con installed 4 years ago. I had it serviced the first year, and then went three years without servicing. It started leaking. I am now $2500 into repairs, and it is still leaking. I wish I would have gone with cast iron. I have become convinced that simple and reliable often works out better in the end than complex and "more efficient".

Hot_water_fan

what is the radiation? Baseboard, radiator, floor, etc?

hot_rod

https://forum.heatinghelp.com/discussion/comment/1829477#Comment_1829477

If you use a conventional tank type water heater I would run it at 120 or 140 and use a mix valve. If you run them at 100°F constantly for years they corrode badly inside.

While not code approved in many areas they are a higher pressure rated vessel with multiple safeties, and can work fine.

I don't think tankless water heaters are ASME either, thousands of them in use for radiant.

Cyclist77

Hot Rod,

I currently have a mixing valve on my coal boiler to control what temp goes out to the zones.

This idea of a separate tank gas water heater is becoming more appealing!

hot_rod

Any idea what the BTU/ hr load is. A typical gas fired water heater in maybe 30,000 btu/ hr output.

Cyclist77

That I don't know. Remember I am just a woodworker!

The boiler is 85,0000btu

Jamie Hall

I think you also mentioned possibly a combi? In some rather limited applications, they can be quite useful. But… particularly in tighter modern houses, they almost always wind up bigger than needed for the heating side if they are to be as big as wanted on the domestic hot water side. While they can modulate down, they may not be able to modulate enough.

In many situations the most efficient possible combination is a "right size" mod/con boiler — and an indirect hot water heater tank, fired by the mod/con.