Indirect heating system

RichardP

I am working on the design for replacing a 100 year old hot water boiler for a gravity heating system in a large house. The boiler was originally coal-fired and converted to oil-fired in 1965. The boiler cracked last winter and needs to be replaced. The owner wants to keep the indirect heating system for the First Floor wherein outdoor is supplied to heating coils inside metal cabinets in the basement which heat the air and deliver it to grilles in the First Floor rooms. I wanted to remove the outside air ducts and return air from the First Floor to the basement with grilles in the floors, but the Owner won't accept cutting the floors. How do you calculate the heating load for the indirect heating coils when supplied with outdoor air? Because the boiler can't operate until it is replaced, it is not possible to measure the airflow through the outdoor air ducts.

The attached photo shows an outdoor air duct (lower duct) supplying air to a heating coil cabinet which then supplies heated air (upper duct) to a room on the First Floor.

Rich_49

No pix

can you attempt to attach them again ?

Paul Pollets

Indirect radiators

I've worked on several of these systems. If you're not replacing the ducts with direct radiators than leave the existing system and size the radiators within the duct boxes for output. Replacement should consider a cast iron boiler, as most of these systems were designed for 180 degree supply temps and a mod-con may not be in the optimum design range.

The cast iron replacement boiler will need protection from thermal shock when re-piped.



http://www.heatinghelp.com/article/333/Indirect-Hot-Water-Heating/73/Indirect-Hot-Water-Heating-FAQ

icesailor

System Design Temperatures:

"" most of these systems were designed for 180 degree supply temps  ""



Did you mean it that way or that the systems were designed to run at a maximum of 180 degrees?  Coal/gravity systems were designed to run water temperatures that were warm enough to equal the heat output of the building. That's what made properly working gravity systems the cheapest and most comfortable systems of the day. All pumped systems are just trying to mimic what true gravity systems did. That's why they will so seamlessly work as ODR systems. They work even better as 4-way thermostatic mixer systems which are another form of primary/secondary so you can control the system temperature as needed and the primary boiler water system at a higher adjustable temperature to prevent boiler condensation when you have warm OAT but cool enough to need heat.

That OAT unit in the cellar can be a heck of a load if it is using un-tempered air.

Paul Pollets

Indirect temps

Any of the systems I've seen (or worked on) did not qualify for a mod-con application. One was connected to a steam boiler.  180 -215 water temps were required.

 In one case, the large home was insulated and windows replaced, which significantly reduced the load, but not in the "sweet spot" of a condensing load. All of the systems we used for replacement had a motorized 4 way mixing strategy with outdoor reset.

RichardP

Indirect heating system - photo attached

Photo of heating cabinet attached.

JStar

Indirect

Icesailor hit it right on the head. These systems WILL work very well with modulation. I would never even consider putting a cast iron boiler in that house. I would also keep the outdoor air ducts the way they are. If you pull return air from inside, you may not have enough of a temperature difference to create the proper draft for good air circulation. And, you may wind up with a negatize Combustion Air Zone. So, a cast iron boiler would definitely spill exhaust into the basement. NO GOOD.

RichardP

Indirect ctd.

My question is: how do you calculate the heating load for the boiler when you don't know the heating capacity of the indirect heating coils and don't know the outside air quantities flowing through the heating coils?

Rich_49

Passive / mechanical

Have a blower door test performed on the home .  Have whomever does the test convert from ???cfm50 to ???cfm nat . This will give you your ACH at worst case .  Just an idea .

JStar

Indirect

Do you have measurements of the enclosures, or pipe sizes?

Zman

Clues

I think you need to take what you do know and work the math a few different ways.

The surface area of the radiator would be extremely helpful.

The duct size and the pipe size are also important.

Have you done a heat loss on the home?

What size nozzle does the existing burner have?

What is the oil consumption?



You are getting some great advise. You need to provide more info.



Carl