High Efficiency Boilers for Fan Coils?

MechEco

Hello, Im HVAC contractor out of Denver, Colorado. We are mainly a service company, often servicing boilers but new to the design/selection side.

I need help choosing an acceptable replacement for an older boiler in a multi-use building. The building owner is wanting to replace the boiler, but I am having a hard time confirming a high efficiency upgrade is possible. The boiler is a 1981 Bryan boiler. Its large, heavy, and on the 3rd floor (second floor with a heated basement)

The space heating is done by a mix of fan coils of various sizes, some ducted some not.

We want to replace the old boiler with a set of new high efficiency boilers, as they are easier to install, and offer some redundancy if one is having an issue.

I've had numerous manufacturer reps out to look at the project and they all are "not sure" if we can upgrade to a high efficiency boiler, recommending only the 80% direct replacement to the one installed in the 80's. The Reps are worried about the loop temp for the Fans coils if the high efficiency boiler is used. This building has changed hands, uses, and gone through remodels in that time. The Fan coil manufactures also have no helpful insight to the situation either.

One of the helpful knowledgeable techs in my area was helping me to understand that some additional piping in the mechanical room like a secondary loop may be the solution.

My question is, can a wall hung, high efficiency boiler work for a fan coil loop? Who can I get sizing, selection, and piping layout help on a retrofit like this? My go to plumbers are happy to offer install help but not helpful in selection and piping design.

The mechanical room is likely in need of a controls upgrade as well as the pumps are on a manual change over switch. Below are some photos of the boiler, fan coils and mechanical room piping

Thanks in advance for your assistance!

Jamie Hall

It's really the typical and very basic questions. How much power must the fan coils put out? And what temperature do they need to run at to achieve that power output? The first question involves doing a heat loss calculation on the space each fan coil serves, and then finding the specifications for that coil (some of the guys here can probably help) to determine what water temperature is needed.

Once you have the water temperature and to9tal power output you need, you can select the boilers. The temperature is the key question on whether you can go to high efficiency boilers, which only achieve those efficiencies at lower return water temperatures.

bburd

Standard practice in fan coil design when that building was constructed was to use a supply water temperature of 180° F. and a return temperature of 160° F. Typically with fancoils this cannot be reduced very much even on mild days, because occupants are likely to complain of cold drafts.

Although high-efficiency boilers will work, they will not condense much if at all, so they will operate more in the range of 85% efficiency than the 90+ percent advertised. They also have a shorter lifespan than a conventional 80% efficient boiler, need more frequent maintenance and generally use expensive proprietary parts.

Bottom line: standard efficiency boilers will almost certainly have lower lifecycle costs. They will also last longer and be easier to maintain and repair. You can still use several boilers to make up the necessary capacity. They have to be piped carefully, primary/secondary is probably the best way.

Hot_water_fan

It’s 100% possible to condense with fan coils. You might use a larger delta T than you typically do, or may consider slowing the fan speed or switch to fan coils specially designed for lower temps. Or some combination. 

hot_rod

Do some more number crunching on the coil outputs and required temperature does the boiler output match the fan coil required output?

Even if not in condensing mode all the time, the condensing boiler can run safely with lower temperature than that hair pin boiler

But the bigger win is the modulation, Id look at two boilers, maybe two 180k that would give you a 20-1 turndown 
Plus the redundancy of having two boilers

EBEBRATT-Ed

The first step is to find the temperature the fan coils need. I agree with @bburb that 180 supply and 160 return will cover the building. And if I am not mistaken with Mod Cons 180 is maximum.

So the question becomes does it need that all the time? Probably not. But taking a bigger delta T to make the boilers condense (basically reducing the water flow) may not cover the building.

There are various ways and control to achieve what you want is to be able to provide the fan coils with 180/160 when it need it and reduce the supply temp when you need less heat so the boiler will condense.

We would need a lot more information and it is probably outside the range of forum members to size anything other than make some good suggestions