Adding radiant to existing Boiler system

CoryF

Hello all… looking for some input on what I’d like to accomplish and my current plans.
Background:
I currently have a 2004 Crown CTPB-45 (160k BTU) oil fired boiler heating 2 floors/zones via fin tube baseboard in an 1880’s farmhouse. Each zone is controlled with separate Taco circulator pumps, with a 3 zone Taco relay panel. Primary hard wood flooring, some vinyl but will eventually be tile. Renovations are slowly under way including sealing and insulating as long term remodeling projects are undertaken. I also this last summer installed an 18k BTU mini split unit, primarily for air conditioning, but the heating has already had a huge impact in fuel oil savings, especially in the shoulder months and warmer winter days in Central NY.

Goal:
As I have hot water heat system already, I would like to add stapled up radiant floor heat with transfer plates to the downstairs, as well as what will be a small bathroom upstairs. Ideally, some individually controlled zones. Bathrooms, kitchen, and “rest”. For now, would be leaving the baseboard in place, as a “backup”, but if the radiant was enough I could remove.

Current “plan”/idea:
With the high temp, non-modulating boiler, and desire for “small zone” radiant floor control and high temp baseboard, I’ve been doing research. I saw some solutions utilizing an indirect water heater as a buffer tank for radiant and with some fortunate luck, found a Well McLain 45 gallon indirect water heater for a very low price. My idea is to use this tank as the “third” zone off the existing boiler/controls as a buffer tank for the radiant loops. The radiant loops would connect via manifold with thermostats/thermal actuators for the zones. This would allow the radiant system to exist nearly independent of the rest of the heating system. I would use a variable speed pump to manage the variable flow. Thus this would have it’s own fill valve, air purge, pressure tank, pump relay etc.

One question I might have, that is specific to this idea, is the temperature set points of the buffer tank. I not sure if this should based on the temperature needed to maintain the floor temp, or, would it be better to have a higher tank temperature to allow for longer boiler burns, and use a mixing valve to lower the temperature to the radiant manifolds.

One other concern after the winter so far, with the minisplit offsetting the boiler quite a bit, the call for baseboard heat, and thus run time on the boiler is much lower than in the previous years. I have some concern that I might not be able to maintain warm floors if the heat is being maintained without the boiler. I’m not quite sure yet how the systems might interact.

I very well could be way off base on this idea, and be missing something but it seems relatively straight forward, with perhaps the only difficulty piping in to the existing boiler piping. I also know a heat loss calc is probably hugely needed, but have been somewhat difficult to determine given the age of the house and changes in insulation/air sealing.

Look forward to any responses (positive and negative!), and appreciate any input, and if you read this far!

Thanks!

hot_rod

If you want the radiant on its own zone, and it is a small btu/hr load, then a buffer is a good consideration.
An even more advanced system would be pulling off the buffer via an outdoor reset control. Properly tuned you could get constant circulation on the radiant loop.

Are you running the boiler on ODR?

CoryF

Hi hot_rod, thanks for the input. Would the load of the radiant be primarily based on length/amount of the loops? Or do I need to try get more accurate heat loss calculations?

Thanks!

Jamie Hall

CoryF said:

Hi hot_rod, thanks for the input. Would the load of the radiant be primarily based on length/amount of the loops? Or do I need to try get more accurate heat loss calculations?

Thanks!

It's more the area covered by the radiant that is a factor, rather than the length of the loops. Not quite sure, honestly, how I'd approach this one, since you can -- and will, most likely -- use the baseboard heat for the coldest days, so an approach based on design day heat loss isn't likely to be helpful. Rather I'd shoot for a reasonable floor temperature and thus BTUh output from the floor, and base calculations on that.

One comment, though. You may need to run somewhat higher than average water temperature, as your floors are likely to be thicker than a modern floor. if there is a subfloor, you may well have a total of 2 inches thickness of wood there. Take that into account.

EdTheHeaterMan

CoryF said:

Hi hot_rod, thanks for the input. Would the load of the radiant be primarily based on length/amount of the loops? Or do I need to try get more accurate heat loss calculations?

Thanks!

As far as the boiler is concerned, the load is based on the amount of tubing connected, and the water temperature in the tubing. As far as the surface area of the floor compared to the actual heat loss requirement... that depends on the structure and the construction material used. There are some buildings where the floor space that is being use as the heat emitter is not large enough to maintain a comfort level below a given outdoor temperature.

This was my experience in my home in Brigantine NJ. The staple up portion was unable to keep up with the load whenever the outdoor temperature was below 21°. The newer. section of the home was tubing in a Slab, and much better construction, insulation, windows and the like. That area was good to about 6° before we felt the chill. Good thing I didn't remove the old gas furnace!

CoryF

Sorry, I thought I had this in my write up before. Total square footage of radiant flooring would be around 1100SF, in an 1800SF house (basically all of downstairs and a bathroom upstairs). I haven't calculated length of tubing runs.

@hot_rod Missed this question, no ODR on boiler.
@Jamie Hall Correct, my "design" goal as it were, would be to have warm floors (obviously), but baseboards can be used if needed. Also, yes old of house has quite thick subfloor, so temperature might need to be higher.
@EdTheHeaterMan Thank you for your experience. As noted, if I could have the benefit of the warm floors and have the baseboards as backup, that would be fine.

hot_rod

A room by room load calc is where I would start. See how the fin tube emitters you have already, match the loads and at what SWT. Also the boiler size to entire home load.

The load calc for the radiant area will determine what is workable. I suggest mid 20's btu/ sq ft is a reasonable output for radiant floors.

How is the home zoned now?

If you have multiple zones or micro zones, you could buffer the entire system with that indirect. Pull the radiant loads off the coil connections. Run the entire system on ODR.

Control does get tricky if you have both radiant and fin tube in a room. Go with one or the other. If the radiant cannot cover a room load at design, a two stage thermostat could call on some fin tube supplement.

It's all in the numbers, start crunching

EBEBRATT-Ed

I agree with the two stage thermostats. 1stg radiant 2d stg baseboard. You need to run the boiler warm enough so it does not condense when running radiant only. A mixing valve off the buffer tank will control the radiant temperature