Running Hydro Coil & Plated Staple-Up Radiant at the same temperature

ILikeEmOlder

Slowly but surely making progress on the plans for the next phase of mechanicals for this old house (timber frame structure dating back to the 1700s, with 1800s balloon frame addition).

Any pros see an issue with using the existing modcon combi gas boiler to serve both a single 8-circuit radiant manifold and a proposed hydro coil?

House details: CZ6; building heat load is roughly 60K BTU/hr @ 1F (25K BTU for the 1st story, 30K BTU for the 2nd story). Plated staple-up for entire 1st story, and ducted for entire 2nd story. All ducts are within conditioned space. Existing gas furnace could be kept for supplemental (2nd stage) heat.

My hope is to run the radiant floor and the hydro coil at the same temp, roughly 120-130F boiler SWT through the heart of the winter (to maximize efficiency without compromising on comfort).

This would represent a very cost-effective way to heat the entire house, which is paramount with all the other money going out to completely rehab this house.

Teemok

If the boiler isn’t cycling and the supply air temp is comfortable enough and you are able to meet set points in a reasonable time period, I don’t see a problem. If it is cycling or and there’s another compliants, then there’s room for improvement. Lower return water temperatures is the way to seek efficiency but creating bigger delta T’s or lowering supply temperatures will approach a min. performance limit.

Jamie Hall

If you can actually make that work, that would be lovely. Big if. Generally speaking, a radiant floor will be much happier almost all the time at a lower, but steady temperature. At the same time, a hydrocoil is almost certain to want a higher temperature to avoid a feeling of — if not the reality of — a cool draught.

Now… all this means is a little extra plumbing and some slightly more sophisticated controls and one extra pump (well, maybe two, for flexibility). Will this make the setup more expensive? It will — but in the overall scope of your project, not that much.

What wants to happen is, basically, three loops: your primary boiler loop, the secondary radiant floor heating loop pulling off that with the usual temperature controlled mixing valve to mix return floor water with feed from the primary loop to get constant flow in the floor at an optimum temperature, and another secondary loop off the primary loop feeding the hydrocoil units. That one doesn't need to be mixed down.

With some intelligent controls and a good mod/con, you should be able to keep the return to mod/con down around 80 or 90 or so, most of the time, run the floor at perhaps 90 to 100 or so, and the hydrocols at as much as 140 to 150. It's all a matter of controlling firing rate and flow rates (and, for the hydrocoils, fan speeds).

hot_rod

grab the spec sheet on the hydro coil and check the btu/ hr output at that AWT

130 SWT at 20 delta gives you 120 AWT. So enter the chart at that temperature

Same concept with the radiant, the R value of the floor and the btu output required determines the floors required SWT.

If anything the hydro coil may need higher SWT. You can buy air handlers with different coil size and capacity based on that SWT

It is easy enough to run a two temperature system.

ILikeEmOlder

Thanks for the input gents, I really appreciate it.

Two-temp systems seem to be the tried and true for cases such as this.

Based on the specs, it looks like I could pull this one off as a single-temp setup. It would be mighty close though. Long story short, I’m on a mission of sorts to use my own house as a test case for the limits of a modcon combi boiler. I turn to folks such as yourselves for the wisdom and knowledge of those who have been there (and probably have the scars to prove it).

hot_rod

the one goal with a mod con is to keep return temperatures low to promote condensing operation. If efficiency is on your list of goals.

130 is getting on the high side of what I would supply a plate radiant.

Staple up, UltraFin, or suspended pex are in the 140f + temperature category

ILikeEmOlder

hot_rod,

No disagreement here. I predominantly work on much older houses, so I’m using my own oldie to test the limits of both modcons and plated staple-up radiant when operated within houses with challenging envelope details, and aesthetic concerns related to maintaining original features, such as rather thick built-up flooring.

In my own house, I have had the boiler SWT for the radiant floor set to 120F this winter. Floor temps (measured with a Flir IR camera) have averaged high 70s in the 1800s addition, and mid 70s in the 1700s structure. Boiler is constantly firing, and maintaining air temps near 66-67.

I’m aiming for 110 SWT for the milder heating season months (when average temps are in the upper 30s to 40s).

Kaos

What I have done is used an oversized fan coil but run at low CFM. This gets you the oversized heat exchanger which gets pretty decent approach temperature. The lower CFM means the air will be pretty close to SWT so you don't get that lukewarm air feel.

You can also put the high temp zone upstream of the floor heat on the primary loop. This lets you run higher SWT but keep the returns low enough to stay in condensation range. If you run the FCU with continuous circulation with a two stage thermostat controlling the fan speed, it will supply some heat most of the time so it should effectively provide the required lower SWT to your floor heat zone. I run something pretty close to this with combination of high temp rads and floor heat.

EdTheHeaterMan

Is the single temperature approach just. to prove a point?  

The fan coils will need to have much bigger coils with low capacity blower fans to make it work.  What will the cost of the unusually configured air handlers, compared to a two temperature setup with standard availability air handlers?  Just an idea to think about.  

Or you can use a regular air handler or hydro coil and hope for the best.  I believe you might not be satisfied with the drafty home on that second floor with 90° to 100° air blowing around.  I am certainly not happy with my heat pump in the attic and the concrete slab floor I have now.   But that is how they do it now.

hot_rod

surface area is the secret to high efficiencies This holds true for radiant panels, radiators, and air coils.