Radiant SWT vs. Boiler Run Time sweet spot

mpdport · February 4

Hello, amateur homeowner here, trying to understand best practices for my new radiant floor and propane consumption efficiency. Antique house with a big renovation/addition. In Maine, so we get some cold nights.
I have new Viessmann high efficiency condensing boiler. It’s able to modulate flame and also controls the mixing valve for the radiant water temp. It has separate outdoor temp curve for both radiant and HW baseboard zones. I Have One x 5 port manifold and one 3 port manifold. Flow rate and temp gauges on both.

Main floor about 2000sf of new radiant floors (wood and ceramic) with Eco-Warm product under the finished floor

https://wbiwarm.com/ecowarm/

Upper floor about 2000sf of original HW baseboard.

Trying to find best way to determine sweet spot for the SWT for the radiant floors vs boiler run time. Is there a trick to how to determine this ???
The engineered wood floors (Kahrs) have max floor temp limit set at 81degrees per manufacture recommendation. Ceramic floors have no limit set.

All my Tekmar smart T-Stats will tell me daily run time. Viessmann will tell me daily gas consumption. So pretty easy to see cause vs. effect (it outdoor temp were constant).

Any advice would be welcome

Thank You!

Jamie Hall · February 4

Presumably this boiler serves both the baseboard and the radiant?

Problem: you need radically different temperatures for those two uses, so the boiler outlet temperature will have to be set to the higher requirement of the baseboards, and that mixed down to the required temperature for the radiant.

For maximum efficiency and comfort, both of those should be controlled for continuous flow 24/7, so the boiler would be modulated by outdoor reset for the baseboard. Then you would have a powered mixing valve to mix that down to the temperature needed for the floors, also controlled by outdoor reset. Now with some additional control wizardry you could arrange it so that when the baseboard was not calling the boiler would drop to the temperature needed for the radiant, and only ramp up to the baseboard when it called.

hot_rod · February 4

The sweet spot could be predicted with a bunch of calculations. Or just adjust it over the course of a heating season. That boiler probably has a powerful control allowing you to really dial it in to you exact needs and wants. An installer would need to make multiple trips back to fine tune the control.

I would get familiar with making the adjustments and tweaks yourself. Read the manual maybe watch some You Tube videos on that control programming.

With ODR dialed in you can get real close to constant circulation.

mpdport · February 4

To Hot Rod, You allude to “constant circulation”. Is that the goal ?
I was wondering since there is no storage tank for supply water. Is the objective to have the boiler modulate its frame and water temp (from the ODR). That it runs most of the time ?

Just trying to grasp the overall most efficient method of operation.
higher temp with cycling

Or, Just the “right temp” to allow pump running most of the time with the boiler running a lower flame/output.

Thanks

GroundUp · February 5

The most efficient combustion will always be the lowest possible SWT and the longest possible run time. If it runs 24/7 at 80* SWT, it's kicking butt and taking names compared to, say, running 4 hours a day at 180* SWT.

psb75 · February 5

I have a Viessmann 222F operating in a renovated 1800's farmhouse in MI, heating cast iron and panel rads on two floors. ~3000 sqft. No thermostats. ODR only. Constant circulation. Condensing all the time.

mpdport · February 5

That is helpful to know. Thank you.
I am just curious - in my mind I am trying to understand the comparison of my old oil boiler, that held quite a bit of hot water in it, and you had to keep it warm All winter. Versus this high-efficiency gas boiler with seemingly almost no water storage…..how does that fit into the equation ? Btw, I have a Low loss header between the boiler and the heating loops. Will he gas unit just cycle and modulate the flame more to try and reach an “equilibrium” on SWT, return water temp, and lots of run time?

Is that also where the return water temp (is that deltaT?) comes into play ? I assume you want to design/control for how much heat is dissipated from the Supply water and factor that into all the adjustments ?

I realize there are all newby, and probably ignorant questions….

Thanks for your patience and generosity of knowledge.

hot_rod · February 5

https://forum.heatinghelp.com/discussion/comment/1840402#Comment_1840402

When you do a design on a hydronic system you chose a design delta. 20° is a common number for fin tube, air handlers. Panel radiators can run 35° or more. Radiant floors design around 10- 15° for residential. Shops or storage radiant may run a larger delta.

It is possible that a system runs different deltas. Maybe 15 for the radiant a wider for other type heat emitters. Radiant floors run a tight ∆ so the floor surface is fairly consistent from beginning to end of loop. That is not so critical with radiators or fin tube, for example.

Now once the system is up and running that delta will move around a bit. On first start up, cold rooms, cold heat emitters that radiant floor may run 20∆ or more. The delta closes as the temperature difference between the load (room) and the heat emitter get closer. With low mass copper fin tube that may happens in minutes. A large concrete slab may take a day or more to stabilize. You may see systems that never run at the exact design delta, ever!

The operating delta is the indication of the amount of heat being delivered at any point. If you know the flow rate the math is 500 X flow ∆

So with a 5 gpm flow at 20°∆ you are moving 50,000 btu/ hr.

It is the building load, heat emitters that drive the operating condition of the boiler, It's operating temperature conditions.

If you want the boiler to modulate the system supply, strap the system sensor about a foot downstream of the LLH. So the boiler can measure and adjust accordingly.

Yes, the larger water content and higher mass of a cast iron boiler gives you some buffer. While the low mass mod cons have less mass and water volume, the modulating function help keep them from short cycling. With modulation and outdoor reset you should get nice long, efficient run cycles. The mod cons can and really like low return temperatures. The lower the RWT the higher the efficiency.

There are a lot of adjustments possible with mod cons to prevent short cycles and low efficiency operation.

Radiant SWT vs. Boiler Run Time sweet spot

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