Boiler Capability for running Radiators AND In floor heating

Andrea44

Hello. We're helping design a new HVAC system in our renovation.
We'd really like in-floor heating. However, with an old home, and cold Canada winters, in floor's maximum capacity for our square footage won't provide enough heat alone, so we will add radiators where needed for the coldest days.
We're quite interested in the Veissman 300 CU3A. - Likely 125 BTU. (Heat losses are roughly about 80-90 BTU)
Before we purchase it, we would love confirmation from those that have used it that it will work for us!

If Circuit 1 is DHW via an indirect tank
Circuit 2 is radiators, and
Circuit 3 (with a mixing valve) is in floor heating

Is is possible to have flow/heat to all circuits at the same time? Or do they have to run one at a time?

Is it possible to prioritize Circuit 3 (in floor) and only use Circuit 2 (rads) when 2 (in floor) has maximized it's load? (Or will we have to guess temperature and set the zones for this?)

Is this a common scenario? To want in floor, but not have enough square footage! What are the most common solutions?

Thanks!

Alan (California Radiant) Forbes

That's a great question! We have only used one CU3A and it's a great boiler because it's old school and has a high water content. We installed it in a house in Berkeley built in the 50's that had a combination of in-slab copper radiant downstairs, old finned tube in the joist bays in part of the upstairs and new Ultra-Fin for the other part of the upstairs.

You could also use the Viessmann Vitodens B2HB with a low loss header with equal results, but it sounds as though you have a good heating contractor making good recommendations.

Depending on your zoning, you can have flow to all your zones that are calling for heat. You can also stage the heating as you suggested: radiant when it's cold and adding radiators when it's very cold. I've never set that up myself, but I know you can do that with a 2-stage thermostat.

Jamie Hall

And remember that a BTU is a BTU. If the boiler has the capacity to output the BTUs you need to heat the structure, then what you are dealing with is purely a matter of plumbing and control -- which can get as simple or complicated as you want it to be.

STEVEusaPA

If you don't have enough floor space you can supplement it with radiant ceiling and/or walls.

Zman

I have done projects like yours using panel radiators sized for low water temps as the second stage. They look great and perform very well.
https://runtalnorthamerica.com/residential_radiators/baseboard_uf.html

GroundUp

I have not used that boiler, but the long story short is yes you can do as you projected as long as it's piped properly.

Paul Pollets

Viessmann boilers are usually set up to run the heating circuits when the temperature is below the WWSD.
DHW recovery will stop the heating circuits for the time the water tank is recovering. When the tank is recovered, the heating circuits will start, if they need to. DHW recovery can be programmed to run without priority, but the tank recovers quickly when the heating circuits are off. I've used this boiler more than a few times, and it has some advantages with it's larger water content and doesn't require a low loss header.

Andrea44

Thanks everyone for the confirmation of running both heating circuits at the same time, and the ideas for added heat sources!
I really appreciate it!

Andrea44

To confirm... the heating circuits can run simultaneously at 2 different temperatures? -- radiators at a higher water temperature and infloor heating at a lower water temperature.
Sorry if that's a simple Q. Our contractor is unsure.

Jamie Hall

Andrea44 said:

To confirm... the heating circuits can run simultaneously at 2 different temperatures? -- radiators at a higher water temperature and infloor heating at a lower water temperature.
Sorry if that's a simple Q. Our contractor is unsure.

Yes, they can -- but you have to pipe it so that the lower temperature of the two circuits (the radiant floor) has it's own pump, and takes it's suction through a mixing valve which is arranged to take some of the return water from the lower temperature return and mix it with some hot water from the main boiler loop to achieve the desired input temperature for the low temperature loop. That mixing valve should be controlled either by a floor thermostat or even better by outdoor reset.

Paul Pollets

HC1 would be the higher temp circuit, HC2 would be the 3 way mixing valve circuit (low temp). each has it's own circ pump. The Vitotrol 300 (or 200) thermostat/sensor is used for zoning.

EBEBRATT-Ed

@Andrea44

Basically when the indirect for DHW calls for the water the boiler will heat the tank regardless of what the heating circuit(s) are doing.

Then I would use a 2 stage thermostat as @Alan (California Radiant) Forbes mentioned above. For example if you set the stat at 70 degrees it brings on the radiant heat, if and when the temperature drops below 70 (say 68) second stage on the thermostat calls and brings on the radiators. When the space temperature rises that sequence reverses

When the boiler water temp is above 120 deg (and it will be at some times) then you must have a mixing valve on the radiant circuit to limit the temperature on that loop.

Alan (California Radiant) Forbes

Andrea44

Thank you! I did not know of 2 stage thermostat setting! I'm excited to move forward with that. I will post an update, and I'm sure a few more questions too!