Snowmelt boiler control

Smyth · February 2021

I posted recently about adding snowmelt to my existing radiant system. I have a few questions and am looking for advice on the best way to control the system.

The current set up is a combi boiler with a Taco 4 zone relay running two zone pumps. I wired the stats/pumps to the Taco and the TT to the boiler. Seems to be working fine 1.5 years later.

I am adding a second modcon boiler to make up the difference between the existing unit and the snowmelt requirement. The two units will be plumbed to a hydraulic separator and then to the zones. Basically how the Caleffi picture shows it.

What I'd like to do is wire this so the new boiler handles the existing zones, so there is no conflict when there is a call for DHW, and only fire the combi when there is call for snowmelt. Does that seem resonable?

I installed the socket for the Tekmar 090 but haven't installed the sensor yet, I also have a Tekmar 662 snowmelt control. What I don't have is strong grasp on how to integrate all of these things. Any help is appreciated.

Smyth · February 2021

So my last post was fairly vague. I've been reading up on the controls and have a couple of specific questions I am hoping to answer.

I plan on adding a staging control probably the 274 to control the boilers. The existing zones are wired through a taco SR504-4, the snowmelt will go through the tekmar 662.

First question: The taco and 662 both have a boiler output but there is only one boiler input on the 274. Can I just wire those both out to the one input? The taco documentation seems to show being able to wire two switching relays this way (in attachment). Basically I'd wire it like the diagram just replacing one of the relay outputs with the 662 output.

The alternative could be to treat the 662 as an expensive thermostat and wire it through the taco.

Second question: Both of the tekmar units have an input for an outdoor sensor, can one sensor be wired to be both units ( it appears to be same model 070) or should they be individual.

ratio · February 2021

If you only want to use the new modcon for snowmelt, why pipe it in with the other boiler?

Smyth · February 2021

I was actually going to do it in reverse, us the new modcon for everything and only turn the combi on for heat when there was a snow melt call. I was thinking that would keep both running.

New plan now after reading up on some controls, is to use the 274 to stage the boilers, so both boilers will be connected to all zones. Just trying to figure out the right way to connect the snowmelt control and the switching relay to it.

Zman · February 2021

Are you planning to use a heat exchanger and 50% glycol for the snowmelt?
I think you are going to need to sketch this out...

If this was mine and I had an existing system that worked perfectly, I think I would pipe the second boiler to the snowmelt only, run glycol through it with no heat exchanger and be done with it. The 662 would trigger the boiler and circ and life would be good

.

I am sure a clever individual could stage a combi and a condensing boiler and make it work, but....
Why would you. My head hurts just thinking about it.

Smyth · February 2021

A standalone boiler would have been the simplest I agree. I went this way for a couple of reasons. A boiler with enough output to handle the snow load by itself is considerably more expensive than one that just covers the difference. Second, was running a standalone boiler and the combi at the same time would put the gas supply over the limit depending on what else turned on. And I can pretend I didn't spend all of that money just to melt snow.

So, maybe I'll regret it but I am going to give this a shot.

For the controls, the 274 seems like it would simplify the whole thing. If I have it in setpoint and it gets a boiler demand from either the taco or 662 it will bring one online, if it doesn't meet the demand it brings on the other.

I am just not clear if connecting the taco and the 662 to the input of 274 is as simple as the diagram makes it look or if there is something I need to stop one from back feeding into the other.

Zman · February 2021

Hooking the 662 to the 274 is the easy part. Yes it will work as advertised.

The snowmelt is what is going to give you fits. When the snowmelt first comes on line, it will crash both boilers to well below 100 degrees and your house won't heat. In fact the snowmelt may actually pull heat from the house. What you are trying to do is far more complicated than you realize.

Using an on/off staging controller for modulating boilers will result in "dueling boilers" that will constantly fight each other. The correct controller will control the boilers modulation.
Tekmar probably has a solution that will limit the output to the snowmelt through delta t limiting or load shedding. The products you are considering don't do that.

I am a big fan of KISS (keep it simple .....)

Smyth · February 2021

I appreciate the help. The 662 snowmelt control has a 'max delta T' for the slab and control for a variable speed pump.

The radiant system isn't a primary heat source for the house. The forced air furnace meets the heat load requirements alone. The radiant is to make the basement more comfortable and to heat the garage when I plan to be out there. I don't follow how it can take heat from the house, from the cold return pipe in the condition space or inducing a flow in the other loops somehow?

Can you explain the "dueling boilers"? My thought was the slab is going to take everything it can get, so both boiler were going to be running flat out anyway. Is that not the case?

PC7060 · February 2021

@Smyth - are you planning to disable the house heating circulators when Snow melt call occurs; similar to priority override for DHW? You want to use both boilers because neither is sufficient on its own for the snowmelt application?

Smyth · February 2021

Yes, the plan is to make the snowmelt the priority. I need the output of boilers to meet the demand of the snowmelt that's right.

pecmsg · February 2021

What temperature water do you plan on sending into that snowmelt system?

Smyth · February 2021

The design calls for 140 degrees

Smyth · February 2021

Does the water temp matter to the controls or what is the reason for asking?

Anyway looking at the 275 control instead of the 274, it can control the modulation and has zone shedding as was mentioned earlier. For the zone shedding I don't know if I follow it's purpose in my application. It sounds like the point is to keep return temps high enough to avoid flue gas condensation in non-condensing boilers. I don't understand how this is different than shutting down the second stage when demand drops. Also, if both boilers are condensing low return temps shouldn't be an issue, right?

Going back to my question, reading the 275 manual it looks like I can run the taco switching relay with the space heating zones to the boiler enable input and the snowmelt to the dhw/setpoint input. If I understand it right this will let me set different temperatures for the two systems if I wanted. The other thing is the outdoor sensor, is anyone aware if I can connect one sensor to both controls? Though if the 275 is actually setting the temp it seems like it should be the only control that needs the sensor input?

Smyth · February 2021

I misunderstood the zone shedding, it's the opposite of what I typed above. If the demand is too high for the boiler plant it will drop zones until the boilers can meet the demand and return temps increase. Low return temps aren't necessarily a problem with condensing boilers but could indicate the system isn't keeping up with demand.

Thank you for pointing this out zman, I wasn't aware of that functionality.

Snowmelt boiler control

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