Vitocrossal 300, CU3A: Parts longevity, next generation, makeup air heater integration...

rossn

Hello,

On the project at my home, I will have a variety of radiant heat... I designed some areas with 130AWT radiators, some areas have Warmboard R, and the basement a tubing-over-slab approach with insulation and concrete overpour.

Due to some issues with the current cast iron boiler, looking to replace instead of repair and my guy is recommending a Vitocrossal 300, CU3A.

Bunch of questions... sorry to be longwinded... any insight on any/all is great:

1. Looks like this generation is around 8 years old. Any idea how long Viessmann commits to stocking parts for this generation?
2. Does Viessmann have a regular interval for releasing new generations? i.e. Any idea if a new generation may come out in the next few years... I could limp along until then if needed
3. I am thinking of having 2 circuits: 1) mid-temp for the rads 2) the warmboard and over-pour slab radiant on the same circuit, as the price tag on an additional mixing valve and controls is more than I can budget right now. Is putting warmboard and slab on one circuit a notable mistake?
4. I also have a kitchen makeup air system and need to provide heat to temper the incoming air. Originally steered away from the hydronic approach, due to complexities, but the 50A service needed for an electric heater may require a service upgrade. Does the CU3A buy me anything for this? I read a comment here that in addition to the circuits there is also the ability to heat a hot tub, but wasn't sure if this also requires a full mixing valve setup, etc. Also didn't know if driving one emitter at high temp would require an overall higher system temp, and therefore drive down efficiency.

Thanks for weighing in!

GGross

I am a stocking distributor for Viessmann, I'll try to answer as best as I can

1. I am able to get parts for viessmann's earliest units that were available in the US still, including their first wall mount units.
2. They wanted to update the CU3A boilers with the new series of controls 2 years ago, I don't know when they would update this model for the US. I imagine there were more logistics related issues as they do have an updated version of this boiler available in some areas in Europe. The updates are cosmetic in nature as far as I can tell.
3. If you are not using the Viessmann motorized valves I would advise using a manual set mixing valve for your lower temp circuits. The boiler can accept 3 separate space heating calls, the boiler will fire at the highest temperature requested. You could at least get it so if only lower temp is calling the boiler would fire at low temp, it would be kind of "cheating" but it does work
4. Kind of the same answer as 3, you could do this (if the capacity works out) and set it as the highest temperature call, the boiler would only fire at this temp when that circuit is calling, you would need to purchase manual style mixing valves, or some other method of mixing for the other circuits, in case those lower temp zones call while the boiler is hot for the makeup air unit

rossn

GGross said:

I am a stocking distributor for Viessmann, I'll try to answer as best as I can

1. I am able to get parts for viessmann's earliest units that were available in the US still, including their first wall mount units.
2. They wanted to update the CU3A boilers with the new series of controls 2 years ago, I don't know when they would update this model for the US. I imagine there were more logistics related issues as they do have an updated version of this boiler available in some areas in Europe. The updates are cosmetic in nature as far as I can tell.
3. If you are not using the Viessmann motorized valves I would advise using a manual set mixing valve for your lower temp circuits. The boiler can accept 3 separate space heating calls, the boiler will fire at the highest temperature requested. You could at least get it so if only lower temp is calling the boiler would fire at low temp, it would be kind of "cheating" but it does work
4. Kind of the same answer as 3, you could do this (if the capacity works out) and set it as the highest temperature call, the boiler would only fire at this temp when that circuit is calling, you would need to purchase manual style mixing valves, or some other method of mixing for the other circuits, in case those lower temp zones call while the boiler is hot for the makeup air unit

Thanks for your insight, GGross!

Good to know on the parts... sounds like a non-issue, and would expect that from an upper level mfg. When did Viessmann start selling units in the US?

My radiant guy does use the Viessmann motorized valves. My current system is on an outdoor reset. I'm assuming by manual mixing valve you are either referring to some thermostatic adjustable valve or some physical valves... either way, I'm assuming I'd lose outdoor reset with that.

How does one chat the boiler into it firing only at the higher temp when there is a call? That might work well for the makeup air system which might only come on once or twice a day. I imagine that won't work well for my mid-temp emitters, as there will be quite a few... but maybe I should walk that back. They are all on TRVs... I'm wondering if they might be able to operate at the lower temp until the outside temps dip low enough, at which point they'd need to run at a higher temp. Assumption here is that the TRVs would allow more flow at the lower temp and with lower BTU need, they would emit enough heat. But, that also assumes the boiler controls are configurable to switch over the core heating temp (meaning primary circuit) below a certain temp outside. Do you know if the unit can do that? Then again, maybe that is overcomplicating... just thinking about efficiency.

GGross

They've been selling here for quite a long time, the first were actually imported from Canada, we still get some parts for those. They have had wall mount units here for around 20 years, and the cast iron stuff before that probably 30+ years.

Basically there will be spots to wire up to 3 space heating demands on the boiler. Each one of those demands can have it's own outdoor reset curve, or can be set to a setpoint temperature call. the boiler will just fire at the highest temperature that is calling at that time.

As for the radiators, you can put those on their own outdoor reset curve, no harm in trying them at a lower temperature and seeing if it works, the curve is relatively easy to adjust should it be a bit too low temp on design day

rossn

GGross said:

They've been selling here for quite a long time, the first were actually imported from Canada, we still get some parts for those. They have had wall mount units here for around 20 years, and the cast iron stuff before that probably 30+ years.

Basically there will be spots to wire up to 3 space heating demands on the boiler. Each one of those demands can have it's own outdoor reset curve, or can be set to a setpoint temperature call. the boiler will just fire at the highest temperature that is calling at that time.

As for the radiators, you can put those on their own outdoor reset curve, no harm in trying them at a lower temperature and seeing if it works, the curve is relatively easy to adjust should it be a bit too low temp on design day

Ok, thank you for sharing your knowledge.

To clarify, is there some additional circuit for heating a hot tub (saw that in a comment in a thread here), or is that misinformation?

GGross

Hot tub heating would require the use of a flat plate heat exchanger or some other means of heat transfer. There is no dedicated hot-tub circuit haha, weird that someone said that, I wonder if they used the DHW side of a combi boiler or something?

Alan (California Radiant) Forbes

In theory, you could heat your kitchen makeup air (small load) and hot tub heat exchanger at a setpoint temperature and your radiators and Warmboard on their own outdoor reset curve. If you've sized those radiators for 130F, I'd be comfortable with the same reset curve for both the radiators and Warmboard.

I would somehow prioritize house heating over hot tub heating unless love is in the air.

rossn

Alan (California Radiant) Forbes said:

In theory, you could heat your kitchen makeup air (small load) and hot tub heat exchanger at a setpoint temperature and your radiators and Warmboard on their own outdoor reset curve. If you've sized those radiators for 130F, I'd be comfortable with the same reset curve for both the radiators and Warmboard.

I would somehow prioritize house heating over hot tub heating unless love is in the air.

Thanks, Alan. No actual hot tub to heat in this case. I was thinking if it had a function for that, it might work for the makeup air heat. The kitchen makeup air heating load would be between 6k-40k BTUs, depending on outside temp and airflow.

When you say radiators and warmboard on their own reset curve, you're suggesting they use the same circuit? If so, I think warmboard puts a max water temp of around 105 to keep the floor < 85F. So, you are thinking to run the warmboard and radiators at that lower temp?

I might be misunderstanding what you're suggesting.

Alan (California Radiant) Forbes

No, you heard me right and maybe I'm wrong. Ultimately, it depends on the floor output you need and the R-value of your flooring goods.