Mixing Valve CV too low - does this approach make sense?

Conifer745

I recently passed inspection on my new heating system and fired up the combi boiler. For now I have a radiator zone serving my top floor which is working well. Each radiator has a thermostatic valve and I have the Alpha 2 set to constant pressure when the thermostat calls for it.   

  

I also have the main floor staple up radiant heating ready to go. The floor heating on the basement level will come later. I’ve quickly realized that I chose the wrong mixing valve during design to temper the water for my radiant flooring zones. I have a resideo thermostatic mixing valve with a Cv if 3.9. My design day flow to the 3 radiant zones (mixing valve outflow) with a delta T of 10 is 12 GPM. Main floor zone is a 7-circuit zone with a Alpha2 26-99 (40k BTU) and the basement is two 2-circuit zones at about 10k BTUs each, each serves by an Alpha2 15-55. All loops are 250-300’

  

For efficiency sake I plan to control the boiler temp with outdoor reset but I realized that is incompatible with a thermostatic mixing valve. I plan to change valve to manual Tekmar 3-way 711 with a Cv of 14. Is it ok that the valve is oversized or would I be better off trying to find a valve with a Cv around 10-12?

 Here is a pic of the current setup. Mixing valve with radiant zones are in the bottom left

EBEBRATT-Ed

The 1" Tekmar is a little borderline if you flow is only 10. Can you mix each loop separately? probably more money in the long run for valves though? Maybe a separate injection pump would better than a mixing valve.

Will see what Mr @hot_rod thinks

Conifer745

EBEBRATT-Ed said: Can you mix each loop separately?

That might be a good option. The 3/4” 710 with a Cv of 7 could handle the main floor zone with 8gpm max and then a 4Cv valve with the 2 basement zones at 4gpm max

Mad Dog_2

Nice, neat job.  What brand of Air Eliminator is that?  I'm a Spirovent man and it doesn't look like a Caleffi...Mad Dog

Conifer745

Mad Dog_2 said:

Nice, neat job.  What brand of Air Eliminator is that?  I'm a Spirovent man and it doesn't look like a Caleffi...Mad Dog

Thank you! It’s a Watts brand

archibald tuttle

@Conifer745 "For efficiency sake I plan to control the boiler temp with outdoor reset but I realized that is incompatible with a thermostatic mixing valve."

I don't think you necessarily want to think of those as incompatible because you can setback your rads as well, same basic idea but still a different temp from the floor.

that said, this is the kind of case for a buffer (direct or indirect) that maintains setback floor temps for constant circ while possibly providing some mixed return to support condensing at the higher rad temps. but if the buffer temp overruns during that function then you need to a second mixing/injection/or bypass function to decrease temp in floor. Buffers aren't cheap although I hover on craigslist for used indirect DHW tank(s).

In the NFN department, I tend to avoid the combi approach and just use a single purpose on demand for DHW although they are finally getting those designs better for the DHW. It has just got to be one really tiny house before extra couple of cubic feet and a little mechanical wall space for an on demand outweighs putting all eggs in one basket if you have the gas delivery capacity (and really doesn't cost much more in my experience because you are paying to get the DHW capacity stuffed into your combi).

And not to go all apocalyptic, but depending on the design temps for your rads I wonder if that is really going to challenge the PVC venting over time.

brian

Conifer745

@archibald tuttle thanks. So you don’t think mixing valves will work in this situation? Your concern is too low of a return temp to the boiler? Could I just mitigate that with a thermos bypass valve? It looks like someone has a 60 gal Amtrol tank near me for $250 so buffer could be an option

archibald tuttle

some kind of mixing is needed, bypass just another way of saying mixing valve, because in a closed system, to mix down temp, you have to have a cooler source, i.e. the return water. no concern about low return temp, concern about high return temp from the rads not being low enough for condensing leading to higher stack temps than that PVC should handle. i'm not sure about the strategy for DHW on that unit which could be another source of high temp if it is plate heat exchanger. iirc, some of the combis are using a preheat coil in the stack flow to handle that problem for DHW.

brian

hot_rod

Here is a 7 Cv valve at 12 gpm, cost you 6.79' of head. I'd go to the larger 3 way tekmar if in fact you need 12 gpm.

https://www.supplyhouse.com/Tekmar-711-1-Brass-3-Way-Mixing-Valve-4865000-p

Tim_D

An oversized (high Cv) valve may possibly hunt and an undersized valve creates unwanted head as HR pointed out. Its all about valve authority. You need enough of the total circuit head in the valve to prevent hunting but not so much as to create the need for a bigger pump. Usually 20% to 30% is good in residential applications.

Big Ed_4

The piping looks wrong , What is the direction flow of that Circulator next to the boiler ? What's with the boiler drains on the top loop ? Where are you going ? ........


Don't over think it... just a tip