Modulating Boiler Question

obg8

So the minimum input of my Viessmann 100w B1HA is about 36,000 btu/h. (LP)

My Aquacoil outputs about 37,000 btu/h at 140 degree water

Is it a complete waste for me to have my heating curve set for less than 140 degrees or am I missing something? If the boiler is going to output 36,000 btu at the minimum and anything less then 140 my aquacoil won't be producing enough btu/h am I just wasting fuel?

Thank you for the help!

Zman

The boiler will begin to cycle if you turn down the temp.
I would run it at 140 and set the circ so that the return temps are less than 130. The boiler will be running continuously in it's efficient condensing mode.

obg8

I am basically wasting fuel if the boiler is heating the water to less than 140 degrees then? For example on my current curve if its 35+ degrees out that boiler water is definitely not at 140.

Zman

As long as the boiler is not short cycling you are not wasting fuel. You just won't make hot water very quickly.
Normally the DHW is set as a priority over the heat. When there is a DHW call, the boiler will stop heating for a bit and increase temp until the domestic is satisfied.
How is your system piped?

obg8

that's not really addressing my question, i have no issues with my dhw. lol

JohnNY

So, your question is basically "Is it wasteful to provide cooler water to a hydronic coil, which advertises 37,000 BTUs @ 140°F, when it is capable of producing more heat with hotter water?"

That's the same question as asking if a boiler that is too small for a house will run inefficiently. And the answer is sometimes.
It depends on the amount of heat loss on the demand side of the coil/radiators at various conditions, i.e. outdoor temperatures/wind/infiltration, solar gain, etc.

obg8

@JohnNY kinda... if my aquacoil is producing 37,000 btu/h with 140 degree water then logic would say its producing less with 120 degree water.

If my boiler is going to output a minimum of 36,000 btu/h would it make any sense to have that boiler water be less than 140 degrees?

JohnNY

Well, then you are asking if less than 37,000 BTUs is enough for what you're doing with your "aqua coil". Nobody here would know that unless you gave a great deal more information. First of all, by "aqua coil" I assume you're meaning some kind of heat exchanger. You would have to know what kind of demand is being put on the heat exchanger and under what conditions. Is it a fixed demand? It rarely ever is. Yours is not a question that is quickly answered.

njtommy

The aqua coil is just a hot water coil that just sits on top the ahu or in the duct work depending on style.

What CFM are you running? That’s a big factor as well. But yes the coil will produce less btus at lower water temps. Unfortunately it does you no good to try that because it will just short cycle your boiler.

Zman

Um, never mind. You are taking about a fan coil. I thought it was an indirect coil.

In the case of a fan coil, run it as low as you can t maintain room temp. Depending on how it is configured, you may get a wind chill effect from it if you go too low....

GW

What’s an aqua coil? I’m not super dumb, I sense that there a water filled coil in the conversation. We talking space heat or dhw?

GW

Obg8, just because your coil is rated for 37000 btu doesn’t mean it’s consuming and therefore producing 37000 btu. Does that make sense?

Gordy

My first question is this. Is the fan coil the only load?

njtommy

I see where your going with this @Gordy. ......

Leon82

To avoid short cycles , yes going below 140 may not help efficiency.

But you can drop to 135 and see how it behaves. And if it does not cycle and puts out adequate heat it will be more efficient than 140.

Gordy

All we have is a boiler with a 36k low end, and one “aqua coil” with a “rated output @ 140”. Is that it? If we use 140 you still see condensing, how much depending on coil delta. It won’t be the 100’s advertised efficiency.

Being matched in the 100’s low end modulation will add some efficiency value.

GW

How many wats does the blower consume to blow air by the coil? What’s the leakage factor on the Ducting?Always more to the picture. Saving 2 cents and spending 3 cents may not be the way to go

obg8

Sorry for the delayed response. The fan coil is the only heating load. Domestic is done by an indirect but that's irrelevant. My heating cfm is around 800 I can't remember exactly. My duct static is about .5 ... At 800+/- cfm and 140 degree water my heating coil puts out 37,000 btu/h. Just wondering if I should leave the boiler water at that no matter what to maximize fuel efficiency... Right now on outdoor reset water temp will be about 110-130 when it's 30+ degrees out.

Gordy

I think it’s the best you can do. You are running ODR. So water temps modulate with outdoor temps. In this case the boiler biggest load is indirect.