Badly short-cycling indirect DHW, lots of data!

esandeen

Hi all -

I have a Triangle Tube Prestige Solo 60 modulating condensing boiler.
in a nutshell, 60kBTU input, 55k DOE, 47k AHRI, minimum modulation 25%.

It is used for space heating our home (mostly via cast iron radiators) and DHW in an indirect tank.

The DHW tank is a Vaughn 550TTPDW, 50 gallon, with a double-walled heat exchanger,

Grundfos Alpha 15-55F pumps, the DHW pump is set to "III" ("Fixed high speed").

The tank setpoint (with sensor connected to boiler controls) is 124F; the boiler setpoint for the DHW zone is 174F.

When there is a call for DHW, it short cycles like mad, as shown in this image of a cycle as monitored via Modbus (space heating calls are on either side of the DHW call.)



When the DHW pump is running, it estimates 3GPM (does that seem low?)


I don't know what to make of this; the only things I can change are storage setpoint, boiler setpoint, and pump speed, right? Does this seem mis-designed, or mis-configured? Any idea what's going on here?
Thanks,
-Eric

esandeen

I guess one other thing to note is that with the temperature sensor in the tank, the boiler uses an "adder" from the tank setpoint to boiler setpoint; max adder is +54F, so with 124F tank storage setpoint, that's a max 178F setpoint. If I use an aquastat (switch) on the tank instead, I can set the boiler DHW setpoint as high as 188F.

hot_rod

What size piping between the tank and boiler. I think those tanks have a small diameter coil, the manual should indicate pressure drop of the coil. Measure the temperature the tank coil and return. With the delta t and flow rate you can calculate btu moving. Although those flow indicators are not 100% accurate. More flow or higher temperatures are the two ways to increase performance if it is piped and controlled properly.

hot_rod

Which tank? Notice the tank performance is rated with 200 SWT

esandeen

1" pipe, lots of right angles which probably doesn't help? I'll be honest, the insulation was my doing, maybe I should remove it and let it scrub off some heat so the boiler can keep running?
It's the 50 gallon top-mount tank.

I don't currently have direct measurements at the tank inlet/outlet but at the boiler, the setpoint is 174 and return bounces from 169 to 174 and just kind of hangs out way up there while the boiler cycles. Close enough as a proxy for inlet/outlet? I could measure directly at the tank.

But as the tank gets close to the DHW setpoint of 124F its like it just can't dump any more heat in at any reasonable rate.

esandeen

Do I have it right that if the SWT is 174, return is 169, that's 5F delta T, and at 3GPM that's 7,500BTU/hr, so with a minimum input modulation of 16,000BTU/hr, it's going to short cycle unless I can up the flow rate, or raise the SWT for more heat transfer?

But with the temp sensor config, I can only set a +54F adder to the DHW setpoint so I guess I can't raise the SWT is effectively limited by the tank setpoint. Maybe flow is all that's left, and the pump is already at max.

hot_rod

1” piping should be able all that boiler can output, properly piped. How old is the tank? When coils scale over they get a bit constipated.
Delta will be wide when the tank is cold and tighten as it nears set point.

esandeen

About 5 years old but it's done this since the beginning.

HomerJSmith

Dumb question. Are you sure that the input and output is correct on the indirect? What happens when you try fixed speed or fixed pressure?

The boiler has to create the heat energy, the pump has to move the heat energy, and the tank has to absorb the heat energy. A screw up in any one of these will short cycle the boiler.

esandeen

Ooh, checking input-vs-output sounds wise, I'll do that, thanks for the idea!

I believe the pump is currently on fixed (high) speed. I could try fixed (high) pressure.

EBEBRATT-Ed

Is there any kind of differential built into the tank set point? If so can it be widened?

When the boiler gets a dhw call is it modulating down?

esandeen

Tank connections look correct. I had high hopes for that, dammit!

EBEBRATT-Ed said:

Is there any kind of differential built into the tank set point? If so can it be widened?

When the boiler gets a dhw call is it modulating down?

I have 2 options for the tank: an aquastat on the tank and a dry contact, in which case the setpoint and differential is set at the tank, or a temperature sensor in the tank connected to the boiler, in which case the setpoint and differential is set in the boiler controls. I'm currently set up with the latter. I think I have it at 8 or 10F differential right now.

If you look at the graph in the first comment, you can see the boiler firing rate (yellow) as well as the set point (blue) and the return temperature (red).

If the tank is very cold after a long draw, the boiler is happily running somewhere in the middle of modulation. If the tank isn't too cold, the return is nearly at the boiler setpoint and the boiler is modulated way down, and cycles on/off. Is that what you're asking?

I could widen the differential and get a longer run during the first part of a call, but regardless, whenever the tank is reaching the setpoint, it seems like heat transfer slows and the boiler starts cycling.

EBEBRATT-Ed

I suppose you could put a mixing valve on the DHW outlet from the tank (some states are I think requiring this now) where you run the tank at 140 and the mixed water at 120. This gives you more capacity (more stored energy) in the indirect.

If you did this you could raise the tank set point and widen the differential setting giving you a longer run time.

Not sure if this is the best approach

Maybe you need a buffer tank