bypass valve into a buffer tank?

hot_rod

This seminar is worth a watch to get some thoughts on 3 pipe buffers.

https://www.youtube.com/watch?v=OyYv4ApJfCc

I've got 5 different drawings on my desk right now on options for piping my a2whp. I have 3 inputs, HP, mod con, and ST.

Loads are the radiant floor, DHW, and one zone and a test wall for playing with product ideas.

The Viessmann VitoCal 100 comes with a 20 gallon buffer, 6 port tank, 1" connections, so I have good piping options. Or use the indirect as a buffer?

I'll shop for a Myson I-Vector 2 maybe for a quick cooling emitter, instead of waiting on the floor radiant.

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seanm10

DCcontrarian, you say

“It would have to be open whenever the flow through the zones is too low, and therein lies the rub: what is the control logic? Do you count how many zones are open, or try to measure flow directly, or something else?”

But isn’t this the exact situation that calls for a differential bypass valve? If the circulator is pushing against closed zones, it will open up and go through the buffer.
hot_rod, that Vitocal unit looks nice, they seemed to do a lot of work to package up everything you need all together. Its output at low temps falls a bit short for my needs, but I looked closely at it.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815418#Comment_1815418

So when all the zones are closed and the set point is satisfied my Chiltrix runs about 1.5 GPM through the buffer tank. If the heat pump circulator is pumping 5.5 GPM and the zone circulator is pumping 4.0 GPM, you get the exact same 1.5 GPM through the buffer tank. In the second scenario, you would want very much to close off the buffer tank, but in the first if you did the heat pump would throw an error and shut down almost immediately.

How do you distinguish between those two situations? It's not going to be by measuring what happens at the buffer tank, you have to measure what's happening in the zones.

Similarly, if all the zones are closed and the setpoint hasn't been met, the heat pump runs at about 4 GPM. If the heat pump circulator were running at 6 GPM and the zones at 2 GPM, you'd get the same 4 GPM going into the buffer tank. Again, in the first situation you need the buffer, in the second you don't. But no way of telling from the buffer.

Kaos

https://forum.heatinghelp.com/discussion/comment/1815398#Comment_1815398

These types of videos are good intro information but they never provide any real world data to back up how and how well it operates. Even our own little discussion here is a lot of hand waving, although DC Contrarian does occasionally throw in some mean spreadsheets.

From my limited experience, I have yet to see a hydronic setup that is not over-pumped. Any kind of parallel buffer with an over-pumped emitter section means the emitter will always get water that is heat pump output mixed down by the return water form the zone. You don't want this with a heat pump as you are loosing COP.

This is why I said the best setup is a single pump that is controlled by smarts (ie the one inside the monoblock) and the rest passive emitters/valves. As long as the emitters are not restrictive ,there is no chance for the installer to mess this up as almost all controls are handled by the equipment.

hot_rod

if it is a system zoned with valves or manifold actuators, a circ in delta p mode should prevent over pumping?

DCContrarian

Thanks for the kind words.

Yeah, a big part of the problem is that we're hacking the control of the heat pump, trying to manipulate its behavior indirectly because we don't have the ability to directly make it do what we want to do.

The circulator in my Chiltrix is capable of 10 GPM. Given that it's nominally 2.8 tons that's ample capacity. The reason I have to have a second circulator is that I can't make the one in the heat pump do what I want, when I want.

My understanding is that in Europe these heat pumps are commonly used in one of two very simple configurations. Both have no buffer tank, no zone valves, no thermostats.

Configuration #1: underfloor heat with outdoor reset. The water temperature changes with the outdoor temperature, that's the only control. To the extent the output of the floor is a mismatch for the actual heating load, the room temperature is allowed to fluctuate. Since the water temperatures are quite low — maybe 95F at full load — small changes in the room temperature lead to big changes in the delta between floor and air, and thus to changes in the output.

Configuration #2: air handlers with thermostatically controlled fans. Water circulates continuously through all of the air handlers, the heat output is determined by the speed of a variable-speed fan. This can be coupled with outdoor reset as well but doesn't need to be.

No zone valves, no thermostats other than in the air handlers. So long as the system has enough water volume to prevent short cycling no need for a buffer tank. If not, you can add volume with a 2-pipe buffer tank.

But the minute we start adding zone valves for more precise control we then need a second circulator and a buffer tank. And we get into a power struggle, where the thermostats on those zone valves should be controlling everything but the internal logic of the heat pump does whatever it wants, we just try to shape that behavior.

Ultimately what needs to happen is that the heat pump controller logic needs to be improved to bring in more information about the rest of the environment, like flow level in the buffer tank. But that's a job for the manufacturers. Until they do that, we have to do what we can.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1815398#Comment_1815398

I'm not a fan of trying to get domestic hot water out of heat pumps, but if that's what you want to do I would suggest using the VitoCal 100 differently. I'd put it in parallel with the other zones, on a zone valve, just another zone.

The problem with trying to use it as a buffer tank is you never know which way the flow in the buffer tank is going to go. If the heat pump circulator is pumping more than the zone circulator the buffer is getting hot water and the DHW coil there would be fine. But if the zone circulator is pumping more the flow is in the other direction and the buffer tank is getting cold water.

Kaos

This is the solar diagram from my unit:

Solar is used only for DHW heat, if the solar keeps the tank above the setpoint from the temp sensor on the heat pump, it will never switch to hot water heat mode.

For the rest I would connect the heat pump output directly to your floor heat, no zone valves. Adjust reset curve till the place is warm or if you unit has indoor temp sensing, use that for controls.

Backup heat is typically piped in parall with the heat pump. Mine has relay out to enable it when temp limit is hit.

Note, if floor heat is your primary heat source, you don't want the mixing setup shown in the diagram. The two way valve in the diagram is for disabling the panel rads and the floor heat for cooling so you don't have to worry about condensation control.

P.S. If you use a plate HX for your heat pump to domestic hot water, you could connect the solar to the coil in the tank.