Hydronic circuit type

[Socrates5]

Hi everyone,

I would like someone's opinion on the secondary hydronic circuit approach that I have chosen for my design.

I have a heating system with a heat pump that serves as a heat generator (primary side) and it's decoupled from the secondary side by a buffer vessel. There are 4 secondary circuits, one of which requires lower flow temperature that the other circuits. Primary HP flow temp and three secondary circuits operate at 45C flow temp, while the fourth secondary circuit requires flow temp of 35C.
The secondary side is a pressurless circuit (no circulating pump between the buffer and the secondary circuits) and each secondary circuit has it's own VSD pump making them constant temperature, variable flow type circuits.

Based on my knowledge, there is just one way to set up the presurless, different temperature secondary circuit and that's via the three port mixing valve with premixing. This setup works and is recommended when the primary and secondary flow temps are different (in my case 45C and 35C respectively), and when the secondary side is a constant flow, variable temperature circuit.

My issue is that my secondary side is variable flow, constant temperature circuit and correct premixing is hard or impossible to achieve given it is a variable flow circuit. So, I have decided to go with a common mixing circuit (three port valve). Given it is a variable flow, constant temp circuit, this three port valve will always stay more or less open in a same position. It will always divert same amount of water through the bypass port. The part I'm unsure about is if this is a good solution from the control perspective? I know 3 port mixing valves are chosen and selected to work throughout the full openable range of valve, but this will not be the case in my setup.

Any thoughts?

 

[LittleInch]

Can you draw this please as my head is spinning from all the words....

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[Socrates5]

Hi LittleInch.

To draw which part?

 

[LittleInch]

The system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Socrates5]

Here

 

[LittleInch]

Looks ok to me providing that there is enough heat loss from your 35C system to make it work well. What is your anticipated return temp from that circuit?

Or put another way, what is the anticipated flow volume percent from the 45C supply vs the return line flow?

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Also: If you get a response it's polite to respond to it.

 

[Socrates5]

The mixing circuit will operate at 35/30C
33% from 45C primary supply vs 66% from 30C secondary return

Heat consumption in the secondary circuit is not the issue, my primary concern is suitability of a simple mixing circuit for the circuit that operates at lower temp from the primary circuit. I know it will work, but how well, that's' what I'm worried about - valve control instability and hunting.

 

[LittleInch]

If the controls go via a PLC or similar then you can tune out wild fluctuations. If the heat flow is steady then the system will become steady state by manipulation of the PID control loop. Might go a bit mad when you first turn it on but with some adjustments should settle down if changes are relatively slow acting.

How sensitive is the load to temperature, i.e. what is your dead band for temperature + or - from a feed temp of 35C?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Socrates5]

PLC (DDC is used in building services) or any kind of control cannot rectify improper selection of a control valve or wrong hydronic setup. This issue boils down to valve characteristic and assuring the valve operates throughout it's full stem range.

Not sure about the dead band, I believe it's usually 0.5C

 

[Alistair_Heaton]

My heatpump system is setup the same.

My ufh is the one that varies when the fan coil circuit is running at 50 degs.

You get built mixer units that do it mixing return to adjust the temp.

When fan coils turn off then the output hp feed regulates to the 35 Deg max ufh so there is no mixing.


If you do a search for vrc700 valliant it's manual has a load of system diagrams in it and there is one for this.

I think my mixing group was 120 euro.

 

[Socrates5]

Alistar,

So your UHF is running at 35C and HP supplies 50C?
Is the mixer unit a unit with the three port valve AND a mixing pump? Do you have a pic or schematic of that unit?
Is your UHF circuit constant volume circuit? Do you have three port valves at the UFH manifolds or any other way of controlling the flow to the manifolds?

My system is 650 kW so a stock control solution won't cut it

 

[Alistair_Heaton]

Oh only other thing is the pipe and the diameter for your feed to the 4. Go big otherwise cavitation may occur.

Some of the ufh loops vary the pump volume to maintain the 3 degs delta T on the return.

If the other 3 are rads with pumps running at max chat they can over power the ufh input.

 

[Socrates5]

Noted on the cavitation, I keep that in mind. In my experience it's usually more the issue of expansion vessel placement.
Starving the other circuits shouldn't be an issue due to pump control via differential pressure sensors

Thanks to both of ye guys

 

[Alistair_Heaton]

It's only certain heatpumps that do this 3 Deg delta T. As a fellow mechie I can sort of understand it when you have an open loop and the pump has weather compensation in play. But when your pulling from one buffer it doesn't compute.

Also look if cooling is going to be in play. I set mine up for that with the fan coils.

 

[Alistair_Heaton]

Btw I cheated and put three expansion vessels in.

There is one on the heatpump by design which is meant to do the primary loop.

I put a another in on the hot feed at the end of the secondary and one on the return from the buffer.

Touch wood I had a bit of an argument with the installers over it but I have had zero issues with pumps or valves. The service technician said I was one of the few on his circuit that was still on his first set without issues.

I suspect more from gut feel and luck I managed to solve the cavitation issue and pressure spikes than knowledge.

 

[Socrates5]

Three expansion vessels... Jesus... There's your issue right there.

Probably no cavitation because your static pressure is high enough and the pressure losses ere really small

 

[Alistair_Heaton]

I don't have issues, the others do.

It has an electronic expansion vessel on the back of the heat pump.

And the other two for me which are more normal.

I might add these are set up for cooling so there are nrv's in the mix to stop convective heat transfer between the cold buffer tank and hot water. If you don't have two you end up with no expansion vessel on the hot water loop to the cylinder.

 

[Alistair_Heaton]

The heatpump rollout locally to me in Europe is an utter mess, Especially the wet systems.

Is it 1 ufh circuit and the rest rads?

 

[Socrates5]

The mixing circuit is for FCUs and the other circuits are serving AHUs, rads and heater batteries