Hydronic circuit type
Hydronic circuit type
(OP)
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?
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?
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RE: Hydronic circuit type
Remember - More details = better answers
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RE: Hydronic circuit type
To draw which part?
RE: Hydronic circuit type
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydronic circuit type
RE: Hydronic circuit type
Or put another way, what is the anticipated flow volume percent from the 45C supply vs the return line flow?
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RE: Hydronic circuit type
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.
RE: Hydronic circuit type
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.
RE: Hydronic circuit type
Not sure about the dead band, I believe it's usually 0.5C
RE: Hydronic circuit type
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.
RE: Hydronic circuit type
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 :D
RE: Hydronic circuit type
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.
RE: Hydronic circuit type
Starving the other circuits shouldn't be an issue due to pump control via differential pressure sensors
Thanks to both of ye guys ;)
RE: Hydronic circuit type
Also look if cooling is going to be in play. I set mine up for that with the fan coils.
RE: Hydronic circuit type
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.
RE: Hydronic circuit type
Probably no cavitation because your static pressure is high enough and the pressure losses ere really small
RE: Hydronic circuit type
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.
RE: Hydronic circuit type
Is it 1 ufh circuit and the rest rads?
RE: Hydronic circuit type