UFH pressure

[MortarThePoint]

I'm interested to know what typical flow and return pressures are in an UFH setup? Not temperatures, but pressures.

 

Here are some useful bits of info:

• Required peak flow rate, Qmax_litres_per_minute = (60 * ASHP_Power_kW) / (4.2 * Termperature_Drop_C)   e.g. (60 * 11.2kW) / (4.2 * 5C) = 32 l/min
• UFH power Watts/m2 = (3.34*(FT - RT) - 14.78) * 0.82^((pipe_centres_mm - 100) / 100)   e.g. Flow temperature of FT=40C, Room temperature of RT=18C -> (3.34*(40-18) - 14.78) * (0.82^0) = 58.7 W/m2   [derived from 1]
• Pump hydraulic power in Watts = (Q_litres_per_minute / 0.6) * (Flow_Pressure_bar - Return_Pressure_bar)   e.g. (32 l/min / 0.6) / (2bar - 1bar) = 53W (random pressures)

[Nickfromwales]

13 minutes ago, MortarThePoint said:

I'm interested to know what typical flow and return pressures are in an UFH setup?

None are typical, they're all the sum of the installation parameters? Length and size of pipe, number of 90o/180o turns, rated power of pump and so on.

Hence the reason each UFH manifold comes with a set of flow gauges that are adjusted during commissioning to balance the system according to its unique characteristics.  

[MortarThePoint]

4 minutes ago, Nickfromwales said:

None are typical, they're all the sum of the installation parameters? Length and size of pipe, number of 90o/180o turns, rated power of pump and so on.

Hence the reason each UFH manifold comes with a set of flow gauges that are adjusted during commissioning to balance the system according to its unique characteristics.  

 

Am I right in thinking the flow gauges act as a restriction to set a flow rate based on the inlet (aka Flow) pressure? Does that mean the majority of the pressure drop is across that restriction. If there is 0.4kW of heat coming from a 100m loop of UFH pipe, then with a dT of 5C the flow would be 1.1 l/min and pressure drop will be around 21mbar, so tiny.

[Nickfromwales]

1 minute ago, MortarThePoint said:

 

Am I right in thinking the flow gauges act as a restriction to set a flow rate based on the inlet (aka Flow) pressure? Does that mean the majority of the pressure drop is across that restriction. If there is 0.4kW of heat coming from a 100m loop of UFH pipe, then with a dT of 5C the flow would be 1.1 l/min and pressure drop will be around 21mbar, so tiny.

I had a ritual burning of my school requisite scientific calculator, and not when I left school.....but 3 days after buying it.

 

OK;

The restriction comes from the resistance against flow from each loop, unique as each one is. Balancing is required; when one short loop decides to allow the pump 'power' to simply run via that loop, ignoring all the higher resistance ones, as we all know about the path of least resistance etc etc........ aka "bypassing". So, if out of 10 loops each is 10% incrementally more resistive than the other, bear with, I've been drinking, then the one with least resistance would have 90% restriction, the next 80% and so on until the loop with the least resistance is 100% open. Ergo, the potential available from the pump would be equally divided amongst all of the loops, so they would each see the same l/p/m flow rate. 

What you would then do, is look at the loop lengths vs m2 of area they command, and then go at this again to incrementally increase the amount of flow the larger areas required, eg to ensure the correct amount ( fair share ) of heat energy was transferred to those loops which had the most amount of 'work' to do. 

Therefore, the restriction for the loop with the lesser duty would be high and vice versa.

 

Hopefully this makes some sense, if not, could you quickly drink 3 bottles of reasonable strength IPA and then come back with your next volley  ?

[MortarThePoint]

22 minutes ago, Nickfromwales said:

The restriction comes from the resistance against flow from each loop, unique as each one is. Balancing is required; when one short loop decides to allow the pump 'power' to simply run via that loop, ignoring all the higher resistance ones, as we all know about the path of least resistance etc etc........

 

Makes sense. I am planning to use the Wunda auto-balancing actuators. They look to maintain a 7C temperature difference when actuated so should make my life easy.

 

The loop itself doesn't seem to drop much pressure if you have a low system flow temperature, like 35C or 40C. The heat comes out relatively slowly so the flow rate has to be quite low (e.g. 1.1l/m for a 8m2 area supplying 400W of heat).

 

It feels like the flow gauge (or auto-balance actuator) is dropping most of the pressure in a low temperature system. I think you'd normally need to cope with the varying number of loops switched in and their relative resistances as you said. If using the auto-balancing actuators, it should be possible to have a low Flow pressure, well under a bar. The actuator could adjust to compensate for another loop switching in and out. Even if the manifold is a 50m round trip away, 28mm pipe should work with only 0.5bar (11.2kW makes for 32l/min flow which has a pressure drop of 244mbar over 50m of 28mm pipe).

 

I am thinking in part about the pump power consumption which if an 11.2kW ASHP system and running at a flow pressure of 1.6bar would be around 100W. That could be as much as 5% of the electrical energy (as the 11.2kW is the heat not the electricity which could be 2.2kW if COP is 5).

Edited October 17, 2022 by MortarThePoint

[dpmiller]

The flow pressure for the loops will not be anywhere near that. I've 600-odd metres over 12 loops and the pump is running at a head of 1.4m and using 12W in the process. I've no actuators installed so this is pretty much constant.

Now the ASHP's circulating pumps Main and boost) are probably taking about 60W at the minute, but the main internal one is on PWM control and consumption falls to under 30W when the heat requirement tails off.

[MortarThePoint]

I've forgotten all the corners in the UFH pipe which will up the pressure in the loop itself, but pressure dropped there should still be low.

 

11 minutes ago, dpmiller said:

I've 600-odd metres over 12 loops and the pump is running at a head of 1.4m and using 12W in the process

 

Is that at the moment or when running full wack? 0.14bar and 12W suggests a flow rate of about 40 l/min. I'm guessing you temperature drop is under 5C.

 

I've seen UFH checklist type websites that suggest pressure should be 1 - 2 bar but maybe they're wrong.

Edited October 18, 2022 by MortarThePoint

[Temp]

Our system is "vented" so neither flow or return pressure is more than about 6m head, about 0.6bar.

[dpmiller]

Loop flow is constant in this case. Frankly I haven't seen the system on long enough yet to see a stable Delta-T for the whole manifold. The loops are in a 50mm flowscreed and the system is *very* responsive.

[Nickfromwales]

If zoning, of course you then add another facet to the scenario where certain loops go from 100% of the available flow, to less or zero flow, which then changes the dynamic at the pump. This can be managed without intervention if an ‘intelligent’ variable-speed pump is used,   

[PeterW]

15 minutes ago, MortarThePoint said:

Is that at the moment or when running full wack? 0.14bar and 12W suggests a flow rate of about 40 l/min. I'm guessing you temperature drop is under 5C.

You can’t use a circulator power in this calculation as you are talking constant volume scenario and you would need a displacement pump to accurately understand the dynamic flow pressure within the loops. Loop pressure or static pressure is also irrelevant in this scenario - it is there to remove air from the system and fluctuates with temperature so again isn’t a constant. 
 

Your other issue is then when you use zoning or a loop reaches temperature that you will have a problem that your volume now changes as the loop is removed from the system - all these are reasons open plate circulators are used and not displacement pumps in heating systems. 

[Nickfromwales]

23 hours ago, PeterW said:

You can’t use a circulator power in this calculation as you are talking constant volume scenario and you would need a displacement pump to accurately understand the dynamic flow pressure within the loops. Loop pressure or static pressure is also irrelevant in this scenario - it is there to remove air from the system and fluctuates with temperature so again isn’t a constant. 
 

Your other issue is then when you use zoning or a loop reaches temperature that you will have a problem that your volume now changes as the loop is removed from the system - all these are reasons open plate circulators are used and not displacement pumps in heating systems. 

+1.... in a nutshell.

Kind of a pointless pursuit imho, but the question was asked  Switch the heating on, balance it all out at the switch on, go to pub, drink beer.