John Carroll

The available volume for any accumulator is (1-((prechargepressure+1)/(chargingpressure+1)) X accumulator volume) For example, in a nonpumped accumulator, if 1.0bar is sufficient for your needs then a 200L accumulator will be precharged to 1.0bar, and assuming a fully charged accumulator from mains at 3.0bar, then the available volume on mains failure is 1-(1+1)/(3+1) X 200, equals 0.5*200, 100L A pumped accumulator like yours, assuming you require a minimum pressure of say 2.0bar would be precharged to 2.0bar and assuming the pumped charging pressure is 4.0bar, then the available volume is, 1-(2+1)/(4+1) X 200, equals 0.4*200, 80L. but remember then, because the diaphragm will now have bottomed out that you you won't get a single drop of any extra water even if 0.5bar is enough in a emergency.

I thought I replied to this, anyway, That pump setting of proportional pressure PP1 is not suitable, apart from being far too low, for UFH, suggest setting to constant pressure CP2, (LED on far right, no4), this is a constant pressure of 3.0M, then set your flowmwters to 2 to 2.5 LPM and see how it goes.

Yes John, The fixed speed is settable, 50% to 100%, and a few tests showed a (vaillant) flowrate of (converted from LPH) 24.3LPM at 100%,and 11.53LPM at 50% which makes good sense, since flow is directly proportional to speed. The dT tests did nothing, there is also a constant pressure option, settable between 100mbar and 400mbar but the tests were inconclusive as 400mbar gave ~ 23LPM but 150mbar didn't produce a much lower flowrate whereas it should theoretically, have been, 23*sqroot (150/400), 14.1LPM.

The rads flow/return are 60C/50C, mean of 55C, so a, (55-20), T35 Rad, which will emit (35/50)^1.3, 63% of a T50 rating, irrespective of what is going on elsewhere, you then have ( UFH), 3.84LPM@32C mixing with (rads), 5.73LPM@50C to give 9.57LPM@42.8C goint to LLH secondary but because the boiler circ pump is circulating 21LPM then the primary side recirculation of 11.43LPM@60C is mixing with that returning 9.57LPM@42.8C, you end up with a boiler return of 21LPM@52.2C. As you allude to, a LLH can becomes a DLH, (Dead Loss Header?) unless the primary and secondary flows are equal., hence the desire to get that dT control working, I have read of other conventional installations (no LLH) where that boiler doesn't achieve this either.

This is what someone is trying to do (if Vaillant can sort out this dT control problem). There is a rad(s) demand of 4kw and a UFH demand of 7.5kw, a boiler flow temp of 60C is required to give the required rad output, this means the boiler return is 52.2C, 7.8dT, not too bad, but just about condensing, this is with a boiler circ pump flowrate of 21LPM, if the boiler dT was increased to 16.5C, then the boiler return temp should be ~ 43.5C with quite considerable condensing, this is with a boiler circ pump flowrate of 11.5LPM, if my calcs, attached, are correct. LLH + Rads and UFH Rev0.xlsx

Don't know if new cylinder is installed but just for interest, OSO technical inform that its cylinders do not have a baffle so removing the dip tube and installing a EV to give that ~ extra 20/25% extra capacity does not leave something to bang about inside.

What is the ufh demand in kw, how many loops?. Coil rating is often based on a flow temperature of 80C and maybe 30/35 LPM flowrates so ensure both coils are suitably rated.

Looking at the OAP's pump it seems to have just 3 speeds so should be just a case of pressing the setting button (arrow) and try speed I, first check and see if its running at setting III.

That makes sense, I think those settings are 4M,5M,6M&7M, so try the 4 or 5m curve.

It states above, "Without PWM signal, the pump runs at maximum speed" so if there isn't another cable plugged into the pump PWM socket and it looks like there isnt, then that pump will be running at a 7M head so can't be set to one of its other heads down to 4 or 5M?. These UPM type pumps seem popular with UFH systems, wonder why, as all the normal type can run in all the different modes/settings. most don't have this PWM socket but its very rarely needed.

Unless this pump has a external (PWM) signal then it will only run at the highest speed, a 7M head. This will more than likely cause noisy operation IMO especially when coupled with Evohome control which continually throttles the motorized rad valves, don't know what the cure for this is.

UFH by its very nature has to run with relatively low temperatures to avoid damage to floor surfaces, so is sized accordingly, Rads are sized on a T50 basis so a required room temperature of 20C would require temps, flow/return/dT, of 75C/65C/10C at whatever the lowest OAT they were sized to. Most of my own would just about deal with a OAT of -10C, consequently I would need to oversize by a factor of at least 2 even with at a OAT of 5C or so and by a factor of 2.5 at a OAT of 0C to enable running with lowish ASHP temps, see attached. ASHP WC Extract.xlsx

Came across this sometime/somewhere where some one was (ASHP) running their rads on WC of 43C@-10C & 35C@15C, must have had mighty big rads, if I ever installed a ASHP I reckon if I installed a extra rad in 3 downstairs rooms the I could probably run reasonably comfortably with a more aggressive WC slope. ASHP WC.xlsx

Can you identify which pump is serving the CH (rads only) by just requesing rads only on and then take a photo of the front of the pump showing its LEDs, which should show what head the pump is producing.

In another thread you also had a Grant ASHP 17 KW, did you have noise while this was installed or is it at another location?