ABVs and Pump performance curves
- Thread starter Woodreeve
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- abv pump overrun
It's perhaps a fool's errand to try and specify an "adequate" flow rate during over-run: there are too many unknowns including boiler properties and settings, as well as the rate of heat loss from the bypass circuit itself.
In principle surely the higher the flow rate during over-run, the better. Beyond a certain point however higher over-run flow rates can only be achieved by also having some degree of bypass opening during normal (i.e. non-overrun) conditions as well. The point at which this occurs depends on the interaction between the characteristics of the bypass valve on the one hand, and the pressure/flow curve of the pump on the other. For pumps with relatively flat pressure-volume curves at low flow rates (which seems to include most of them) there may have to be a compromise between (i) the need for whatever is deemed sufficient flow during over-run and (ii) the accompanying unwelcome bypass flow during normal operation. This discussion assumes the pump is running on a "constant speed" setting - whatever that actually means in practice.
It also begs the question of what is an acceptable bypass flow rate during normal running. I can see no clear answer to that other than it being one that does not impede adequate heating of the radiators.
I suppose one should also consider the possibility that different makes of bypass valves might not all have the same degree of sensitivity to pressure changes. But as they all seem to rely on compression of a spring, and have an operating range of between 0 and 0.6 bar or so, it seems unlikely there is much difference in properties between them.
In principle surely the higher the flow rate during over-run, the better. Beyond a certain point however higher over-run flow rates can only be achieved by also having some degree of bypass opening during normal (i.e. non-overrun) conditions as well. The point at which this occurs depends on the interaction between the characteristics of the bypass valve on the one hand, and the pressure/flow curve of the pump on the other. For pumps with relatively flat pressure-volume curves at low flow rates (which seems to include most of them) there may have to be a compromise between (i) the need for whatever is deemed sufficient flow during over-run and (ii) the accompanying unwelcome bypass flow during normal operation. This discussion assumes the pump is running on a "constant speed" setting - whatever that actually means in practice.
It also begs the question of what is an acceptable bypass flow rate during normal running. I can see no clear answer to that other than it being one that does not impede adequate heating of the radiators.
I suppose one should also consider the possibility that different makes of bypass valves might not all have the same degree of sensitivity to pressure changes. But as they all seem to rely on compression of a spring, and have an operating range of between 0 and 0.6 bar or so, it seems unlikely there is much difference in properties between them.
Generally speaking, pump overrun is only required when all roomstats (or cylinderstat) are satisfied or the boiler is timed/programmed Off, if so, then its recommended that the pump overruns for a period of time, 3 minutes is often the default time, then circulation has to be maintained either by a ABV or if configured, reopening a zone valve for this purpose, another relatively simple solution is to use a NO (normally open) motorized valve as a bypass which closes when any zone valve opens.
As stated, ABVs do not work well with the modern A rated pumps as its difficult if not impossible to avoid some recirculation at all times which doesn't help boiler efficiency as the condensing effect is reduced because it raises the boiler return temperature.
A ABV installed as far away from the boiler/circ pump as possible with the pump running in CP mode may work reasonably well, because the constant pressure is only maintained at the pump and the head (pressure) falls the further down the line as the flowrate increases and vica versa, the head at the ABV will then be the same as the pump head when all zone valves shut so should open if set properly and reclose once the (a) zone valve reopens.
Thanks.
Presumably, if a normally-open MV is fitted instead of an ABV, it is run off terminal 10 (boiler SL) in the wiring box.
The only drawback I can see is if the MV fails in the activated (closed position). I have experienced this with normally-closed MVs. In the latter case, one tends to notice quite quickly because the system never shuts down. One might not notice so soon with a bypass MV failure, unless the boiler started kettling audibly.
I have recently had a 10 year old Vaillant ecoTEC plus replaced with an ecoFIT pure. While the Installation instructions for the ecoTEC plus specified that a bypass had to be fitted, there is no mention of this in the instructions for the ecoFIT pure. I have sent a query to Vaillant about this, and at the same time have asked whether (assuming a bypass is needed) whether they recognise an MV as being suitable. I would rather not invalidate my 10 year guarantee!
Yes, I would think off terminal 10.
There is AFAIK, no auxiliary (end) switch on these valves, the only way I can see it failing in the closed position is if it seizes in this position because if the power failed or the motor burned out then it will fail open.
There is AFAIK, no auxiliary (end) switch on these valves, the only way I can see it failing in the closed position is if it seizes in this position because if the power failed or the motor burned out then it will fail open.
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