Pump overrun while valves are shut

[akist]

Could anyone please offer some advice?

I have :
a conventional boiler, one pump, two valves (CH and HW), one room thermostat, one HW cylinder thermostat and one timer module.

When the room or the HW thermostat opens, immediately the corresponding valve shuts off, while the main pump is operating in the "overrun", forcing a very tight, and short loop around the boiler and through a pressure valve that closes the loop.

I would like it so that the valves remain open while the pump is operating thereby never going through the pressure valve. Is that possible by altering the wiring connections, and is it common or is it avoided?

 

[Madrab]

I presume by a pressure valve you mean an auto bypass? If it's set up as an S plan then that is the way it should work.

You mention that one valve closes when the other stat calls for heat. The only case where that should really happen is if the system is in a Priority HW system (PDHW), so when the HW calls then the CH shuts down, the HW 2 port opens and the boiler ramps up to max output. Once the HW is satisfied then the HW stat shuts down its 2 port, the boiler winds down and the CH opens up. It shouldn't happen if the CH calls at the same time as the HW.

 

[akist]

Thanks, yes "an auto-bypass" seems to be it. And I presume it is an S plan. Two independent 2-port valves, in parallel, one valve controlling the CH loop and the other controlling the HW loop.

The two valves operate independently of each other. They can be both open or closed or any combination.

The boiler is not being told why it's being asked to fire, is what I think. I have asked to have full flow temps when HW is needed and low flow temps when CH is needed, but I was told it is not possible. However I can modify flow temps from the boiler's front panel so it must be theoretically possible. I also think that perhaps when HW is on, CH ought to be off, especially if the boiler had switched to high temp flow, but again I am not sure of the possibilities.

All this while I have purchased a Wiser kit 2 ( main hub plus some radiator valves and a room thermostat ) and want to explore the possibilities.

 

[Madrab]

When the room or the HW thermostat opens, immediately the corresponding valve shuts off

The two valves operate independently of each other. They can be both open or closed or any combination

Ah ok ... this is where the confusion came from. The 2nd behaviour is A-typical of an S Plan.

However I can modify flow temps from the boiler's front panel so it must be theoretically possible

You want Priority Domestic Hot Water but that doesn't mean the boiler can support a smart system control that modifies the boiler flow temperature automatically when the HW is calling. This is done though a control protocol, the main one is called Open Therm (OT) though other manufacturers implement this control through their bespoke protocols (e.g. WB - EMS).

What boiler do you have - make - model

 

[Mottie]

I have asked to have full flow temps when HW is needed and low flow temps when CH is needed, but I was told it is not possible.

I’m not a heating engineer but I can only choose one flow temp for when the boiler is running whether it be for heating or hot water. I have a Vaillant 418 EcoTEC plus boiler using hive controls. I set my flow temperature at 62° which is fine for the heating and water. HOWEVER, if I were to purchase and fit the Vaillant controls, I could choose different heating and hot water temperatures. I personally don’t think it worth the expense though.

What boiler do you have?

 

[Harry Bloomfield]

The boiler is not being told why it's being asked to fire, is what I think. I have asked to have full flow temps when HW is needed and low flow temps when CH is needed, but I was told it is not possible. However I can modify flow temps from the boiler's front panel so it must be theoretically possible. I also think that perhaps when HW is on, CH ought to be off, especially if the boiler had switched to high temp flow, but again I am not sure of the possibilities.

Your system (external to the boiler) doesn't support making use of the two temperatures, you would need to upgrade the controls to a much more sophisticated system, to include that. Once upgraded, your boiler will only be able to serve either, the CH, or the HW, at one time, with HW heating having the priority.

 

[akist]

I have a "Worcester Bosch Greenstar 8000 Life 40kW Regular Gas Boiler"

Worcester technical support have told me that it is not possible to tell the boiler to increase flow temperatures. We even discussed if it might be made possible with a new boiler motherboard. Which does not yet exist. I was puzzled because you can adjust flow temps from the display panel on the boiler, so definitely some functionality already exists.

 

[MNW67]

When the room or the HW thermostat opens, immediately the corresponding valve shuts off, while the main pump is operating in the "overrun", forcing a very tight, and short loop around the boiler and through a pressure valve that closes the loop.

I would like it so that the valves remain open while the pump is operating thereby never going through the pressure valve. Is that possible by altering the wiring connections, and is it common or is it avoided?

Are you basically saying that you want the zone valves to stay open for the time (often three minutes) that the pump is on overrun?

 

[Harry Bloomfield]

I have a "Worcester Bosch Greenstar 8000 Life 40kW Regular Gas Boiler"

Worcester technical support have told me that it is not possible to tell the boiler to increase flow temperatures. We even discussed if it might be made possible with a new boiler motherboard. Which does not yet exist. I was puzzled because you can adjust flow temps from the display panel on the boiler, so definitely some functionality already exists.

To make use of the separate HW versus CH capabilities of the boiler, then you would need to install the specific control system for the boiler. For instance, I have a similar system, though Vaillant, which I installed and set up myself....

A heat only boiler. I added the fancy Vaillant controls to it. A module in the airing cupboard, connecting to pump, 3-port valve, cylinder actual temperature sensor - that connects via a data bus, to the boiler, plus either the bus to a wired control, or in my case a wireless control, plus wireless outdoor sensor, with a wireless unit in the boiler. The contol then takes care of everything, replacing time-clock, room temperature sensor, and cylinder heating temperature.

Instead of the boiler running flat out, until the stat clicks off, it has the boiler gradually modulate down, so it precisely brings the house to the set temperature, no more, no less, then perfectly holds it there, working at maximum efficiency. The valves remain open, and the pump running, long enough to dispose of the boilers heat too.

The control system is much easier to install, than a more basic, older system.

 

[akist]

Are you basically saying that you want the zone valves to stay open for the time (often three minutes) that the pump is on overrun?

Exactly that. I think it is one minute in my case.

 

[Madrab]

This is just getting even more confusing. You said that you want priority hot water (PDHW)?

I have asked to have full flow temps when HW is needed and low flow temps when CH is needed

If so why do you want both valves open at the same time? That will not give you PDHW.

You don't need to have both valves to give the pump what it needs on overrun, that's the autobypass' job, i.e. provides min flow through the boiler.

 

[Harry Bloomfield]

Exactly that. I think it is one minute in my case.

Your problem with that is....

The way your system presently works is - the time-clock, feeds the two thermostats, the thermostats feed the valves, the valve (when open) signals the boiler to run. The boiler itself, feeds the pump, so it can run after the boiler is shut down, to cool it down.

There is no way for either valve to be open, without the demand going through to your boiler, with your present, basic control system.

 

[stem]

I was just trying to workout how to describe that and struggling a bit...... Good explanation @Harry Bloomfield

 

[Mister Banks]

The only way to do what the OP wants is to replace the CH motorised valve with a normally open valve to give a PDHW setup but without the ability to change flow temps in the boiler.

 

[ScottishGasMan]

1) Your boiler doesn't support priority hot water with variable flow temperatures. So without a new boiler & compatible control system that's out the window.

2) You can't rig your valves to stay open as its the open valve that tells the boiler to run, not the Cylinder or Room Thermostat. The thermostats open the valves and the valve has an internal switch which it presses to activate the boiler. So keeping the valve open for say 2 more minutes would mean the boiler fires for 2 more minutes and still needs its overrun afterwards. (You could actually rig it to do this, but it would be some pretty complicated wiring and relays for zero gain)

3) Your system sounds like its working exactly as it was designed to do. The auto by-pass is doing its job allowing the pump overrun a loop to circulate round so that it can level out the flow/return temperatures after use. There is no reason to change it from how its working.