The Efficient Home Forum

I have just completed an Evohome installation controlling both heating and hot water via an S-plan.
The Evohome controller interacts with the S-Plan control rather than with the boiler or pump directly.
I understand how the heating system works.
I am looking to set the hot water parameters under the installer's menu.

I understand the functions of the HW temperature and the hot water differential.

However what exactly does the hot water overrun setting do?
I offer two possible explanations below but they are just my ideas of what mighty be sensible explanations.
I have included more details than may be needed to answer this question.
I do so, in the hope of reducing any confusion over exactly how the system is set up.

The boiler is a Baxi Megaflo System 18.
This boiler is wired with a permanent live and has an integrated pump.
The S-plan controls the boiler by activating a switched live.
Put simply the Evohome calls for heat or HW; this opens either a heating 2-port valve or a HW 2-port valve.
Once open, the action of opening the valves triggers a switched live cable to the boiler.
This switched live, first turns on the pump and, after a period of time, the boiler fires.
When the switched live turns off, the boiler pump has a built in and fixed 3 minute overrun.

If I set the hot water overrun to, say, 5 minutes, are either of the following statements true (they can not both be true!)?
Note: I have arbitrarily selected 5 minutes so as to be different from the 3 minutes of the actual pump overrun and hence to make any explanations clearer.
1. The Evohome controller sends a control signal via the HW BDR1 to turn off 5 minutes after the hot water reaches the set HW temperature?
In my case this will just mean that the water continues to be heated for 8 minutes after the HW temperature is reached and will likely lead to hotter water than required.
OR
2. The Evohome software calculates the rate at which the hot water is rising in temperature and hence the likely rise in temperature over an anticipated 5 minutes of overrun.
The Evohome controller sends a control signal via the HW BDR1 to turn off 5 minutes before it thinks the HW temperature will be reached.
In my case, this would likely lead to the HW not quite reaching the required temperature as the actual overrun is only 3 minutes.

If statement 2 is true, I would obviously set the HW overrun to be 3 minutes to match the boiler and the system would work OK.

If neither statements 1 or 2 are quite correct, please can someone explain exactly how the HW overrun works.
Since in most set-ups the Evohome controller just turns BDR1s on & off, ideally I would like the explanation in terms of when the HW BDR1 is turned off.

The idea of the hot water overrun is that the hot water valve remains open for the set duration after the 'boiler demand' relay has been switched off. This is to stop heat blocking of the boilers heat exchanger - or so is the theory.

I never use it, neither do I get my customers to.

Thank you. I too will set the HW overrun to zero minutes.

This said, I can see the merit in allowing a boiler to dissipate the heat when demand is shut off.
The problem with a typical S-plan or Y-plan is that the cessation of demand is implemented by act of closing the valves thereby cutting the obvious channel for heat dissipation.

In my case, the Baxi boiler has a 3 minute pump overrun.
With the valves closed, I can only assume that the pump pushes hot water around the boiler's internal bypass, which the boiler manual claims is installed.
I trust that this is enough dissipate any excess heat.

The control wiring would need an overhaul to cut the boiler whilst leaving the valve open for a period of time to dissipate heat.

An internal bypass is not always sufficient, so always check with the boiler manufacturer when on an S Plan or S Plan Plus system!

fg61044 wrote: Put simply the Evohome calls for heat or HW; this opens either a heating 2-port valve or a HW 2-port valve.
Once open, the action of opening the valves triggers a switched live cable to the boiler.

This switched live, first turns on the pump and, after a period of time, the boiler fires.
When the switched live turns off, the boiler pump has a built in and fixed 3 minute overrun.

If your boiler is fired by the switch in the zone valves then you must be using a 2x BDR91 relay configuration, with no "boiler control" relay configured.

In that case the hot water overrun feature cannot be used, because there is no way for the Evohome to shut off the boiler but keep the hot water zone valve open. The setting will still be visible but it won't work, so in your case you should set the overrun to zero minutes.

To use hot water overrun, you need a 3x BDR91 configuration with a dedicated boiler relay, as I do, (or OpenTherm + 2x BDR91) so that the Evohome has independent control over boiler firing and the hot water zone valve.

Without this the rest of your post is somewhat moot, however I'll reply anyway to satisfy your curiosity.

If I set the hot water overrun to, say, 5 minutes, are either of the following statements true (they can not both be true!)?
Note: I have arbitrarily selected 5 minutes so as to be different from the 3 minutes of the actual pump overrun and hence to make any explanations clearer.
1. The Evohome controller sends a control signal via the HW BDR1 to turn off 5 minutes after the hot water reaches the set HW temperature?
In my case this will just mean that the water continues to be heated for 8 minutes after the HW temperature is reached and will likely lead to hotter water than required.

Because you have no 3rd boiler control relay this is probably what will happen, yes. Normally with hot water overrun the boiler control relay turns off exactly when the hot water temperature is reached and the hot water zone valve relay waits an additional X minutes before turning off. If there is no boiler control relay (I haven't tested this) I suspect it will still delay the turnoff of the hot water zone valve relay and because this is indirectly also controlling the boiler it will lead to overshoots. So set it to zero if you have no boiler control relay or OpenTherm.

OR
2. The Evohome software calculates the rate at which the hot water is rising in temperature and hence the likely rise in temperature over an anticipated 5 minutes of overrun.
The Evohome controller sends a control signal via the HW BDR1 to turn off 5 minutes before it thinks the HW temperature will be reached.
In my case, this would likely lead to the HW not quite reaching the required temperature as the actual overrun is only 3 minutes.

No it doesn't work this way, there is nothing predictive about how it works, its a simple timer that delays the turn off of the hot water zone valve relay (but not the boiler control relay) after the set point is reached.

Also it wouldn't make sense for it to work the way you describe - the purpose of the hot water overrun is to allow the water to keep circulating between the boiler and the hot water cylinder for a few minutes after the boiler stops firing so that latent heat can be transferred from the boilers heat exchanger into the cylinder. This is to prevent the flow temperature of the boiler itself from overshooting the set flow temperature if the boiler is going full blast dumping heat into the cylinder and then suddenly the flow is shut off by the hot water zone valve and only a small loop of water is flowing between boiler and automatic bypass valve. (or perhaps even an internal bypass)

Whether this causes a problem is highly dependent on the boiler and system design. On a modern low thermal mass heat exchanger boiler operating in a pressurised CH system its probably not an issue, but on an old vented, high mass (cast iron) boiler system like mine the temperature overshoot of not having the hot water overrun can lead to kettling in the boiler when the hot water demand is satisfied..

If neither statements 1 or 2 are quite correct, please can someone explain exactly how the HW overrun works.
Since in most set-ups the Evohome controller just turns BDR1s on & off, ideally I would like the explanation in terms of when the HW BDR1 is turned off.

As above, you need a boiler control relay for it to work, but assuming you had one:

1) If there is no heating demand, (the boiler relay would otherwise be off if it wasn't for hot water demand) when the target hot water temperature is reached the boiler relay turns off immediately but the hot water zone valve relay stays on for the configured number of minutes then turns off as well. There can be a small overshoot of the hot water temperature maybe 1-2 degrees depending on the thermal mass of the boilers heat exchanger as the latent heat is transferred to the cylinder. (I see about 1 degree typically)

2) If there is a simultaneous heating demand that is causing the boiler control relay and heating zone valve relay to come on, when the hot water temperature is reached the boiler relay stays on (if heating demands it) and the hot water zone valve closes immediately. Essentially hot water overrun is disabled if there is simultaneous heating demand as there is no way for it to turn off the boiler as heating is demanding the boiler stays on, and if it were to still apply the hot water overrun time to the zone valve relay it would cause a significant hot water overshoot. Also there's no need for hot water overrun when water is also flowing through the radiators as they will take away the latent heat of the boiler turning off. Hot water overrun is only needed when there is no heating demand and the only alternative flow route after the hot water zone valve closes would be a bypass valve.

I have mine set to 1 minute, and I find it helps stop kettling if I have the flow temperature set above 70 degrees but only have hot water demand. (EG heating turned off) But then I have an ancient, vented, cast iron boiler that has a tendency to want to kettle when pushed to its limits anyway.

Hope that explains things.