Proposal: limit boiler demand from "low-priority" rooms

[candlerb]

Hello,

I have a feature request that I'd like to present for discussion.

Short version: don't turn on the boiler when there is only demand from an "unimportant" room, to avoid wasted heat through bypass radiator.

Here's the long version!

Background

I have a very traditional system: gas boiler, hot water circuit and tank, heating circuit. There are 13 radiators: 12 with control valves, and one permanently-open bypass radiator in the hallway, close to where the old thermostat is.

Now I've fitted the Wiser Hub (2 channel), and replaced 10 of the TRVs with Wiser Smart Radiator values. Two unused rooms remain on TRVs.

I don't care about the temperature in the hall, so I've put the Wiser room thermostat in the living room.

The problem

One of the rooms with a Wiser Smart Radiator value is the downstairs bathroom. This has an undersized towel rail as its radiator, and it takes a long time to heat up - see the attached temperature plot.

Once the rest of the house has reached set temperature, the boiler can remain firing for 2 or 3 hours more just for this one room. During this time, the bypass radiator in the hallway is also heating - and as I mentioned before, I don't really care about the temperature here. This is basically wasted heat.

I'd rather the bathroom were a bit colder, than to fire up the boiler just for this room.

Workaround

For now, I've decided to replace the Wiser radiator valve with the old mechanical TRV. It's actually "smarter" only to heat this room when the boiler is firing for another reason.

Of course, I now can't vary the set temperature on a schedule, but this is no worse than the old system. In effect, the bathroom radiator now acts like another bypass, but which turns itself off if the set temperature is reached.

Proposal

I would really like to be able to use the Wiser valve properly!

What I want is for this bathroom to heat "opportunistically". That is: if the boiler is firing for some other room already, then by all means let it heat; but don't fire up the boiler just for this room.

Idea 1

This is the simplest option which expresses my requirement.

Have a toggle switch for each room:

Boiler demand: (*) Normal    ( ) Disabled

(defaults to "Normal").

Rule: the CH relay only closes if one or more rooms with "Normal" priority are demanding heat (or if any room has boost enabled)

Idea 2

This is a slightly more sophisticated take on the same idea. Have a slider control for each room:

Boiler demand: 0 <======> 1
(in 0.1 steps, defaults to 1).

Rule: the CH relay only closes if the sum of the boiler priorities of all the rooms requiring heat is 1 or more (or if any room has boost enabled)

This is more flexible, but perhaps harder to understand. For example, you could set some less important rooms to 0.5, and then if any two of those rooms demand heat, then the boiler will be activated.

Idea 3

Have a slide control for each room

Boiler demand offset: <Normal -0.5 -1.0 -1.5 -2.0> degrees
(defaults to 0).

Rule: each valve opens as normal when below the set temperature. However the boiler is only switched on if the temperature is "offset" degrees below the set temperature.

Let's say I set this for -2.0 degrees in the bathroom: the boiler will only fire for this room if it is at least 2 degrees below the target temperature. However, if some other room is already calling for the boiler, then the valve will open and heat the bathroom as normal (until the set temperature is reached).

Conclusion

Thanks for taking the time to read this.  I think this would be a nice addition to the system - but I do realise every feature has to be carefully evaluated for its benefit, versus the complexity seen by the user.  And maybe I'll just have to live with the old TRV!

[Erisian]

I strongly support your request.

I am baffled that a 'basic disable call for heat' is not already built in.

I would be happy with just a simple toggle.

[PhilHornby · Accepted solution]

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@candlerb wrote:

Short version: don't turn on the boiler when there is only demand from an "unimportant" room, to avoid wasted heat through bypass radiator.

Proposal

What I want is for this bathroom to heat "opportunistically". That is: if the boiler is firing for some other room already, then by all means let it heat; but don't fire up the boiler just for this room.

I have the same "issue" - also with the downstairs toilet. The room has two walls, on the north corner of the house, so it's the coldest room, left to its own devices. I have an Oil boiler - so no boiler modulation happening. I fitted an iTRV here - rather than leaving the mechanical one in place - precisely so it could keep this room warm. I know I'm paying over the odds for this in Oil though.

In my case, it doesn't take long for it to come up to temperature - but it does have a proper radiator. I did wonder about treating it and the adjoining Utility Room as a single room (which would then just take the average of the iTRV readings).

I believe you can choose, in some systems, whether iTRVs can demand heat, or just limit the temperature. If you're just limiting though, it hardly seems worthwhile (to me) swapping out the mechanical valves. 

[masonben]

This is not a solution. The solution would be to implement a “only heat if there is demand from another stat” mode.