Andrew Gabriel wrote in
news:vfbibq$288jk$1@dont-email.me:
> On 21/10/2024 13:21, Theo wrote:
>> Regular mechanical TRVs aren't able to call for heat, they can only
>> control flow from a boiler that's already running. If the boiler is
>> off but one room is cold, it can only get colder even with the valve
>> wide open. If the boiler is running it'll be wasteful if all the
>> TRVs are throttling the flow. If a TRV can call for heat, it can
>> request the boiler run to actually change the room temps, but only
>> when needed.
>
> I've mulled this over in the past.
>
> One thought is to have a bypass loop with a pressure differential
> bypass valve, set to open only when all the rads are off. Also in this
> loop, include a flow switch. Any flow detected in the bypass loop can
> kill the demand for heat signal for, say, 10-15 mins. Obviously, you
> need the pump to run to detect if all the rads are closed.
>
Nice idea although I feel that with current boiler modulation ranges that
the bypass will want to come on sooner than on the smallest radiator
calling for heat.
> If I was implementing this, I would have the pump operate momentarily
> perhaps every 15 mins and checking the bypass flow switch, just to
> test if there's a radiator open and it's worth actually demanding heat
> from the boiler. This will depend how integrated the pump control is
> with the boiler.
>
Neat. On my last boiler change (we both used to run Keston Celsius) I
went heat only with fully modulating Grundfoss Alpha pump outside,
although I don't think it is as clever as they make out.
> In practice, boilers are not great at driving just one rad at a time,
> and you really want some logic which says something more like, when
> there are enough zones calling for heat to absorb a significant
> proportion of the boiler's lower output power, then demand heat,
> unless there's a zone sufficiently far below its setpoint which can't
> really wait for more zones to call for heat before demanding heat from
> the boiler.
>
If you have the room I think the most sensible way of coping with small,
and particularly unsynchronised, heat demands is to use a thermal store,
heated by a boiler or some other means. I have one zone per room
capability here that has not been fully implemented for fears of boiler
cycling. I have a 200 litre store waiting to go in so that I can smooth
out the bumps and fully implement it.
> Having a setpoint range for each zone (which can also be used to
> implement hysteresis) gives more flexibility. When one zone drops to
> the lower acceptable setpoint, demand heat for that zone, and when the
> boiler has got that part of the heating circuit hot, bring in more
> zones which are below their upper setpoint and coming closest to their
> lower setpoint, such that the open zones can dissipate the boiler's
> lowest modulation power. I did hack this up in software controlling my
> system.
>
Yep, neat again but it's not for Joe average.
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