Boiler control

[Steve]

Is there a device available off the shelf that will switch an output (for example a boiler) only when it recieves more than one input (for example zone valves).

Just an idea I've got in my mind about having proper control over the heat in every room of the house, but obviously you can't just open one zone valve for one radiator and fire up the boiler onto it.

So some control would be necessary to ensure more than one valve was open before the boiler fired up.

 

[EFLImpudence]

Is there a device available off the shelf that will switch an output (for example a boiler) only when it recieves more than one input (for example zone valves).

That is how most boilers work - thermostat and timer.

Just an idea I've got in my mind about having proper control over the heat in every room of the house, but obviously you can't just open one zone valve for one radiator and fire up the boiler onto it.

Yes you can.

So some control would be necessary to ensure more than one valve was open before the boiler fired up.

Bit lost there
It all depends what you want to do and how it is connected.

 

[bernardgreen]

I am in the process of creating a control system for the heating system in my cottage.

In theory if any one zone ( or radiator ) valve is open then the boiler needs to fire up. In practise it is not so simple as the radiator must be able to dissipate the heat that the boiler is producing otherwise the boiler will start cycling ON and OFF. A good boiler will modulate its heat output down if the return water ( from rads ) is too hot suggesting they are not dissipating enough heat. But there is a limit as to how low the boiler can go so the minimum amount of heat that the selected radiators can dissipate must be greater than the boiler's minimum output to prevent cycling ON and OFF.

 

[Steve]

So some control would be necessary to ensure more than one valve was open before the boiler fired up.

Bit lost there
It all depends what you want to do and how it is connected.

one zone valve per radiator so that each room can have accurate timed and thermostatic control.

I understood that letting a boiler run on one radiator would cause cycling as most boilers will not modulate down to only 1kw or so and would be inefficient.

What I want is for the boiler to only come on when more than one room is calling for heat.

 

[AndyPRK]

You don't normally control it down to one room.

Maybe control it to a floor or couple of rooms with a zone valve.

The thermostat opens the zone valve, and the micro switch then activates the boiler and pump when its open.

 

[bernardgreen]

I understood that letting a boiler run on one radiator would cause cycling as most boilers will not modulate down to only 1kw or so and would be inefficient

Three years ago I had a Reno HE16 Heat Only (Johnson and Starley )installed and from memory that modulates to around 1 Kw but that model is no longer produced. When only one small radiator is on line the HE16 runs without cycling and the flue gases are almost cold to the hand.

What I would say is take time over the design, what I thought was the "perfect" design needed a few tweaks and changes after living with it for a couple of years. My cottage is small and my life style is OK with 16 Kwatts in the middle of winter. If you need a large boiler but also need to modulate heat down below the that boilers minimum output then consider two boilers. Complicates the plumbing and control but it has the added advantage that a single boiler failure does not leave you without heating.

 

[Jackrae]

Contacts wired in series from 2 zones valves will do what you want. However if you want "Any-2-from-N" then you will either need a complex relay system or a multi-input logic controller. You might be far better using decent control valves on your radiators. Out of interest, what's your objection to your boiler cycling in a controlled manner.

 

[ericmark]

One zone valve per radiator is exactly the same as having TRV on each radiator and that is what the modern boiler is designed for.

Either built into the boiler or near the boiler there should be a by-pass valve. The idea is with all radiators off hot water returns to the boiler is turns it off.

After a set amount of time the boiler switches on and if water returns hot it extends the time before trying again.

If water returns cold then it reduces the time before next try and runs flat out until the return water gets warm. As the return water gets hotter and hotter so it reduces flame height until it reaches minimum when it turns off.

Some times a timer or thermostat is added the latter placed where it will get morning sun so on a prospective hot day it stops the house warming up so there is no need to use the air conditioning unit latter in the day.

Where the problem lies is where some one puts the thermostat in the coolest room instead of where it gets morning sun then something else happens like wind coming form opposite direction. I have this problem with mothers house for some unknown reason there is no TRV in the hall where the wall mounted thermostat is mounted.

If the prevailing wind swaps direction then the hall gets hot before the TRV's in other rooms have started to work and so all rooms are cold only the hall gets warm. Quick cure is open internal doors to let hall cool.

If only one radiator needs heat then likely the by-pass valve will lift slightly and you will get a mixture of hot feed water and cold return water which will reduce the flame hight in the boiler.

Be it a zone valve on a thermostat or a TRV with a LAN head connected to thermostat or a simple TRV the effect is the same. However with all zone valves you could connect all the micro switches in parallel so the boiler does not have to test fire to see if required.