Mid position mishap valve help.

[Limestonecowboy]

Hi. Newbie !
Just bought a property as a project. Has an old oil fired (no gas here) mistral combi.

All was good now no hot water.

I’ve just given it the once over and ch heating works fine.

There’s no room stat ! (Not yet anyway) Just a timer control panel. And there’s no water cylinder.

I’m presuming the hot water is heated by a valve diverter in the boiler pushing circulating water through the thermal store and up through the heat exchanger.

There is a flow switch on the hot water out flow pipe from the heat exchanger, which appears to be working .

When hw demand is triggered by control panel and flow switch the boiler fires and heat exchanger is getting warm but cools quickly with cold water flow through. The thermal store also doesn’t appear to be getting very hot either. Just warm.

I’ve stripped everything down and all seems fine with the exchanger. Store and pipe work.

This leaves the 3 way diverter valve as the main culprit, which is located below the pump in front of the thermal store. It’s an old Honeywell 3 wire 3 way diverter.

The ball valve was freeish, but a bit sticky and perished, also the cogs on the motor mech were knackered, worn and almost stripped.

I’ve gone and bought what I thought was a replacement but isn’t ! . I’ve bought a Drayton MA1 mid position ! 5 wire. Rookie mistake.

Is there anyway to wire it to work as a diverter until I can get a replacement? Which is looking like a week !!

Bearing in mind I cannot take a feed from cylinder stat satisfied circuit to the grey wire because I don’t have one, just a stat dial on control panel. (I believe the grey wire needs 3 connections)

Can it be done ?

I’m an ex component diagnostic engineer so I know my way around a circuit diagram. But I’m wary that some of the connections on the valve become live depending on valve position so I need to know where to tie in.

Any help appreciated.
I probably need a new boiler but this thing is built like a tank and probably has some service to give. That I haven’t got seven grand for a new one.

 

[Johntheo5]

So, you just require a 230V signal to open one port only and then the spring return will return the valve to its original position (port B on a mid position valve). Suggest connecting the mid position valve grey&white wires together and connect the switched live to them, the valve will (should) then move fully over to open port A.

 

[Johntheo5]

For safety, put a connector block or tape off the mid position valve orange wire as thats live when the valve is fully over.

 

[Limestonecowboy]

Cheers Johntheo5. Thanks so much.
I’ll give it a go.

Do I leave the orange disconnected and insulate as The boiler will fire from control panel and the flow switch.

As I understand it I only need power to the valve to actuate it.

 

[Limestonecowboy]

You answered my last question as I was typing.

Lol

Great stuff. Much appreciated.
I’ll let you know how I get on.

 

[Limestonecowboy]

Valve swap working.
Still no hot water.

Temporarily hot water from tap for 10 seconds

boiler fires when demand from flow switch triggers. But rapidly stripping heat from exchanger. Water store below it hot.

Working on the boiler today I noticed that the pipes through the exchanger would occasionally and suddenly go cooler. The store was up to temp and no demand from hw or ch.

Then a relay clicked. And the pump ran for a few seconds and the heat exchanger rapidly became hot. Pump over run ?

There’s no continuous circulation from the thermal store into the exchanger when there’s HW demand and I believe that’s the problem.

So I have a good boiler, fully serviced and cleaned inside and out. Good burner, Good pump. All thermostats, flow switch, gauges, prv, overheat trip, 3 port valve, actuator, heat exchanger all replaced, or tested working.

This mistral unit has an old relay type control board. So I don’t know if the relay has failed and if so what my options are if it has.

Can I build a new hw system that simply uses an additional pump to push store water through the exchanger on demand?

I’d like to solve it without drastic intervention though. I need a shower !

Didn’t have a clue a week ago.
At this rate I’m going to know every in and out of old oil combi boiler systems before I’m done. But I do love learning. Even if its the hard way . My Easter weekend has been, erm educational


Tomorrow I’m going to check there’s voltage to the pump when HW is selected an flow switch demands.

 

[Johntheo5]

One of your schematics shows a mid position valve and a programmer with a HW OFF contact??.

Is the DHW supplied from a PHE (plate heat exchanger?, you should have a separate circ pump started by the flow switch for this which circulates primary water from the Thermal Store.
What controls the TS temperature?, does it run at the boiler flow temperature.
Post any details of the PHE.