PT100 cylinder probe wireless connection

[JohnMo]

Have ASHP and cylinder, currently operating on an on/off signal from thermostat, with a signal being sent wirelessly.

 

Want use the digital input terminals on the heat pump for a different purpose, so need to connect a PT100 probe. 

 

Does anyone know how I can transfer a PT100 probe output wirelessly to the heat pump - is it possible.  I have home assistant if that's any help

[Alan Ambrose]

Well anything is possible, but you probably won’t like the cost and/or performance. The problem is that RTDs are usually used as small-signal devices to avoid self-heating  - think tiny cables, screening, short distances to avoid noise pick-up etc. Sure, you can convert to digital and send IP/serial etc over wireless. But you’ll probably need a custom-made device to convert from digital back to small signal again with PT100 curve characteristics.
 

Any other input options to your HP?  RS485, Modbus, 0-10V etc?

Edited November 17, 2023 by Alan Ambrose

[JamesPa]

21 hours ago, JohnMo said:

Have ASHP and cylinder, currently operating on an on/off signal from thermostat, with a signal being sent wirelessly.

 

Want use the digital input terminals on the heat pump for a different purpose, so need to connect a PT100 probe. 

 

Does anyone know how I can transfer a PT100 probe output wirelessly to the heat pump - is it possible.  I have home assistant if that's any help

I spent quite a while looking for a ready made (or easy to make) solution to this one and failed.  So I ran a wire.  It was a bit if a pain but by going outdoors and through the loft no floors or walls were disturbed.

Edited November 18, 2023 by JamesPa

[JohnMo]

23 minutes ago, JamesPa said:

I spent quite a while looking for a ready made (or easy to make) solution to this one and failed.  So I ran a wire.  It was a bit if a pain but by going outdoors and through the loft no floors or walls were disturbed.

Starting to come to the conclusion, I either leave as it is; a wireless on/off switch, or I need to run a external cable.

[JohnMo]

Solved. Thanks to an unrelated post by @ProDave

 

Basically the heat pump using a NTC-10kΏ probe is just looking for a resistance, above a given resistance DHW starts below a given resistance it stops.

 

So the default settings are 44 degs heating starts, 48 degs heating stops. So long as the heat pump sees resistances in the range it expects it is sorted.

 

So solution is two resistors in parallel, the second brought into and out of circuit by a simple normally closed relay. Two resistors one 5.6kΏ always in circuit and a 15kΏ resistor switched in/out by the relay.

 

5.6kΏ probe is equal to 41 degs

5.6kΏ and 15kΏ in parallel is 4.1kΏ is equal to 51 degs.

 

So relay in a deactivated stated would have the resistors in circuit, powering the relay opens it's contacts and removes the second (15k resistor) from the circuit.

 

So now I can simulate a probe but use use wireless switching. Will cost more in postage than parts

[ProDave]

It will work, but you will miss out on being able to read the tank temperature.

 

I like on mine when someone has had a long shower and almost emptied the tank, being able to read the tank temperature on the ASHP display to see how it is doing re heating the tank and is it ready for another shower yet.

[JohnMo]

1 minute ago, ProDave said:

but you will miss out on being able to read the tank temperature.

That's not really an issue, I have a direct readout on the probe in the cylinder plus I have a further 4 gauges on the cylinder so I know where the thermocline is located.

 

My heat pump has a simple external on unit controller and no readout in the house.

[JohnMo]

On 17/12/2023 at 15:32, JohnMo said:

Solved. Thanks to an unrelated post by @ProDave

 

Basically the heat pump using a NTC-10kΏ probe is just looking for a resistance, above a given resistance DHW starts below a given resistance it stops.

 

So the default settings are 44 degs heating starts, 48 degs heating stops. So long as the heat pump sees resistances in the range it expects it is sorted.

 

So solution is two resistors in parallel, the second brought into and out of circuit by a simple normally closed relay. Two resistors one 5.6kΏ always in circuit and a 15kΏ resistor switched in/out by the relay.

 

5.6kΏ probe is equal to 41 degs

5.6kΏ and 15kΏ in parallel is 4.1kΏ is equal to 51 degs.

 

So relay in a deactivated stated would have the resistors in circuit, powering the relay opens it's contacts and removes the second (15k resistor) from the circuit.

 

So now I can simulate a probe but use use wireless switching. Will cost more in postage than parts

So took the time to install my simulator for a cylinder DHW probe and it works a treat. 

 

5.6kΏ resistor across two terminals of the wiring centre and a 15kΏ resistor connected to normally closed side of a relay.

 

A call for heat opens the relay contacts and the heat pump thinks the water in the cylinder is 42 degs, so starts a heat cycle. Once the thermostat on the cylinder is happy the relay contact closes and the heat pump thinks the cylinder is at 51 degs.

 

The cylinder and heat pump are not connected to each other a wireless MainsLink switch.

 

This has released a set of contacts that now allows a double set into be used and two WC curves.

 

 

 

62c460aeda4aa444DS-MAINSLINK-9.pdf

[jack]

Really nice to see this working.

 

This is almost exactly how I've been considering modifying my system to allow my (Loxone) home automation system to control tank heating.

[ProDave]

1 hour ago, jack said:

Really nice to see this working.

 

This is almost exactly how I've been considering modifying my system to allow my (Loxone) home automation system to control tank heating.

That was my original design  bodge, when I realised my LG ASHP had no "correct" way to turn HW on and off from an external device.