Changing Polyplumb Thermostat for Manual Thermostat with Bathroom Air Sensor


Postby umpers » Tue Mar 15, 2016 10:15 am

Hi.
Im looking to change my existing Polyplumb digital room thermostat ( PBPRP) with a simple manual thermostat.
It is a 2 wire installation with a facility to add an air sensor cable ( 2 wires) to wet room next door.
Any suggestions please as to a suitable manual replacement of any make for this configuration that can take an air sensor for wet room as above ??

Many thanks, Mark
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Postby ericmark » Wed Mar 16, 2016 10:28 am

Much depends on how wired most thermostats suffer from hysteresis one or two have it built into the thermostat to learn so swapping one which has some anti hysteresis software for one without it may mean it also needs relocating.

Also the one you have can work with under floor heating and with under floor heating you need to monitor floor temperature as well as air temperature.

So without a detailed wiring diagram showing how wired at the moment plus a plan of the house sorry no easy answer. Most people would want to change in the other direction.
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Postby umpers » Thu Mar 17, 2016 12:20 pm

Hi
Thanks for your reply.
Need to give this some more thought then. Not quite as straightforward as i thought......
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Postby ericmark » Fri Mar 18, 2016 12:37 am

I bought a cheap wireless programmable thermostat that instead of switch central heating on/off it has 5 temperatures per day so it turns temperature down at night rather than off.

Unlike most the unit was free standing I could put it where I liked. And I found the position was so critical. My son however got an intelligent thermostat either hive or nest and with that one the position was far less critical as it learn by how much it over shot and adjusted to compensate.

This is the problem in most houses near the radiator it does not over shoot but tends to set room on the cool side, away from radiator gets room to set temperature but then it over shoots.

The normally method with water filled radiators is to fit a TRV to the radiator they are marked 1 to 6 no attempt to say what the temperature is as centre of room temperature will vary house to house. The draw back is you can't really set so down stairs heated in day and upstairs at night.

There are some programmable heads for the TRV which will allow times to be different room to room, however it can upset the boilers built in anti cycle software so is a bit hit and miss if it works with your boiler.

The honeywell evohome redresses this problem each TRV head talks to the main controller which in turn tells the boiler when to run. The bees knees but be seated when you look up the price.

In other countries they use hot air central heating, I actually had a house in Wales with this fitted, it circulates the air so this problem with over shoot does not happen. You can close the output vents manually but there was no real way to control rooms independently. However this system works well with the hive or nest thermostats as a single thermostat controls the whole house. The hive and nest however don't really work well with water systems. They do allow you to remotely turn on and off the heating but don't allow each room to be controlled independently, it would seem only the honeywell evohome does that.

I considered either a cheap cheerful thermostat like I got for around £65 or full honeywell evohome the half hearted systems like nest and hive just seem a lot of money for some thing which can only control one room.

Your thermostat is some where between the hive/nest and the full honeywell evohome it does monitor the outside temperature and has some very good systems built in. The problem is not heart of winter or hight of summer it's spring and autumn where if it is going to be a warm day you want house kept cool as once heated up very hard to cool again the best is not to heat it up. The outside sensors should do this.

What I wonder is why you want to get rid of it. What have I missed other than taking it to next house don't understand why you would want to replace it. Also out of interest what type of heating do you use? I would guess some underfloor heating?
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Postby umpers » Sun Mar 20, 2016 7:56 pm

The house only has UFCH throughout and the Polyplumb thermostat was already in situ when i bought the house . It is situated outside the bathroom with the floor sensor cable from it sitting in a box inside the bathroom on the wall and not into the floor itself.
The only reason i want to change the Polyplumb digi thermostat is i find it very difficult to read the screen. Im after either a manual or a very basic digital thermostat that is basically easy to operate but obviously must have provision for a floor sensor cable to be connected. I never use the timer or settings functions on this one so a billy basic type is all i need.
Many thanks.
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Postby ericmark » Mon Mar 21, 2016 8:58 pm

I am assuming electric under floor heating? This can have a huge load so only thermostats designed for under floor heating can be used. Without knowing the load sorry can't answer the question.

It will not be cheap. I have under floor heating in mothers house in the wet room and it is simply not used. It was a total waste of money. The sensors have blown and the controllers have blown many times. Last time I gave up and never repaired it.

The efficiency of electric heating other than inferred and heat pumps is based on the time taken to heat up. So if we for easy maths assume a room is used for 3 hours and it takes an hour to warm up it is classed as 75% efficient. If the same room is used for 1 hour then 50% efficient and same room for 9 hours then 90% efficient. The off peak electric is the worst however the power costs half that of normal power so that is a special case. Next you have guessed it is the under floor heating as it takes so long to heat up.

Using clever thermostats which allow for the warm up time do help, but to be honest they don't help much. Only when the family is at home 24/7 can it really work because the heat up time is so long.

So you would need a clamp on ammeter or similar to work out power used and you may need more than just a simple controller. So the big question is it worth it.

Google "UFH thermostat" and there are quite a few start around £50 up to around £80 for the basic model. They use words like "Intelligent Control" however my experience is not really what I would call Intelligent control.

As a job I had to heat pipes in a controlled way to 700 deg C at 100 deg per hour hold for 2 hours then cool at 50 deg per hour and the cooper heat controller was really an intelligent controller, It worked out the lag between switching on and the pipe getting hot and compensated for that lag. It would inch up to the point where it changed to hold and also the point where it started to cool and work out the delay. However most of the so called intelligent central heating controls just simply switch off as the temperature is reached there is no mark/space ratio change to ensure no over heat.

In the main the controller has three functions a clock, and two thermostats it heats the floor to 28 deg C and regulates it at that temperature until the room is warm enough, it is simply to thermostats. In the main contacts are rated at 16A.

Once the floor hits 28 degs the relay is switching on and off all the time depending how close the sensor is to the heating cable. This switching on and off all the time means life of the thermostat is very short. With the more expensive type however they use solid state switching a large field effect transistor and their life is 20 times longer.

We got one 25 year warranty, it failed so I returned it, then they said unless fitted by one of their authorised dealers the warranty was void.

You can google as easy as me. But I would consider first if throwing good money after bad. Sorry
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