Wireless Room stat and Thermal Store

[retiredbri]

Hi, we moved house 2 years ago and are very pleased with the Thermal Store that was already installed. Running costs (y/e 2015) for a 4 bed large house built 1991 are £70 per month total for elec (including elec oven) and gas (for ch and hob).

I have installed a wireless programmable stat that just controls the pump (alpha) to the rads/trvs. It's a very quiet system.

The tank stat (set to 76deg C) controls the boiler that is set to 82degC. The boiler controls the separate pump (for over run etc). It too is very quiet.

The question relates to the wireless stat and in particular to the thermostat proportional bandwidth (was this originally hysteresis?).

With the stat set to say 19degC, the ch pump starts and the room heats up. The pump will not stop till the stat reads 19.7degC. Once off, the room cools to 18.3degC before the ch pump starts again. Although a lot better than the original wall stat in the hall, we were hoping for less of a room temperature swing.

We could not get at the fixed wiring to alter hall stat and being older bodies, move the stat to the bedrooms at night to save going downstairs when we wanted a warmer "morning" bedroom.


Are programmable stats (wired or wireless) optimised for boiler controls (cycling etc) so as to not be so suitable for thermal stores? Or do all stats have this wide “dead band". I think that the 210 litres of very hot water in the store provide a buffer against short cycling that would be essential with a boiler/pump/rad/trv system.


Hope someone can help here.


retiredbri (was electronics engineer - not a heating expert)

 

[Phill Peck]

What thermostat have u fitted.
Some have adjustable (let's call it) sensitivity

I can see your theory on the cycling but I doubt this is the case. Usually this is controlled by the boilers own anti cycle programme

 

[D_Hailsham]

Proportional bandwidth and Hysteresis and completely different.

Hysteresis is the difference between on and off temperatures. On modern digital stats it's 0.5 to 1.0 degC.

PB is the temperature range within which the TPI function is operative. On most thermostats it is 1.5 degC. When the room temperature is below the PB (i.e 19.5C for a 21C setting on the room stat) the boiler, or pump in your case, will run continuously. When th room temperature enters the PB the boiler (pump) will be switched on and off at a rate which depends on the difference between actual and desired temperature. The closer the actual temp is to desired, the shorter the on time.

Another factor is the cycle rate/time. This is set to 6 cycles per minute for a gas boiler, giving a cycle length of 10 minutes. The on time and off time add up to the cycle time. To complicate things further, the cycle time can be lengthened by the logic of the stat.

You say the pump is not stopping until the temperature is 0.7C above the set temperature. How are you measuring the temperature?

 

[retiredbri]

You say the pump is not stopping until the temperature is 0.7C above the set temperature. How are you measuring the temperature?

Thanks - I can see the difference now between Hysteresis and PB.

The temperature is shown on the stat display. The receiver for the stat has a red "on" light and when this on, the Alpha Pump green light is also on. I can only think that the programmable stat I have doesn't have PB.
This is because even at stat set temperature, the red light (and pump) stays on and only goes out when the stat shows [set temperature +0.6 degC]. The lights do not show any sign of the cycling.
But, not having PB with a Thermal Store shouldn't be a problem since stopping and starting the pump would not do anything.

The programmable stat is a Time Guard TRT307.
The "span" can be 1 degC or 0.5 degC. From my experience, I think this is +- 1 degC or +-0.5 degC.

I will look at better sites for the stat. (Because it is easy to move, so far I have just put it on shelves in the lounge and bedroom). I can see that if it can experience a free air flow, it would react more quickly. I'll also get a thermometer to measure the room temperature at various points.

Although it will take some time, I'll report back.

Regards retiredbri

 

[D_Hailsham]

The programmable stat is a Time Guard TRT307.
The "span" can be 1 degC or 0.5 degC. From my experience, I think this is +- 1 degC or +-0.5 degC.

I assume you mean TRT037.

I am not familiar with this product but, having read the mfrs literature, I can find no mention of Proportional Bandwidth. Are you sure it has that feature? It does have Optimal Start, which adjust the boiler on time according to the temperature (i.e overnight heat loss). If this is turned on, you set the first time to when you want the room to be up to temperature, not when you want the boiler to turn on.

The 'span' is defined in the literature as the difference between on and off temperatures. So a 'span' of 1C is actually ±0.5C.

 

[retiredbri]

Thanks again.
Because I have a Thermal Store and because the stat just controls the pump for the rads, not having Proportional Bandwidth is just fine. I probably thought it had Proportional Bandwidth when I was trying to understand what was going on - sorry for the confusion.
I have started monitoring the air temperature and comparing it with the stat indicated temperature. The low thermal mass air temperature probe shows a fixed 0.4 degC difference under stable temperature conditions. However, the stat sensor is very sluggish (probably has a high thermal mass burried inside the body of the stat) so laggs the air temperature as the room heats and cools.
I'll check the span but know that the jumper is set to the 0.5 degC position and not the 1 degC position. Maybe it is the marketing interpretation of "span".

Regards

retiredbri

 

[retiredbri]

I've had time to check the "span".
When the timer comes on in the morning and with the room at 17.5 degC (air temp) and with the stat set to 19 degC then the pump starts and the reds get hot. The air temp rises and when at 19.5 degC air temp, the stat indicates 19 degC (lags), When the air temp gets to 20.2 degC, the stat reads 19.5 degC and demand is removed. The room then cools to 17.9 degC air temp before the stat reads 18.5 degC and demand is on again. The 0.5 degC stat span equates to a air temp span of +-1.15 degC

I've checked the jumper and it is set to 0.5 degC span.
I'll contact the manufacturer to see if this is correct.
Do all programmable stats have sluggish responses because maybe the sensor is fixed to the pcb and has, therefore a large themal mass?

regards Retiredbri

 

[D_Hailsham]

Presumably you are using a separate thermometer to measure the air temperature. If so, how accurate is it?

Are you checking the air temperature next to the room stat or in the middle of the room?

 

[retiredbri]

Thanks
I tried two, separate room thermometers. Neither agreed with each other but the probe one tracked the stat readout but with +0.3 degC difference. It is a probe with low thermal mass and is located next to the stat vent slots (top left hand corner). Because the thermal mass is different to the stat, I calibrated over long periods at constant temp.

This probe allows me to to see how the air temp compares with the stat readout, how much the stat lags readout lags the air temp reading when rising and how much the probe thermometer leads the stat readout on falling air temperatures. It also tells me what the air temp is when the stat turns on and off (regardless of what the stat readout is).
The offset has been built into the figures given in it earlier post.

Hope this helps.