Digital Roomstat, the answer to high fuel costs?

John.G

I installed one of these resently serving a (now combined) dining + sitting room, 2 rads, combined rated output 2.65kw. Its nonTPI and and has a settable hysteresis of 0.3C to 2.0C, present setting 0.3C.

When the boiler fires up it will have increased the mean rad temperature to 50C or 1.2kw output in ~ 4/5 minutes, in another 11/12 minutes the room will have increased by 0.3C (stat hysteresis) and boiler cut out, the roomtemp will then overrun by 0.2C (and 0.1C subsequently) so total roomtemp rise 0.5C after first fire up, here's the conundrum, it will then take at least a hour for the room temperature to fall by 0.5C and boiler refire???. When house up to temp. it will take even longer to fall to the SP. I have monitored/confirmed these temperatures with a digital thermometer.

Doolittle51

Sounds like a good setup, what's the conundrum?

I have a digital thermostat in each room, except the bathroom. Different rooms have different setpoints and it works really well, but I dont have such good control over the hysteresis. Mine are fixed at 1 degree. So if SP is 19, room gets up to temp, but overshoots by 0.4 or 0.5. Stat doesn't call for heat again until the room temp drops below 18. Still quite comfortable, but would be nice to have some TPI stats and adjustable hysteresis.

What thermostat do you have?

Wearb

I have my hysteresis set at .5. I get roughly the same results as John. The odd time -when there's nobody moving about disturbing the air- I might just notice the temperature drop, just before heating kicks in. This is so rare that I've not dropped there hysteresis down. Anyway if needed I can turn up the stat .5c from my phone.

This along with programmable rad stats gives me great control over my heating.

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As above, what's the conundrum?

John.G

https://www.boards.ie/discussion/comment/119818581#Comment_119818581

The conundrum is why does it only take 15 minutes or so to raise the room temperature 0.4C but it takes over a hour for the temperature to fall by the same anmount.

Wearb

https://www.boards.ie/discussion/comment/119819074#Comment_119819074

I would use an electrical kettle to explain this. We boile the kettle and leave it stand after the element switches off. After x amount of time it cools by 10C. Now lets wrap this in a large anount of insulation. It will take a lot longer to cool by 10C.

If we were to boil it with the insulation in place, it would not boil much faster (a nominal amount only) than without the insulation.

EDIT The heating up time is related to the heat input. Cooling down time is related to the rate of heat loss.

John.G

Think I know the reason now.

The rads emit heat both during heat up and cool down, the heat emitted on heat up averaged ~ 1.5kw and on cool down ~ 0.33kw, a factor of 4.55 so cool down period ~ 15X4.55, 68 minutes.

youtheman

https://www.boards.ie/discussion/comment/119819074#Comment_119819074

It because of heat mass balance. Your radiator is able to put heat into the room much quicker than you are losing it out through the walls. If you had a tank with a small hole in the side and if you are pumping water into it with a large pump then it might take 1 minute to raise the level by one inch. Turn off the pump and it may take much longer for the level to drop the same 1" (if the pump is much larger compared to the hole).