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Post by Clinton Cool on Jun 3, 2024 14:51:53 GMT
This discussion is a little too technical for me but if I were to simply things, bit of layman's language:
If I set the thermostat at, say, 18 would the boiler kick in when the temperture dropped to 18, or a different figure? And them, how much above 18 would the temperature need to rise, before the thermostat switched the boiler off?
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Post by Jim on Jun 3, 2024 14:57:34 GMT
This discussion is a little too technical for me but if I were to simply things, bit of layman's language: If I set the thermostat at, say, 18 would the boiler kick in when the temperture dropped to 18, or a different figure? And them, how much above 18 would the temperature need to rise, before the thermostat switched the boiler off? I've skipped to the end, but imho a degree or so. There's been a recommendation around for a while to set the rad flow temp on a condensing boiler to 55deg . I've set the HW temp to a similar temp. Improves efficiency of boiler. I'm not bothered about legionnaires, water in pipes is cycled/replaced frequently. |
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Post by Telemachus on Jun 3, 2024 15:01:59 GMT
Not my cage, yours, but then you know that. So to precis, both systems will cut off when the space temperature is reached. A mechanical thermostat on an older system will do the same. on older systems, if the flow temperature gets high, the burner cuts out with the room stat but the pump still runs until the space temperature is reached. One expects some improvement in the passage of time as has been shown. I dont pretend to know anything about modern (or even not so modern) helicopters, I used it to show that throwaway remarks can be made. No, you still don't get it. I will try one more time... A modern system doesn't cut off when the space temperature is reached. When the space temperature is reached, a modern system is designed to go to a lower flow temperature such that the heat losses from the house are matched by the heat input from the system, and thus the temperature remains rock steady. As it does with our system unless it is fairly warm outside such that the minimum boiler rate is still too high. An old fashioned system has a set flow temperature, this is maintained until the thermostat clicks off and the burner stops. Some time later the temperature has dropped a bit (not much, thanks to the heat anticipator) and the burner resumes with the same target flow temperature. So it is very much an on-off type of regulation rather than a progressive one.
Returning to my original point for which I was foolishly hoping for some useful suggestions, my mother's system has a modulating boiler, 3 zone valves, a 3 channel timer and on the downstairs zone, a room stat. When both the timer and the room stat are calling for heat, the motorised zone valve is powered and opens. When the valve is fully opened a microswitch is operated that makes the boiler fire. When the thermostat clicks off, the zone valve closes. So even though there is an overrun on the boiler pump, the flow has nowhere to go except via the automatic bypass valve. This seems a poor design because the thermostat makes the electromechanical zone valve operate on each cycle and the flow is blocked when the valve is closed / thermostat up to temperature.
Clearly the reason why it is like this is to ensure that at least one zone valve is open before the boiler fires, but in other respects it isn't good. Although it does seem to have been fairly standard practice for boilers intalled 1990s -2010s (and probably before that)
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Post by Clinton Cool on Jun 3, 2024 15:03:14 GMT
This discussion is a little too technical for me but if I were to simply things, bit of layman's language: If I set the thermostat at, say, 18 would the boiler kick in when the temperture dropped to 18, or a different figure? And them, how much above 18 would the temperature need to rise, before the thermostat switched the boiler off? I've skipped to the end, but imho a degree or so. There's been a recommendation around for a while to set the rad flow temp on a condensing boiler to 55deg . I've set the HW temp to a similar temp. Improves efficiency of boiler. I'm not bothered about legionnaires, water in pipes is cycled/replaced frequently. Mine's an old boiler, I don't think there's any option to set the flow temperature. Only a knob which you can turn from high to low when the want the radiators to be hotter (or cooler) but this doesn't give any indication of any temperature. |
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Post by Telemachus on Jun 3, 2024 15:07:48 GMT
This discussion is a little too technical for me but if I were to simply things, bit of layman's language: If I set the thermostat at, say, 18 would the boiler kick in when the temperture dropped to 18, or a different figure? And them, how much above 18 would the temperature need to rise, before the thermostat switched the boiler off? I think this has already been covered at length! Without the heat anticipator (which will only come into play if the N is connected) you can see the hysteresis (difference between cutting in and cutting out) by slowly rotating the knob and hearing when it clicks on the way up and on the way down. There will probably be 2 or 3 degrees difference. So in use, the room temperature would slowly fluctuate by this amount. Which is a lot!.
With the heat anticipator (N in place) it will contribute to the temperature measured by the stat when it is on, but not when it is off. This means that the room temperature will vary by much less than the 2 or 3 degrees, probably more like 1 degree or less, but it depends on a whole lot of facto
Another minor issue is that a thermostat measures the air temperature, whereas humans are sensitive to both air temperature and radiant heat. So a room at 20C with a hot radiator will feel much cosier than a room at 20 with a cold radiator, assuming you have line of sight to the radiator. This is another advantage of load compensation, the radiators tick over steadily at warm as opposed to cycling between hot and cold.
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Post by Telemachus on Jun 3, 2024 15:15:24 GMT
I've skipped to the end, but imho a degree or so. There's been a recommendation around for a while to set the rad flow temp on a condensing boiler to 55deg . I've set the HW temp to a similar temp. Improves efficiency of boiler. I'm not bothered about legionnaires, water in pipes is cycled/replaced frequently. Mine's an old boiler, I don't think there's any option to set the flow temperature. Only a knob which you can turn from high to low when the want the radiators to be hotter (or cooler) but this doesn't give any indication of any temperature. The knob sets the flow temperature of the water in the rads. It just doesn't have a temperature scale. The thermostat sets the desired air temperature. The former affects how quickly, if ever, the room will reach the desired air temperature. Turn it up, the rads are hotter and the room heats quicker, but the thermostat will still turn the system off at the set temperature. If the flow temperature is very hot, there will probably be more overshoot and oscillation in the room temperature. If the flow temperature is set too cool, the room may take ages to heat up and in fact never get to the desired temperature on a cold day. If the flow is set cooler, and assuming it is a condensing boiler, the efficiency is better. So it is all a compromise. I would suggest setting a fairly high flow temperature in very cold weather and turning it down quite a bit on those days where you barely need the heating to be on. Having set as low as you can commensurate with it actually being able to heat the room enough, is best for efficiency / economy. However if the boiler also heats hot water in a tank, you need to consider the risk of legionnella bacteria growth. If you are going to have a very cool flow (in summer) then from time to time, say ever couple of weeks, heat the water to a much higher temperature by turning the knob up when the hot water circuit is on. Of course if it is a Combi (I will check in a minute) it doesn't matter.
Edit: It's a Combi so the knob only affects radiator temperature, not hot water temperature. No need to worry about Legionnaires.
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Post by brummieboy on Jun 3, 2024 18:51:09 GMT
We actually agree.
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Post by Telemachus on Jun 3, 2024 18:52:04 GMT
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Post by Clinton Cool on Jun 4, 2024 9:06:43 GMT
Connected it up this morning and gave it a go. All seems well, works as it should.
Thanks Brummieboy and Telemachus for your help with this.
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