Explanation of thermostat vs control temp

[Johntheo5]

Thats the key to the whole thing, get the temperature down, you cant get a anti cycle time increase which will/would certainly do the trick, but if you can increase that pump speed, do.

 

[Johntheo5]

I see what you mean.

Jonathan, here's a little calc for you, how long would it take to increase the flow temperature by 5C assuming rads demand of 3kw, boiler output 23kw and a flowrate of 21LPM.

 

[karlbbb]

Thats the key to the whole thing, get the temperature down, you cant get a anti cycle time increase which will/would certainly do the trick, but if you can increase that pump speed, do.

As seems to be the growing theme, there's no obvious option to do that on the Ideal's. Unless I need to open it up and flick some kind of switch somewhere... at which point I need a GasSafe engineer as the front panel is head-to-toe room seal.

 

[JonathanM]

Jonathan, here's a little calc for you, how long would it take to increase the flow temperature by 5C assuming rads demand of 3kw, boiler output 23kw and a flowrate of 21LPM.

I'm going to say 19 seconds.

 

[Johntheo5]

As seems to be the growing theme, there's no obvious option to do that on the Ideal's. Unless I need to open it up and flick some kind of switch somewhere... at which point I need a GasSafe engineer as the front panel is head-to-toe room seal.

How long did it take the boiler temperature to fall by 10C when burner stopped firing?

 

[JonathanM]

As seems to be the growing theme, there's no obvious option to do that on the Ideal's. Unless I need to open it up and flick some kind of switch somewhere... at which point I need a GasSafe engineer as the front panel is head-to-toe room seal.

I think this is the right manual:

https://idealheating.com/uploads/documents/Logic-Max-Combi-Installation-Servicing-Guide-222362-1_lowres.pdf

On page 40 it looks like you can change the pump modulation using the front buttons. It also looks like you can get a read out of the current pump speed, which might be worth doing first.

I really would emphasis that Johntheo5 knows a lot more than me about whether this is safe!

 

[JonathanM]

I think this is the right manual:

If yours is an earlier version it might have a smaller LCD display.

You mentioned service mode or perhaps it's called installer mode. Does that page of the manual mention the pump speed at all? I think if it is a 2017 boiler it will have the modulating pump, because of a 2015 EU directive.

 

[Johntheo5]

I'm going to say 19 seconds.

I'm finding it difficult but the way I'm thinking is that if I assume say that a total of say 60 litres (circulating system contents) has to be increased by 5C, then 20kw net will achieve this in 63secs and so on??.

 

[JonathanM]

I'm finding it difficult but the way I'm thinking is that if I assume say that a total of say 60 litres (circulating system contents) has to be increased by 5C, then 20kw net will achieve this in 63secs and so on??.

I'm thinking about it. Isn't it just heating the amount that's passing through the heat exchanger in the moment, rather than whole circuit. I also find it difficult.

EDIT: otherwise I can't see how it would go up 10C in 20 seconds, like in the case we are discussing.

 

[Johntheo5]

I'm thinking about it. Isn't it just heating the amount that's passing through the heat exchanger in the moment, rather than whole circuit. I also find it difficult.

EDIT: otherwise I can't see how it would go up 10C in 20 seconds, like in the case we are discussing.

Yes, I've always used .... dT = BoilerkwX860/60/LPM. so, 23X860/60/21, 15.7C, if the boiler return temp is 34.3C, then the flow temperature is 50C for a very very short time since the heat demand is only say 3kw, but the boiler return must reach 20.7C before the flow temp reaches 55C and burner trip so should one then consider the circulated rad contents as a "tank" that must be heated by 5C??.
I think the reason that its flying up in 10 secs or whatever is that the flowrate is very low, this happens regularly if TRVs are used, and even if the pump speed was at 100% then the (if) TRVs would still only allow the exact same flow circulation by throttling down?. No real problem for oil fired boilers as their 20/25 Litre Hx acts as a mini heat store.

 

[JonathanM]

I'm finding it difficult but the way I'm thinking is that if I assume say that a total of say 60 litres (circulating system contents) has to be increased by 5C, then 20kw net will achieve this in 63secs and so on??.

I've had to think really hard about this.

My workings are always rather laborious. But this is what I did. 21 litres of water pass through the boiler in 1 minute. We have a 23KW boiler. So in 1 minute it gives out 0.384KWh. Multiply by 3600 and divide by 4.2, gives 328.57, then divide by the 21 litres, gives 15.65. So in 1 minute the boiler will raise the temperature of that 21 litres of water by 15.6 degrees. So it will raise it 5 degrees in 5/15.6 minute = 19 seconds. I've forgotten what the units going through the calculation are, but it always works out right.

EDIT: Your 15.7 and my 15.6 must be the same, just rounded differently.

 

[JonathanM]

Yes, I've always used .... dT = BoilerkwX860/60/LPM. so, 23X860/60/21, 15.7C, if the boiler return temp is 34.3C, then the flow temperature is 50C for a very very short time since the heat demand is only say 3kw, but the boiler return must reach 20.7C before the flow temp reaches 55C and burner trip so should one then consider the circulated rad contents as a "tank" that must be heated by 5C??.
I think the reason that its flying up in 10 secs or whatever is that the flowrate is very low, this happens regularly if TRVs are used, and even if the pump speed was at 100% then the (if) TRVs would still only allow the exact same flow circulation by throttling down?. No real problem for oil fired boilers as their 20/25 Litre Hx acts as a mini heat store.

Rather than a "tank", I've been thinking of it like an instant water heater. You only need to consider the amount of water that's actually flowed through the heater, and see how much that has been raised.

EDIT: if water exiting the boiler was already at 45C before the addition of this extra heat, it will be at 50C after 19 seconds of this extra heat.

I'm always happy to be wrong! Helps me learn.

EDIT 2: Or when I think about radiators, the first one starts to get warm whilst the last one is stone cold, because only the first chunk of water has been heated. Until the whole 60 litres has been through the heat exchanger, it's not uniform.

 

[Johntheo5]

Interesting, one way or the other, a gas boiler is certainly a instant heater since its contents may only be 2 litres or so, at 21lpm, 2 litres will have passed through it in less than 6 secs, the Hx metal will take some finite time to heat up however little but once this has heated up then in less than another 6 secs, the 23kw will be transferring to the water and the burner will almost trip instantly (from water at 45C) but not so from water at 34.3C, this must rise by another 5C to reach 39.3C, with a boiler dT of 15.7C = 55C, burner trip.

 

[JonathanM]

Interesting, one way or the other, a gas boiler is certainly a instant heater since its contents may only be 2 litres or so, at 21lpm, 2 litres will have passed through it in less than 6 secs, the Hx metal will take some finite time to heat up however little but once this has heated up then in less than another 6 secs, the 23kw will be transferring to the water and the burner will almost trip instantly (from water at 45C) but not so from water at 34.3C, this must rise by another 5C to reach 39.3C, with a boiler dT of 15.7C = 55C, burner trip.

Throughout the burner off time, the pump is still running I think, and water is passing through the heat exchanger. Somehow, the return water (at whatever temperature) is being turned into flow water at 45C, even with the burner off. Presumably this has to be from residual heat in the HX. So when we turn the burner on we are adding extra heat, over and above that which is already taking the return water up to 45C. So I think we do base it on water starting at 45C.

 

[Johntheo5]

Its being turned into flow water at 45C because the flowwater which was at 55C is giving up its heat to the rads and the reason that its not going any lower is because the burner is cutting back in at 45C (SP-5C) due to the anti cycle time being too short, it then short cycles as the OP states until it drops still further, IMO it has to fall to ~ 34/35C to stand a good chance of getting away on the first fire up after burner shutdown.