Flow temp vs power rating

[Johntheo5]

Hi John
Yes it takes around 9 minutes to drop to the anti hysteresis that I have set. If I set it any lower then yes it takes ages, but this way I’m guaranteed a drop no matter how long the boiler fires for.
The Valiant boilers anti cycle software obviously works differently

One of the main reasons for anticycling timing is to allow the flow temperature to fall sufficiently to avoid the boiler reaching SP+4/5C and burner trip after refirig, if it does trip then repeat recycle(s), some boilers do require 2 or even more recycles to get away, and thats with the full recycle time so difficult to see the logic in using the remaining recycle time as its very unlikely to get the boiler away, it would be interesting to set the hysteresis to say - 2C and set the time just a minute or so less than the actual observed time required to get the boiler away in one attempt and then see what happens on next or subsequent refiring(s).

 

[Johnytuono]

One of the main reasons for anticycling timing is to allow the flow temperature to fall sufficiently to avoid the boiler reaching SP+4/5C and burner trip after refirig, if it does trip then repeat recycle(s), some boilers do require 2 or even more recycles to get away, and thats with the full recycle time so difficult to see the logic in using the remaining recycle time as its very unlikely to get the boiler away, it would be interesting to set the hysteresis to say - 2C and set the time just a minute or so less than the actual observed time required to get the boiler away in one attempt and then see what happens on next or subsequent refiring(s).

I’ve tried it John, it obviously depends on how high the flow temp is as to how fast it reaches -2c hysteresis. On 50c flow it’s a couple of minutes, meaning it’s more or less firing until room temp is reached but the boiler fire time will also be only a few minutes. That’s what I observed anyway

 

[Johntheo5]

Obviously, if the heat demand is lower than the boiler's minimum flowtemp then the boiler will reach SP+4/5C and burner trip then anticycle etc, if it doesn't trip on the first cycle what was/is the recycle time setting, 5 minutes??. and I presume the flowtemp will reach SP-2C before the 5 minutes or whatever has elapsed? which would mean that a anticycle time of 5 minutes is sufficient?.

 

[Johnytuono]

Obviously, if the heat demand is lower than the boiler's minimum flowtemp then the boiler will reach SP+4/5C and burner trip then anticycle etc, if it doesn't trip on the first cycle what was/is the recycle time setting, 5 minutes??. and I presume the flowtemp will reach SP-2C before the 5 minutes or whatever has elapsed? which would mean that a anticycle time of 5 minutes is sufficient?.

The boiler might fire for 5 or 6 minutes which would then mean it's literally off for a couple of minutes, it may even be seconds (until -2 hysteresis is hit) before going through the same cycle scenario again. Is this better than a longer burn a la waiting for the anti hysteresis to drop the flow temp. Personally I thought the longer burn (not silly long) would be better? I'm only experimenting and curious.
Obviously I'm talking about when it's cold(er) and the heat demand is greater, where 5 minutes fire time wouldn't be enough. The first burn is much longer, even as much as 30/40 minutes, then a drop to -2 AH, wouldn't let it cool down much and it would easily reach SP+4 again quite quickly, infinitum until room stat temp is reached. More efficient?

 

[Johnytuono]

Obviously increasing the heating ON time will effect internal warmth most, it will be warmer for longer.

The emitters can only emit what goes in and the property can only absorb what it can until the thermostat indicates it's warm enough the turns the boiler OFF.

Increasing flow temperature will put hotter water into the emitter which means its heat up of the space will be faster. Increasing the power of the boiler will put more heat energy into the same amount of system water.

Both these excesses will increase the likelyhood of heat input being too fast and too great for the load over time (the amount of heat the property can absorb before a room or boiler thermostat says stop).

Both increase cycling, increasing setpoint reduces the likelyhood of the boiler operating in a condesing mode.

If you are able to, slow the pump speed and restrict the lockshield valves on the radiators to lengthen the time water spends in the emitter, try to widen the delta T across the boiler flow and returns and stretch the anti-cycle time if it is adjustable. Reduce the boiler setpoint as low as possible without compromising comfort.

A poorly insulated 1930's three bed semi will have a heat loss probably less than 9kW and that only when it's say -3°C outside which thankfully it rarely is.

Wouldn't there be a case of strangling a little too much though and it (the boiler) then can't get rid of the heat quickly enough, so that the same scenario arises? I'm not an expert, just curious and learning the best economically and comfort wise.

 

[vulcancontinental]

Wouldn't there be a case of strangling a little too much though and it (the boiler) then can't get rid of the heat quickly enough, so that the same scenario arises? I'm not an expert, just curious and learning the best economically and comfort wise.

You're stuck with what you have, if it were new the pipework could be sized to suit your desired delta T, flow rate and heat loss..

Slow flow means water gives up it's heat over a long period and the return to the boiler is cooler (the temperature entering the radiator doesn't change but the return temperature does. Fast flow rate and the return gets back to the boiler quicker still hot or with a narrow delta T, boiler cycles.

Strangling flow is not good, it's crude but it's what you've got. Slow the pump if you can, stretch the anti cycle if you can and you'll lengthen the burn. The water still circulates giving heat to the property during anti-cycle.

If your burn is longer that three minutes it's about a 1-1.5% hit on the efficiency of the boiler. If it's 10 seconds it's a 12% hit according to a study.

 

[John Smith Esq.]

Interesting posts.

I have the pump head setting on the lowest (1) on mine for that reason - i.e. slowing flow. I also lowered the rating down to about 12KW, from 24KW.

I will have a check on the timings of mine tmw and report back how it is working for me.

It seems to heat the house reasonably well but doesn't do that psychotic thing any more of constantly going off and on as it hovers around the set flow temperature. It'll go off for a bit and then once triggered on again, begin a gradual build up back to the set temperature.

I notice that the smart meter reading isn't too extortionate even though I have the boiler running for about 8 hours per day, in three blocks. It is on £5.20 now for gas and electric use and I work from home. Temperature has averaged 17.5C - 18C in the different rooms over the past week (although there is a lot of variation with over night temps, when the boiler is on, etc). I let the rooms get up to about 20C when the boiler is running, apart from the thermostat room which is colder and the set temperature on that is seldom reached (which is another good reason for me to force boiler to take breaks with anti cycle).

We do have a room with a wood burner which we keep more toasty in the evenings during winter.

 

[Johntheo5]

Wouldn't there be a case of strangling a little too much though and it (the boiler) then can't get rid of the heat quickly enough, so that the same scenario arises? I'm not an expert, just curious and learning the best economically and comfort wise.

The only reason that a boiler will trip the burner at SP+5C (we'll use 5C since most boilers use this figure) is 1, when the heat demand is lower than the boiler minimum output and 2, on refiring, if the flowtemperature is too high, then the burner will trip within a minute or so, this can be prevented by selecting a suitable anticycle time or/and if available like with the WB a suitable negative temperature hysteresis. If you take a example of say a 25kW boiler with a minimum turndown to 5kW and a flowtemp SP of 50C, if the heat demand is 5kw or higher then the boiler will modulate to maintain a flowtemp of exactly 50C, if for some reason like a zone shutting down to reduce the heat demand to say 2.0kW then the 3.0kW of excess boiler power will raise the system contents by 5C and burner trip, a system circulating contents of say 50L will only require 0.29kWh to raise the temp by 5C, burner will continue to fire for, 0.29/3.0*60, 5.8, say 6 minutes, On refiring, if the flowtemp is reduced to 40C after anticycle then there will be ~ 16kw (25*65%) available initially and then 20kw, say a average of 17kw to increase the flowtemp to its 50C SP so ~ 50*10/860, 0.58kWh, 0.58/17.0*60, 2 minutes or so, so, if calcs are anyway correct, a total boiler run time of, 6+2, 8 minutes with a heat demand of 2.0kW. Range rating the boiler might add another few minutes to this run time.

 

[vulcancontinental]

Not thoought about it before but some boilers allow adjustment of the hysteresis.

 

[Johntheo5]

Not thoought about it before but some boilers allow adjustment of the hysteresis.

Yes, as does the @Johnytuono one, above, settable, I think to (minus) 2C to 15C.

 

[Johnytuono]

Interesting posts.

I have the pump head setting on the lowest (1) on mine for that reason - i.e. slowing flow. I also lowered the rating down to about 12KW, from 24KW.

I will have a check on the timings of mine tmw and report back how it is working for me.

It seems to heat the house reasonably well but doesn't do that psychotic thing any more of constantly going off and on as it hovers around the set flow temperature. It'll go off for a bit and then once triggered on again, begin a gradual build up back to the set temperature.

I notice that the smart meter reading isn't too extortionate even though I have the boiler running for about 8 hours per day, in three blocks. It is on £5.20 now for gas and electric use and I work from home. Temperature has averaged 17.5C - 18C in the different rooms over the past week (although there is a lot of variation with over night temps, when the boiler is on, etc). I let the rooms get up to about 20C when the boiler is running, apart from the thermostat room which is colder and the set temperature on that is seldom reached (which is another good reason for me to force boiler to take breaks with anti cycle).

We do have a room with a wood burner which we keep more toasty in the evenings during winter.

I tried pump speed 1 but the water wasn't hot enough entering the rads, on speed 2 I'm only gaining a couple of degrees extra drop at the boiler so I put it back to speed 3.
Our heating is set to run most of the day for a block of 15 hours (18c from 0700 and 19c from 1730 until 2200), for 3 days it's also on until 0230, the setback temp. the rest of the time is 16c, and this month we're averaging around 80 kw a day, inc of cooking, showering etc., which I don't think is too bad considering the weather and the length of time the heating is running. I think it uses about 5kw an hour (according to my smart meter) on 50c flow.
We keep all the trv's on full so that we are using the full heat load, as I heard it can be a disadvantage to turn them off, I don't even think it's an advantage to even turn them down in our case as all rooms are used often.

 

[John Smith Esq.]

Mm... these different ideas and ways of doing things. I think the TRVs fully open works with your strategy of keeping the boiler on for longer periods but to lower temps.

In my case, I have the TRVs set to about 3 (varies by room because of individual TRV quirks, placement of furniture, etc), in one case 4, and this means rooms don't go over 19C-20C. Only the one in thermostat room is fully open. This is all the more reason for adjusting anti cycle settings since some room radiators will reject heated water after a time and cause the boiler to go over.

I see the TRV use as a good thing in my case as I'm not burning as much gas to over heat rooms. But my strategy is different. Three blocks a day of 2-3 hours and I have the flow temp set to a comfortable level (stingy end of comfortable

 

[Johntheo5]

I tried pump speed 1 but the water wasn't hot enough entering the rads, on speed 2 I'm only gaining a couple of degrees extra drop at the boiler so I put it back to speed 3.
Our heating is set to run most of the day for a block of 15 hours (18c from 0700 and 19c from 1730 until 2200), for 3 days it's also on until 0230, the setback temp. the rest of the time is 16c, and this month we're averaging around 80 kw a day, inc of cooking, showering etc., which I don't think is too bad considering the weather and the length of time the heating is running. I think it uses about 5kw an hour (according to my smart meter) on 50c flow.
We keep all the trv's on full so that we are using the full heat load, as I heard it can be a disadvantage to turn them off, I don't even think it's an advantage to even turn them down in our case as all rooms are used often.

IMO, its better to keep the head/flowrate "up" which ensures all rads are getting enough flowrate without more and more balancing, for example if you require 5kw say 25% of T50 rated rads, you can achieve this in a number of ways, let the pump speed as is and just reduce the flow temperature, a quite normal flowrate would be ~1.0LPM/kW rad output (T50) to give a dT, of ~ 14C, if the flowtemp is reduced to 39C then the return will be 35.4C, dT 3.6C, flowrate still 1.0LPM to give 25% output or set the flowtemp to 47C (47.2) and reduce the pump speed/head to give a flowrate of only 0.18LPM to give a return temp of 27C (27.2) and the same 25% output, or, as above, use TRVs whereby you can still have a high pump head ( to reach all rads) but the TRVs will just keep throttling the flow to give the required (25%) output. the first method is how WC operates generally.

 

[John Smith Esq.]

So I've monitored my boiler this afternoon. It doesn't make sense...

So flow temp is 60C
Anti fast cycle flow temp is -14
Anti fast cycle time is 15 mins

12:00 - boiler comes on. Flow temp initially goes to 43C, then settles down to 30C and builds from there.

12:29 - reaches the 60C flow temp and continues at that for a while.

13:09 - hits 61C as one or more of the TRVS presumably reaches setting temp and starts to close radiator. The boiler has been running continously since it started and burning gas. House temp has increased by 2.5 - 3C.

13:19 - hits 64C. Boiler stops burning gas after about 30 seconds. Fine. It is from here that I am interested to see what the anti fast settings do. From the point it stopped burning gas it continues cycling for about 5 mins. Flow temp falls quite quickly to 59C.

13:23 - cycling stops. 57C. Boiler now idle.

13:29 - cycling starts again, no gas. Temp goes from 59C - 51C.

13:33 - cycling stops. 50C. Idle.

That's two 5 min cycles without gas, 5 mins apart.

13:38 - 50C, boiler fires up, burns gas. Temp goes to 58C quickly, but then settles down to 53C.

13:40 - gas stops, but cycling continues.

13:44 - 46C, cycling stops. Boiler becomes idle.

So this is the end of a third cycle without gas although the third one began with a brief period of burning gas. So about 15 mins of cycling. And the -14C point has also been reached...

13:46 - 47C. Idle.

13:50 - another cycle with no gas. Temp goes from 48C to 41C.

13:53 - the gas comes back on and now the boiler seems to begin again a proper burn back towards set temperature. Gets up to 56C by 14:00 before turning off with timer.


So I'd appreciate thoughts. It didn't seem to fire up at -14 below set temperature. Or fire up properly at -14 below SP+4. And it isn't clear how the 15 minute setting came into effect either. What was that mini burn all about at 13:38?

 

[Johntheo5]

Presume you can assume that because the boiler shut down normally that the AFCtime was 15 minutes and the AFChysteresis was SP-14C, 60-14, 46C (flowtemp), so you say the boiler refired after only 3.5 to 4 minutes and at a flowtemp of 57C?