Why range rate a modulating boiler?

[Ltnman]

So if my new boiler can modulate for load compensation why can it be range rated as well? As the boiler has an CH output of between 4.6Kw to 26kW, if the boiler was range rated would the minimum output also drop to mantain the ratio? I have worked out that my rads have an output of around 12Kw.

 

[muggles]

No, the minimum output would stay the same but the maximum output would be altered to match the maximum demand of the system. This will prevent the boiler overpowering the system and cycling

 

[Johntheo5]

Unfortunately, most boilers don't modulate to much below 5kw and most cycling can occur when the heating demand is below this, what with zoning, outside temperature compensation and increased levels of insulation. The Viessmann Vitodens 200 W can modulate between 35kw and 1.9kw, this is outstanding IMO especially where in most cases there is a mismatch in CH demand and DHW demand (if a combi) most 35kw boilers will not modulate below 7kw (5:1 turndown) leading to still further cycling.

 

[Ltnman]

Unfortunately, most boilers don't modulate to much below 5kw and most cycling can occur when the heating demand is below this, what with zoning, outside temperature compensation and increased levels of insulation.

Excuse my ignorance but to avoid cycling on a boiler without sufficient modulation could the boiler not just be turned down to say 50C to 55C via its dial so it operates for longer with a background heat or am I missing something?

 

[DP]

Excuse my ignorance but to avoid cycling on a boiler without sufficient modulation could the boiler not just be turned down to say 50C to 55C via its dial so it operates for longer with a background heat or am I missing something?

Best way to describe range rating is sitting in a car at the lights, light goes green, you put your foot on the accelerator all the way to max- engine is now at full power. You reach the road speed limit and ease off on the pedal to maintain set speed. Result is wasted fuel

Alternative. At green light, move off to reach the speed limit. Result is fuel used more economically

A combination boiler is rated for hot water delivery minimum being 24kw ( anything less means hot water delivery becomes dire). An average house seldom needs 24kw. Manufacturers may use electronic trickery to limit application of full power to heating load or the flow temperature quickly puts boiler into low output mode. If flow temperature keeps rising,burner shuts down and ant-cycling timer starts.

 

[DP]

Unfortunately, most boilers don't modulate to much below 5kw and most cycling can occur when the heating demand is below this, what with zoning, outside temperature compensation and increased levels of insulation. The Viessmann Vitodens 200 W can modulate between 35kw and 1.9kw, this is outstanding IMO especially where in most cases there is a mismatch in CH demand and DHW demand (if a combi) most 35kw boilers will not modulate below 7kw (5:1 turndown) leading to still further cycling.

In almost every case ( I stress not all makes), heating output is pegged at around 28Kw when it is a high output combi boiler. There are cases where a 24kw combi boiler may have heating output of say 18kw

 

[Ltnman]

Best way to describe range rating is sitting in a car at the lights, light goes green, you put your foot on the accelerator all the way to max- engine is now at full power. You reach the road speed limit and ease off on the pedal to maintain set speed. Result is wasted fuel

I have been thinking about this answer for a few days. If the boiler is modulating, as it doesn’t need full power isn’t it already easing off the pedal, as you have put it?

Also I have assumed the Ideal Vogue can be range rated but there is nothing about this in the installation manual. Can the Ideal range of boilers be range rated?

 

[Johntheo5]

Almost all boilers will fire up at ~ 60 to 70% of their full output for up to a minute irrespective of any range rating and some have difficulty in getting away especially if using outside temperature compensation, range rating may/might help to alleviate this problem. Some, like Vokera will automatically reduce the output to 75% of their max output for 10 minutes on each burner fire up.

Don,t know if the Ideal ra ge canbe range rated.

 

[TeaAndBiscuits]

The Ideal Vogue cannot be range rated.

 

[vulcancontinental]

Vokera not only limit htg output to 75% for the first 15 minutes but the limit is 75% of the range rated heating output not the maximum htg output.

The latest entry level Vibe Max combis will be 25kW to HW and 19.6 to htg unless range rated. It can be range rated to minimum, 2.9kW but obviously not for 75% of that.

Anyway boilers fire at ignition fan speed which is not minimum then adjust after reliable ignition, this has led to micro-firing with the Viessmann apparently which has an ignition fan speed higher than minimum modulation so it starts quite a bit above 1.9kW then cycles at periods of extreme low demand. There are ways to ameliorate this but fiddly.


As long as the burn time exceeds 3 minutes the hit on efficiency is less than 1.5%

 

[Johntheo5]

Interesting points above.

I've often wondered about this 75% limit for 15 minutes or whatever, there is no limit if a combi is on HW duty and the dT through the boiler Hx is likely higher than that on CH. I have read that one reason that Vaillant keep the ignition settings high for ~ 1 minute is to get the combustion chamber temperature up rapidly to reduce emissions.
Relation has a 8 year old Vokera Vision 20S which I can hear modulating down within seconds of achieving ignition.

 

[mpgw]

The Ideal Vogue cannot be range rated.

But you can set its max flow temp

 

[amcluesent]

I've ranged rated my 30kW Worcester down to 45% for CH and set flow temp to 55°, the Nest "Time-to-Temperature" is set to allow up to 60 minutes of running to achieve a scheduled temp. As outside temps drop, I'll be assessing if these settings can maintain comfort levels of 21°C in the house.

 

[mrbg07546]

I've ranged rated my 30kW Worcester down to 45% for CH and set flow temp to 55°, the Nest "Time-to-Temperature" is set to allow up to 60 minutes of running to achieve a scheduled temp. As outside temps drop, I'll be assessing if these settings can maintain comfort levels of 21°C in the house.

Interesting. I have a large 5 bed and range rated my 27kw vaillant system boiler down to 6kw. Seems to be fine. I might even try a bit lower

 

[JonathanM]

Interesting. I have a large 5 bed and range rated my 27kw vaillant system boiler down to 6kw. Seems to be fine. I might even try a bit lower

Do you have a different output for the hot water?

EDIT: I suppose I'm asking if you have priority hot water?