Do you need to range-rate a modulating boiler?

[Poogle]

Hi folks, just a regular member of the public here, learning all he can before having a new combi-boiler and radiators installed in my old victorian flat. For years I've just had some fan heaters! But now it's time to upgrade.

I'm fairly techie, but I can't see why your would range-rate a boiler, if it modulates anyway. I've read and watched as much as I can; but the answer eludes me. I'm most likely misunderstand something, so would be so grateful for your help.

Thanks!

 

[Brambles]

Modulating boilers do not need to be range rated. Some manufacturers (Vaillant and others) allow “Engineer settings” to limit the maximum output for heating (only), rather than heating and hot water - I think on combis.

Having said that get a reputable installer to confirm that a Combi will deliver the hot water flows to showers et al that you require

 

[oz-plumber]

Heat only boilers are generally sized to the heatload requirements.
Combi boilers are sized to the hot water requirements.

You may need a 30 kW combi boiler for your hot water requirements, but only 12kW for your heating requirements.
So you would adjust the boiler to perform at the required level for heating and adjust the boiler to perform at the required level for hot water.

So to answer your question, yes, a combi boiler needs range rating.
Non condensing boilers via manometer on burner pressures.
Condensing boilers via a Flue Gas Analyser

 

[Dr Doolittle]

British heating is just so far behind the world its embarrasing. Just go with a boiler that has the ability to switch between room temperature and system flow temperature heating methods. This allows the use of low temperature water flow which will reduce your gas consumption by at least 40% while giving your home a lovely steady constant and even heat but has the added benefit of being able to heat really quickly when needed. These "smart" boilers have built-in weather compensation capability so you can even select the "you sort out the heating for me" button. If you want the best system, then split off the heating and DHW with 2 boilers. This means heating is always on (not with a combi) and hot water is unlimited at whatever temperature you set. If you have the space, this is the way forward. Tankless DHW heater and combi would still be less than £1,500

 

[doitmyself]

I can't see why your would range-rate a boiler, if it modulates anyway.

It all depends on the internal control logic of the boiler.

Let's say the boiler has a much higher output (say 30 kW) compared to the system requirement (e.g. 15 kW). If the boiler''s logic makes the boiler go to max output every time it re-lights, the flow temperature will rise very quickly and the boiler will be turned off by the internal stat and start cycling on and off. So it takes a very long time for the water to get up to the required temperature. And don't forget that, for most of the year, the requirement will be much less than 15 kW. If, however, the boiler restarts at minimum output, or even at the same output as when it was turned off by the internal stat, the cycling will not occur so often and the water will reach the required temperature much faster. Range-rating the boiler which goes to max on restart will have a noticeable effect.

Modulation was not designed to provide the engineer with a way of avoiding setting the boiler output to meet the system requirements..

 

[John.g]

Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C, I've watched a relations 20kw Vokera firing up on HW duty only when the coil demand is just above the min boiler output of ~ 5.5 kw and its quite impressive as it only overshoots the SP by 2C or so and then runs away at the SP, even when the cylinder is almost up to temperature where the coil demand is lower than the boiler min output it will still modulate down to minimum and after a few minutes will creep up to its cut out of SP+5C.

 

[Dr Doolittle]

Look like the obvious has to be stated then....did you calculate the peak demand required @ (-2)? If not then its always a guess
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Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C, I've watched a relations 20kw Vokera firing up on HW duty only when the coil demand is just above the min boiler output of ~ 5.5 kw and its quite impressive as it only overshoots the SP by 2C or so and then runs away at the SP, even when the cylinder is almost up to temperature where the coil demand is lower than the boiler min output it will still modulate down to minimum and after a few minutes will creep up to its cut out of SP+5C.

set point 18 degrees. Overshoot by 5 degrees is nearly 30% differential...that's way off. Thats why creating the heating flow SP is the only way forward.

 

[doitmyself]

Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C,

Yes it should do that; but the problem arises when the flow temperature rises faster than the boiler can modulate down. When the anticycle time is taken into account, the aveage water temperature is low, so the house does not rise very vast, with the inevitable result that the occupants complain about the house taking ages to heat up and never gets up to temperature.

Vaillant 400 series were notorious for this, particulrly the versions with an "Auto" setting in the range-rating parameter.

 

[John.g]

Yes, the modulating down time from fire up seems to be a bigger problem for some makes, wonder is it anything to do with the fan speed etc on start up which is ~ 70%? of max output so even with de rating the start up settings are probably unchanged to obtain stable/safe light up. Also it looks like some manufacturers (all?) seem to employ the same size burner for a range of power outputs, I think I have seen power outputs ranging from say 38 kw to ~ 20 kw with the same minimum output which implies that the same size burner is employed and is actually range rated before it goes to the customer, the fact really is that gas boilers do not like cycling because of the high output start up coupled with the very small volume of water in the Hx, so no buffer like a oil fired boiler with a Hx of 20/25 litres.

 

[Poogle]

Many thanks for your kind replies, and although I confess I don't grasp some of the more detailed tech lingo, it sure gives me confidence that people here know their stuff!

I'm going for a Vokera Vision Plus 30C (combi-boiler), CH 25kW, DHW 31.6kw, Flow Rate 12.9L/min. My flat is 2 very large bedrooms, 1 very large living room, and 1 shower room. The estimated heating requirement at delta 50 is approximately 16kW (3.4m high ceilings!). So I easily could go for the 25C, which is CH 20kW, DHW 25kW, Flow Rate 10.2L/min. But I thought I would go for the better flow rate; my water pressure is approximately 20L/min.

According to the tech spec of the Vision Plus:

Vision Plus C Data Sheet

This does make sense to me. If there was no modulation, then I could see why you would want to limit the max heat output of the boiler. But with modulation, I can't see why you would do this. Indeed, in some circumstances you would want to boiler to go to max.

eg: You come back from holiday in mid-Jan, and it's minus 5C outside. The heating has been off for 3 weeks, and everything is stone cold. So you want to heat the CH water as quickly as possible, and the radiators are shedding large amounts of heat, as the rooms are so cold. The boiler can work at it's max, until the temp starts to climb, and then the modulation can throttle the boiler back, to ensure a gentle arrival at the set-point. If the boiler had been range-rated at say 70%, then even with or without modulation, the property would take much longer to heat-up.

In practice though, I guess one should at lest set the heating to at least a level to protect against frost; but even then, the max output will help. And yes, I'm getting a fancy Wiser control kit, so I can turn on the heating at home when I take off from the Bahamas!

As it's now December 1st, I wish you all a very Merry Christmas, and a Happy New Year.

 

[John.g]

The vision 20S restricts the output to 75% for 15 minutes on each start up so a automatic measure of output, desirable to you or not.
You want to check this out on your combi model because you certainly require full output for your shower.

"With the selector switch in the heating & hot water position and any additional controls (time clock, programmer, room thermostat, etc.) calling for heat, the appliance will operate in the heating mode. The pump and fan will be activated via the flow temperature sensor. When the fan is sensed to be operating correctly (tacho signal), the ignition sequence commences. Ignition is sensed by the electronic circuit to ensure flame stability at the burner. Once successful ignition has been achieved, the electronic circuitry increases the gas rate to 75% for a period of 15 minutes "