Range rating, modulation and manually setting the CH temperature question.

[Ltnman]

Range rating a boiler means adjusting its output to more closely match the heat requirements of a property, while on the other hand boiler modulation also reduces the output of the boiler to match the heat requirement of the property, as it gets close to its desired temperature.

Range rating is performed to reduce cycling and extending boiler life but is this also not done by just turning down the CH temperature to say 50 or 55c, which just sets the maximum temperature that modulation can occur so again reducing cycling?

Is modulation and presetting a lower CH temperature a form of range rating for those without range rated boilers?

 

[muggles]

No, range rating limits the maximum power whereas turning the temperature down limits the maximum temperature

 

[Ltnman]

But isn’t the limiting of the maximum power is what modulation is also doing?

 

[vulcancontinental]

I've pasted this text I wrote for a FaceBook group (I was promoting a new boiler but the principle applies to all boilers).

A three bed semi may require 9kW and you can have a 25kW combi with 20kW output to the heating out the box.

The combi is over twice the output that the house can absorb so boiler comes on and hits setpoint very quickly and cycles; delta T closes affecting the efficiency of transfer in hex, boiler is slightly noisier as fan runs faster at higher outputs.

Modulation takes place after the rapid heat up so comes to the party late.

Some boilers have a reduced startup of say 70% then slowly ramp up.

The trouble is the reduction is only from the max output so 75% of 20 is 15kW; still almost twice what the house can absorb and remember the 9kW it requires is only when the weather is at its coldest, say -3°C. For most of the heating season, 18 of the 20 weeks, it needs perhaps 4 to 6kW.

Best is to have slow incremental startup; not from max output but from the range rated output, say 60% for the first 15 minutes, then if demand from the controls continues the boiler automatically adds 10% to its output. After another 15 minutes, if the heat demand continues, another 10% and so on till 100% of range rated output or controls stop the boiler firing or it reaches setpoint.

When the property is up to temp that's where good modulation comes in so in the example above the boiler is range rated to 9kW, it starts at 60% (just under 6kW) then ramps up in stages 10% at a time reducing cycling.

When property approaches the required space heating setpoint good modulation, say down to below 3kW keeps the boiler better matched to the load rather than stop start.

A bonus is that without swings of on/off generally the customer can run the room control a degree or so cooler as temperatures are more stable and they will not faff with the stat as often and just let it do its job.

Dual temperature weather comp or OpenTherm help in this but are not a substitute for soft start and modulation by themselves.

BTW, all the above is as standard with the new Vokera Vibe Max 26C stainless combi.

Range rating is simply button pressing, slow startup from the range rated output and modulation to below 3kW is automatic.

It's also included with the Vokera excel-i from Wolseley and the Vokera easiheat-i from independents.

Just thought I'd mention it.

 

[Johntheo5]

But isn’t the limiting of the maximum power is what modulation is also doing?

You could say that range rating a boiler reduces its modulation ratio because it doesn't and cannot reduce its minimum output. Ie a combi boiler might be say 40kw with a modulation ratio of 8 to 1 but if its range rated to meet a heating load of say 20kw then it's effective modulation ratio is 4 to 1?. If I had a gas fired combi boiler I wolud think I would range rate it to the heating load + 25%. Some boilers like Vokera limit their output automatically to 75% of max every time they fire up, for 10 minutes.

 

[JonathanM]

Thanks for this information. It's very hard to find elsewhere.

Would you know what sort of initial fire up pattern the Ideal Logic boilers have? Specifically, the current model 24KW Heat and 24KW System?

 

[ericmark]

Thank you @vulcancontinental most interesting, although it seems the only control I have is the maximum temperature. I feel my radiators are too small, if all rooms are heated then yes I may be able to put the full 20 kW into the house, but since I only heat rooms when required, the boiler cycles on/off throughout the day. Clearly it should cycle once house it up to temperature, but it is cycling while still warming up, and I have considered fans next to radiators to increase output.

But the big problem is to get my wife to close doors. Having nine programmable TRV heads is all well and good if doors are closed, but in the main they are left open.

 

[Johntheo5]

What boiler make/model and how many minimum rads/size are on when cycling in heat up mode?, Boiler cycling shouldn't be of huge concern except that the anti cycle time is very long resulting in the rads actually cooling down when they should be heating up as the circ pump does not or should not stop, it doesn't have a huge effect on boiler efficiency, @vulcancontinental "As long as the burn time exceeds 3 minutes the hit on efficiency is less than 1.5%"

 

[vulcancontinental]

What boiler make/model and how many minimum rads/size are on when cycling in heat up mode?, Boiler cycling shouldn't be of huge concern except that the anti cycle time is very long resulting in the rads actually cooling down when they should be heating up as the circ pump does not or should not stop, it doesn't have a huge effect on boiler efficiency, @vulcancontinental "As long as the burn time exceeds 3 minutes the hit on efficiency is less than 1.5%"

Yes, everything now is incremental, the boilers are at maximum efficiency, improvements are one percent here a few more percent there getting the thing to work as best it can with the system and the user. Last year however if 3% represented £30, this year it's £60.

Plenty of claims out there of "I saved a packet" or "my customer saved 20% with weather compensation or OpenTherm" whatever. I had one guy claim WC saved a customer something like 20%, I asked "what, just adding a sensor?" No, he said, he also changed the boiler, some pipework radiators and a flush!!

Without full record of before and after and exactly what was done it's just talk.

 

[Harry Bloomfield]

Plenty of claims out there of "I saved a packet" or "my customer saved 20% with weather compensation or OpenTherm" whatever. I had one guy claim WC saved a customer something like 20%, I asked "what, just adding a sensor?" No, he said, he also changed the boiler, some pipework radiators and a flush!!

Without full record of before and after and exactly what was done it's just talk.

It's near impossible to do a like for like comparison anyway, with so many variables like outdoor temperature, humidity, wind and even variables such as people entering and leaving via the outside door.

 

[mpgw]

I've pasted this text I wrote for a FaceBook group (I was promoting a new boiler but the principle applies to all boilers).

A three bed semi may require 9kW and you can have a 25kW combi with 20kW output to the heating out the box.

The combi is over twice the output that the house can absorb so boiler comes on and hits setpoint very quickly and cycles; delta T closes affecting the efficiency of transfer in hex, boiler is slightly noisier as fan runs faster at higher outputs.

Modulation takes place after the rapid heat up so comes to the party late.

Some boilers have a reduced startup of say 70% then slowly ramp up.

The trouble is the reduction is only from the max output so 75% of 20 is 15kW; still almost twice what the house can absorb and remember the 9kW it requires is only when the weather is at its coldest, say -3°C. For most of the heating season, 18 of the 20 weeks, it needs perhaps 4 to 6kW.

Best is to have slow incremental startup; not from max output but from the range rated output, say 60% for the first 15 minutes, then if demand from the controls continues the boiler automatically adds 10% to its output. After another 15 minutes, if the heat demand continues, another 10% and so on till 100% of range rated output or controls stop the boiler firing or it reaches setpoint.

When the property is up to temp that's where good modulation comes in so in the example above the boiler is range rated to 9kW, it starts at 60% (just under 6kW) then ramps up in stages 10% at a time reducing cycling.

When property approaches the required space heating setpoint good modulation, say down to below 3kW keeps the boiler better matched to the load rather than stop start.

A bonus is that without swings of on/off generally the customer can run the room control a degree or so cooler as temperatures are more stable and they will not faff with the stat as often and just let it do its job.

Dual temperature weather comp or OpenTherm help in this but are not a substitute for soft start and modulation by themselves.

BTW, all the above is as standard with the new Vokera Vibe Max 26C stainless combi.

Range rating is simply button pressing, slow startup from the range rated output and modulation to below 3kW is automatic.

It's also included with the Vokera excel-i from Wolseley and the Vokera easiheat-i from independents.

Just thought I'd mention it.

Does this soft start & modulation functionality exist in the Vokera Vision Plus S or Unica Max S system boilers?

 

[vulcancontinental]

Yes and no.

First the Vision Plus, this boiler starts at 60% of the range rated heating output for the first 15 minutes then increases output in 10% increments until a timer or thermostat stop it or it reaches temperature setpoint. The modulation is 1:7. I had to go check this myself as it's not in the manual (I range rated the output on a 35kW combi - 30kW to heating to 10kW and timed it watching fan speeds).

The Unica Max is also a 35kW combi model that I range rated to 12kW. It starts at 75% of the range rated heating output and ramps up to 100% after 15 minutes. The fan is also subject to being part of the active combustion control so is dynamically involved in fuel/air mix. Modulation is 1:8.

I don't have system boilers on the test rigs but the software is the same.

This was an interesting part of my day as there is no training today

 

[JonathanM]

First the Vision Plus, this boiler starts at 60% of the range rated heating output for the first 15 minutes then increases output in 10% increments until a timer or thermostat stop it or it reaches temperature setpoint.

At what point does it try to modulate down? I thought a boiler would modulate down as it approached the set point, rather than increasing further?

EDIT: I think I read your comment a bit too literally. Thought you meant it kept ramping up until setpoint then just turns off.

 

[Johntheo5]

It's worded badly in the manual, relations vision 20s starts to modulate down within seconds of firing up.

 

[ScottishGasMan]

A lot of boilers will only modulate down when there within about 5°C of their target temperature, so on a boiler significantly oversized aiming for say 65°C, it may hit 60°C and start to modulate, but the temperature is already climbing to 65 and overshoots causing it to cycle off before it has time for the modulation to work.

Where range rating alters the maximum heat output to the system, so lowering this will initially take longer to warm up the system water, the boiler will then be able to glide into its desired temperature while modulating to suit.

If its the spring/autumn and the heat load is lower than the minimum output of the boiler, then it will eventually have to cycle off anyway. Where in winter if the required heat is lower than the boilers minimum modulation, then it will be able to run pretty much indefinitely without cycling.

Cycling is inevitable for any heating appliance unless it's grossly undersized for winter load, the amount of cycling and response to cycling is what makes the difference. Old boilers used to be able to light and go out, then light again, all in the space of 1 minute, which meant high losses from unburnt gas leaving the flue at ignition. Modern units "usually" have better control over this where they will leave longer in-between cycles intentionally. Some will over heat the water i.e.: aiming for 65°C, they will heat to 70°C so that when they cut out, they can stay off for longer, maybe until the water is at 55°C before coming back on.


The University of Salford performed a test in a house built within a climactic chamber, their results were something in the order of a 13% saving using compensated controls to lower the flow temperature to suit the house. I'll caveat this by saying, I don't think the tests were performed over a long term, and it was one boiler type, one control type, and one house type, so the results to me, only show that there is a cost and CO2 saving to be made, but I'd never claim compensation controls can save that much for everyone.

A properly condensing boiler, might save in the region of 5-7% just from the fact it's condensing (Theoretical max saving is 10%) The rest will come from a mixture of things, from cycling to how long the heating has run for and the likes.