Help understanding boiler modulation

kittennose

I’m looking at new boilers and a bit confused about efficiency and modulation.

from my understanding a combi can be set to have 2 different flow temperatures, hot water and heating.

a standard heat only boiler will just have one, which will need to be 65c or more as the hot water storage needs to be that high for legionella.

so how would the heat only boiler modulate to a lower temp or go into non-condensing mode if it’s set at 65c?

FreeBear

With a heat only (or system) boiler, you do not need to run it at 65°C and higher. Legionella is a very low risk (only some 400 cases reported annually), and as long as the hot water is maintained at a temperature of 50-55°C, the bacteria will die. Only in a commercial or care environment do you need a DWH temp of 60°C (along with thermostatic controls on all hot taps).

So in a domestic environment, a 60°C flow temperature is perfectly acceptable - Assuming a temperature drop of 13-14°C on the return, the boiler will start to condense. To maximise efficiency, you'd be wanting a lower flow temperature of around 50°C or even 40°C. But to make best use of such low flow temperatures, the radiators need to be a lot larger. With a heat only boiler, the control would (should ?) command a higher flow temperature so that the DHW tank gets heated to the set point.

Modulation is pretty standard on gas boilers, and the smarts that handle it are all internal. All you need to do, is tell the boiler what temperature you want it to run at, and it will automatically modulate down when required. The problems start when the boiler can not modulate down low enough to match gas burnt with the heat load, so it starts to short cycle. To avoid this, you need to look at the minimum boiler output and match it to your radiators.

I have a Viessmann 050 which will modulate down to 3.2KW and a total radiator load of some 7.5KW@50°C flow temp. Not seen any sign of short cycling, but if/when I do, control parameters will be tweaked.

Oh, and I have the DHW set for 45°C (Legionella risk is virtually zero with a combi).

Edit to add - One other advantage of running at a lower flow (and DHW) temperature, particularly if you are in a hard water area, is reduced build up of scale.

ThisIsWeird

What a darned good question, Kitten.

And, although largely answered by FreeBear, it does leave some interesting points about heat-only and system boilers.

First, tho', just to make clear - 'modulating' is not the same as 'setting the output temperature of the boiler'. Early boilers - going back beyond 15-odd years - were simple water heaters with a burner under the block that held the system water. The burner came on full blast, and heated up the water until a thermostat went 'click'. That thermostat set the water temp required, and this was usually adjustable by the user - ie, you could set the water temp you wanted. When the water reached that temp, the burner went off. When the temp dropped, the burner came back full on. So, if you only had a couple of radiators running, the boiler would be switching on and off regularly, with every 'on' being full blast! Not very efficient.

'Modulating' boilers have adjustable gas flames, a bit like a gas hob. So, as the required water temp is reached, it doesn't 'switch off', but instead turn itself down, ideally trying to match the demand. They constantly measure both the flow and return temps, so it has a good handle on the actual system 'demand'. As your house heats up, and the return flow is also warmer as a result, the boiler will turn down its flame to meet this reduced demand - far more efficient and far less wear; the boiler will be 'ticking over' for much of the time.

That's 'modulating'.

And, as you say, a Combi boiler will also have separate CH and DHW output controls.

You are asking an important Q about Heat-Only and System boilers, and how they can answer the different demands of CH and DHW. The answer is, I don't know.

FreeBear points out that 60+oC DHW isn't 'necessary', but I'd still personally like this, as you need 'hot' water for dishes, baths, showers, and the ability to fine-tune the temp by blending some cold. And even if you were able to manage with a reduced 55oC DHW, that is still higher than ideal for your most efficient CH.

I think that some current HO/System boilers can have separate temp outputs, controlled by the programmer's demand? Ie, when a DHW demand is made, it'll switch to a higher output, and vice versa for CH?

Or, perhaps Opentherm or similar control protocols will be able to get the 'higher' boiler temp down quite quickly when running the CH system, so as to get into fully-cond mode as quickly as possible?

But, I don't know. And as you point out, Kitten, a seeming flaw.

FreeBear

ThisIsWeird said: 'Modulating' boilers have adjustable gas flames, a bit like a gas hob. So, as the required water temp is reached, it doesn't 'switch off', but instead turn itself down, ideally trying to match the demand. They constantly measure both the flow and return temps, so it has a good handle on the actual system 'demand'. As your house heats up, and the return flow is also warmer as a result, the boiler will turn down its flame to meet this reduced demand - far more efficient and far less wear; the boiler will be 'ticking over' for much of the time.

That's 'modulating'.

Modulation curve from my boiler - Control requested 52.5°C at 07:00 to raise the temperature of the house by 1°C from an initial 18.5°C. For the first ~15 minutes, running at 35% (about 8.4KW) before slowly dropping down to 10% (should be 12% or 3.2KW). According to the Bright app, using 7.4KWh of gas.

With my old Baxi back boiler (vintage non-condensing), I would have used around 12KWh of gas for the same temperature increase (the excess heat being dumped in to the hot water tank).

ThisIsWeird said: I think that some current HO/System boilers can have separate temp outputs, controlled by the programmer's demand? Ie, when a DHW demand is made, it'll switch to a higher output, and vice versa for CH?

Yup. Controls like Hive & Nest should have a parameter for DHW and command a higher flow temperature when heating the tank. Shouldn't make any difference whether it is OpenTherm, EMS (Worcester Bosch), or eBUS (Vaillant) - They all do the same job in as much as telling the boiler what temperature to operate at. The smarts inside the boiler will handle the modulation and turn down the flame to suit the load.

WobblyJelly_1

ThisIsWeird said:

First, tho', just to make clear - 'modulating' is not the same as 'setting the output temperature of the boiler'. Early boilers - going back beyond 15-odd years - were simple water heaters with a burner under the block that held the system water. The burner came on full blast, and heated up the water until a thermostat went 'click'. That thermostat set the water temp required, and this was usually adjustable by the user - ie, you could set the water temp you wanted. When the water reached that temp, the burner went off. When the temp dropped, the burner came back full on. So, if you only had a couple of radiators running, the boiler would be switching on and off regularly, with every 'on' being full blast! Not very efficient.

'Modulating' boilers have adjustable gas flames, a bit like a gas hob. So, as the required water temp is reached, it doesn't 'switch off', but instead turn itself down, ideally trying to match the demand. They constantly measure both the flow and return temps, so it has a good handle on the actual system 'demand'. As your house heats up, and the return flow is also warmer as a result, the boiler will turn down its flame to meet this reduced demand - far more efficient and far less wear; the boiler will be 'ticking over' for much of the time.

That's 'modulating'.

Exactly the information that I was looking for but definitely not yet sure that my description/question below is right - so any further clarity would be very appreciated....


I have a relatively old (2011) modulating gas combi (Vaillannt Ecotec Pro 28). 

It's currently only used for DHW (no space heating) but am considering using it as the fill-up & maintenance hot water source for a very well insulated 1 metre cubed hot tub (0.125U on all faces) with top cover the same and that only removed for use.

At the minute i'm trying to analyse whether to initiallly fill/create the 1 metre cube with 40C water using:

(i) it's internal water to water heat exchanger (i.e. it's native DHW exchanger via an insulated hose emptying into tub)
 or
(ii) to purchase a second external water to water heat exchanger to connect the existing CH flow / return to that and heat the hot tub using recirculation.

For info - being very unfamilair with hydronics/thermodynamics the combi was historically used for space heating I have no memory of ever adjusting the CH flow temp set point. Space heating to all bungalow radiators was treated as one zone with air temperature single thermostat in a room with no TRV's.

Anyway, the aim, related to this comment, is to fill the tub as quickly as reasonably possible - e.g. 3kW in-line electric/ASHP is totally unsuitable for the speed convenience sought.

Looking at point (ii) only for now, in detail (i.e. purchase of a new external heat exchanger) the Combi boiler space heating is stated in the install manual at 60c flow / 40C as 9.3kW to 24kW.

The external heat exhanger brochure I am currently looking at has a "standard" output of 30kW. The "Transfer Table" for this item states:
a/ 26kW Transfer rate at delta T of 40C
b/ 15kW Transfer rate at delta T of 20C.

Question 
  
Assuming, for discussion, that:
The hot tub has been filled with cold water just above freezing & the combi CH flow set point is 60C  

Could the following be a reasonable imagination of what happens when the hot tub's in water thermomenter calls for combi heat until it's set temp of 40C is realised?

1/ Combi gas flow/fan are maximised to provide CH thermistor flow at 60C as quickly as possible. Appoximately 26kW very quickly starts to transfer to the 1m3 of pool water.

2/ As the 1m3 pool water begins to heat the combi CH return thermistor registers a gradually increasing increasing temperature and modulates both the combi flame and fan down to exactly match the hot tub load as seen by the new water to water heat exchanger.

3/ Finally, at hot tub water temp of 40C just prior for the call for heat to end the rate of transfer to hot tub is 15kW and that created by the combi gas to water exchanger has been modulated down to this same 15kW.

If 1/ to 3/ are true then is the entire boiler modulation (flame size/fan speed) logic managed entirely by readings supplied by only the combis CH flow and return thermistors?

(PS: my understanding is that the new external water to water heat exchanger will consume approx 10% of power available from the combi but have ignored that in above comment) 
   

kittennose

6 Months into the new boiler, and the energy saving over our 1980s boiler is marginal at best. As suspected it isn't enough to justify the cost of the new boiler as the small saving each month will take longer than the average lifespan of the boiler to pay off - £20-£30 a month that's all.

But were still happy with a combi

ThisIsWeird

Thanks for the update.
Absolutely - the 'MoneySaving' way is to keep your old boiler running for as long as possible, until repairs can no longer be justified.

kittennose

ThisIsWeird said:

Thanks for the update.
Absolutely - the 'MoneySaving' way is to keep your old boiler running for as long as possible, until repairs can no longer be justified.

I agree - we didn't swap so much for cost savings and knew the numbers didn't add up. But I think a lot of people get conned into modern boiler's "energy saving" benefits which simply don't materialise into actual savings. My mum has British gas telling her she needs to upgrade because her 10 year old boiler is inefficient and she's throwing money away....

That's why I updated it, just for people looking to switch. We were at the extreme end, a very inefficient 36 year old boiler and the savings are very minimal.

FreeBear

I'd been whittling down the consumption on my old Baxi back boiler (installed 1988/9), and had got it down to around 3500kWh per year. With the new Viessmann 050, current forecasts are around 2200kWh, but last winter was relatively mild and I had been using the wood stove on colder days.

Saving ~£60 a year on gas at current prices, I won't see a return on investment in my lifetime. But having instant hot water and much better control over heating goes some way in making up for it. The extra space freed up by removing the DHW cylinder is a bonus.

Oldwind

My boiler a Worcester B 2000 30kw is a compact. When in heating mode then a demand for hot water i.e. for washing hands the burner stops then restarts to heat water. If there is some hot water in the pipes I’ve noticed a hot-cold-hot supply from the tap because of a slight delay during switching from heating to DHW. Of course if the boiler has modulated down while heating the rads the delay to get hot water is greater. The hot taps can be turned on and off then a ‘pre heat’ of water takes place, turn tap on and hot water is supplied a lot faster and with better efficiency.
Having become used to how the boiler functions the timer for morning heating turns off before showering etc. A quick on and off of the hot tap will heat water to the set temperature and while heating is off there is no delay for more hot water.
During summer or when no heating needed there is no hot water delay. The anti seize function, runs heating pump with no burner once a day, can steal hot water stored so setting this to occur during the night is a good option.
I’ve had this boiler for 4 years and have assumed this is how most if not all combi compacts function and I’m sure It’s saved me money if not more with my boiler habits

ThisIsWeird

Oldwind said:

My boiler a Worcester B 2000 30kw is a compact. When in heating mode then a demand for hot water i.e. for washing hands the burner stops then restarts to heat water. If there is some hot water in the pipes I’ve noticed a hot-cold-hot supply from the tap because of a slight delay during switching from heating to DHW. Of course if the boiler has modulated down while heating the rads the delay to get hot water is greater. The hot taps can be turned on and off then a ‘pre heat’ of water takes place, turn tap on and hot water is supplied a lot faster and with better efficiency.
Having become used to how the boiler functions the timer for morning heating turns off before showering etc. A quick on and off of the hot tap will heat water to the set temperature and while heating is off there is no delay for more hot water.
During summer or when no heating needed there is no hot water delay. The anti seize function, runs heating pump with no burner once a day, can steal hot water stored so setting this to occur during the night is a good option.
I’ve had this boiler for 4 years and have assumed this is how most if not all combi compacts function and I’m sure It’s saved me money if not more with my boiler habits

That's interesting - I'd never heard of this 'quick tap' feature before. Very clever.