Why can't my Greenstar 24Ri EPR+ boiler not run Low Temp?

[Heman86]

I got a Worcester Greenstar 24Ri boiler with my house and I'm looking into making it as efficient as possible before taking the plunge at some later date to get a smarter boiler like a two pipe Intergas HRE 18 SB with weather comp and PDHW. But short term, I want to resize my radiators upstairs to accommodate low temp heating. (downstairs is UFH that runs on 55deg max). The smallest CH pipes I have are 15mm feeding the radiators. Plenty of 22mm pipe otherwise, so other than a few undersized rads, I should be able to run low temp heating at 55deg. The 24Ri is a basic regular boiler that doesn't even have clear markings for flow temp output. Just a dial with markings of 35degree min and 82 deg high. Max output is 24kW and (I might be reading the installation manual wrong) min output is 8kW. Not sure how much modulation this boiler can actually do or if it's just full power on or off for timed intervals, depending on what temp flow you set the dial to.

Looking through the manual it explicitly says the boiler can't be run on a low temp system. See image below. Anyone suggest why this may be? I was planning on measuring the surface temp of the flow pipe to get a gauge of where 55deg flow would be and manually setting and monitoring temp to keep it in that region.

On a related note. I installed drayton auto balancing valves on all the return sides of all my rads and I'm having trouble getting a differential pressure of min 0.1 as recommended in the manual. Pump fluctuates between 0.0 and 0.1. I only get 0.2-0.4 when I crank the pump all the way up to max power. I have the pump on constant pressure rather than modulating as I'm sure I read somewhere the valves won't balance correctly on a modulating pump. My boiler also only runs for 30secs interval with the valves (a lot of cycling). Not sure if this is my tado TRVs trying to create an artificial TPI type environment or if there's too much resistance in the pipes and return flow is being affected going back to the boiler, causing it to shut off.

Any help or thoughts on any of this would be most appreciated.

 

[Why Not Indeed]

If you're trying to understand how it works, disable all the smart stuff first. TRVs wide open.

Max output is 24kW and (I might be reading the installation manual wrong) min output is 8kW. Not sure how much modulation this boiler can actually do

Then it modulates down to 1/3rd.

 

[Heman86]

Tried that myself. Unfortunately, rad system is hooked up to Tado wireless receiver, so even when I remove all TRVS and have valves fully open, Tado Wireless thermostat is still controlling things. So I can't 100% isolate from the Tado controls. I reached out to Drayton to see if they'd have any ideas why the boiler is running on short intervals since fitting the valves but just got back standard instructions on how to install them. No answer as to why the delta P wouldn't go above 0.0. They just assumed I was talking about delta T.

Any ideas why Worcester would advise this boiler not to be used as a Low Temp Boiler? Cause of the limited modulation? 3:1 isn't a great ratio but surely would still allow for some low temp running.

 

[MNW67]

Any ideas why Worcester would advise this boiler not to be used as a Low Temp Boiler? Cause of the limited modulation? 3:1 isn't a great ratio but surely would still allow for some low temp running.

I found this definition. It sounds like it might be something to do with EU Regs:

https://bpec.org.uk/downloads/CE30%20-%20Domestic%20heating%20by%20gas.pdf

Low temperature boiler: A non-condensing boiler designed as a low temperature boiler and tested as a low temperature boiler as prescribed by the Boiler Efficiency Directive (i.e. the part load test was carried out at average boiler temperature of 40°C)

 

[ericmark]

I started to read the document, and found it hard going, but when it stated

6. What is a thermostatic radiator
valve (TRV)?
TRVs sense the air temperature around them and regulate the flow of water through the radiator which they are fitted to. They do not control the boiler.

I stopped, as clearly written by someone who wants to over simplify the way things work to the point where they get it completely wrong.

The boiler heats and pumps water around the system, and it measures the return water temperature, as the TRV heads close, first they force more water around those still open, then it opens the by-pass valve which means the return water gets hotter and hotter, the boiler used this to work out how much to modulate, so the TRV does control the boiler by the temperature of the return water.

OK electronic TRV heads can connect to a hub, and the hub using OpenTherm or something like that, can control the boiler better than just using the return water, but as to how much better, that is not so easy to work out.

The big problem is the use of on/off controls, many boilers if switched off, when switched back on again, start working out the output required from scratch again, so where possible controls need to be analogue, but tests in a warehouse as to how efficient do not really relate to the real home. I know in my home the radiators are far too small, I often only want the living room heated, and to do that the boiler is cycling on/off all the time.

The buzz word is recovery time. From the time you decide you want to use a room, how long will it take to heat it? If my wife looks at the TV guide, and sees nothing worth watching, she will want to use her craft room, want that to re-heat within an hour, this seems a problem with modern systems.

 

[Heman86]

I found this definition. It sounds like it might be something to do with EU Regs:


https://bpec.org.uk/downloads/CE30%20-%20Domestic%20heating%20by%20gas.pdf

So you reckon it might just be a bit of beaurocracy? EU regs consider a "low temp boiler" to have an average flow temp of 40deg and be "non-condensing"? So because the 24Ri is a condensing boiler its not classed as as a low temp boiler?
Doesn't make sense to me why a low temp boiler has to be definied as non-condensing. But anyway, the 24Ri should technically be able to run a system at 55deg flow right?

 

[Madrab]

Ultimately it comes down to what is considered to be low temp. If the boiler wasn't able to run @ a 35deg flow temp then the boiler wouldn't be able to be adjusted down to that temp by it's controls, to me it really is as simple as that. My understanding though, when a low temp heating system is mentioned, is a system that can be run between 55>35deg and will constantly condense.

Of course that will have a significant impact on the radiators output, if the heat loss calcs and rad output sizes were originally calculated and surveyed @ ΔT50

 

[Why Not Indeed]

The Boiler (Efficiency) Regulations 1993

These Regulations implement Council Directive 92/42/EEC (“the Directive”) concerning efficiency requirements for new hot water boilers fired with liquid or gaseous fuels and Council Directive 93/68/EEC (“the amendment Directive”) amending the Directive. The Directive specifies standards of...

www.legislation.gov.uk

“low temperature boiler” means a boiler which can work continuously with a water supply temperature of 35 to 40°C, possibly producing condensation in certain circumstances, including condensing boilers using liquid fuel;

 

[Heman86]

The big problem is the use of on/off controls, many boilers if switched off, when switched back on again, start working out the output required from scratch again, so where possible controls need to be analogue, but tests in a warehouse as to how efficient do not really relate to the real home. I know in my home the radiators are far too small, I often only want the living room heated, and to do that the boiler is cycling on/off all the time.

Appreciate this. An Opentherm compatible boiler with the ability of modulating power output as low as 2kW or even less would be ideal. But even with Tado maybe trying to create an artificial Opentrm/TPI system, I still wouldn't expect the boiler to only be running for 30sec intervals. I think my issue is more around the new auto balancing valves I fitted to rads.

I've played around with pump settings again and I'm pretty confident the issue is differential pressure (DP).
As per Drayton instructions I've fitted and set the valves for a flow and return at a △T of 20° across each individual rad. The rads power output has to be △T50 (the rads aren't oversized for the rooms so require full output). Checking the pump, it's getting a DP of 0.0. The boiler is running in such short intervals because it's not getting any return back. I'm assuming the valves are creating too much resistance. Currently the only way to overcome this is to set my pump to max power. I then get a DP of 0.4. But this obviously is not the solution. Not only does that create noise but its a ridiculous amount more power than should be needed. I've only got 5 small-med sized rads and a towel rail. Max power on the pump allows for 30 rads to be run. Anyone have experience with these auto balancing valves?

I'm going to try changing the settings on the valves for a △T of 10 for flow/return across the rads, that probably opens up the valves creating less resistance. But puzzled as to why I can't set them at △T20.

 

[Heman86]

Ultimately it comes down to what is considered to be low temp. If the boiler wasn't able to run @ a 35deg flow temp then the boiler wouldn't be able to be adjusted down to that temp by it's controls, to me it really is as simple as that. My understanding though, when a low temp heating system is mentioned, is a system that can be run between 55>35deg and will constantly condense.

Of course that will have a significant impact on the radiators output, if the heat loss calcs and rad output sizes were originally calculated and surveyed @ ΔT50

Rads are sized for ΔT50 so I'll have to upsize but just confirming the 24Ri boiler would be alright to run at 55deg max flow before I commit to buying larger rads. I assumed the boiler would be able to until I saw that line about low temp boilers in the manual. But shouldn't be a problem cause the boiler can be manually set to temps between 35deg>82deg and is condensing.

 

[Johntheo5]

You should have no problem In running the boiler at 55C if the dT is 10C to give a return temp is 45C, you will have T30 rads with 51.48% of their T50 rating,. At 8.0kW output, the boiler flowrate must be 11.47LPM, 0.69M3/hr. If you run with dT of 20C, flow/return, 55C/35C, then you have T25 rads with 40.61% of their T50 rating. At 8.0 kW output the boiler flowrate will only be 5.73LPM, 0.344M3/hr, requiring 1/4 of the pump dP at the 10C dT.