Glow worm Ultracom 30sxi not modulating low enough for "smart" TRVs?

[Risingson]

Hi everyone, I'm looking for some advice please.

My system has 5 Drayton Wiser TRVs. There are 8 rads over 3 storeys and a 170l unvented cylinder. One of the attic rads is in the same zone as the cylinder so just has a "dumb" TRV on it. The two bathroom rads also remain without TRVs and I ensure that at least one of them (first floor bathroom) is always open. Boiler also has an internal bypass. At 30kW I suspect the boiler was somewhat oversized for this terraced house when installed, but it was previously a heavily occupied house share and one or two radiators have also been removed when it was subsequently reconfigured (there is a woodburner downstairs.)

The smartness of the Wiser system is confined to on/off Relay mode. I've read that I can make the boiler do Opentherm using a board sourced from another country. However I gather it won't be smart enough to heat the cylinder to the required temperature when there is a lower flow temperature for the CH - unless some other modifications are made to the system. So I haven't bothered with that.

Once the house is warm and perhaps only 1 or 2 TRVs are calling for heat (without the cylinder), I've noticed that the boiler fires up and the flow temperature reaches the preset level at the boiler too quickly - causing boiler to go straight into pump overrun/anti-cycling. This seems to be because the boiler fires at the full 30kW for the first minute or so, and only modulates downwards after that. Even if I set a lower heating load (say, 18kW) in the diagnostic menu, it has no effect for the first minute or so.

Ideally, I'd like to avoid such frequent overruns - as a few minutes later another radiator might be calling for heat but the boiler won't come on until the end of its anti-cycling mode. And I'm concerned that this constant overrunning is shortening the life of the components. It actually tempts me to increase the flow temperature at the boiler so that it doesn't reach that temperature before cutting out - but that obviously makes it less efficient...

Is the reason for this entirely down to the boiler not being able to modulate for the first minute (which isn't helped by the boiler probably being oversized in the first place)? Is this a known issue with the Ultracom boiler and can the software on the PCB be reflashed to fix it? Or, would it go away if I was able to get the boiler to work in Opentherm (and separately address the need to heat the cylinder to a higher temperature)?

I'd also be grateful to know if the internal bypass in the boiler is having any impact on this. The radiator calling for heat does seem to get warm before the boiler cuts out - i.e. it's not being bypassed completely. And it won't be the only radiator open (although the other is a chrome towel rail which won't be losing a lot of heat). The boiler manual says the internal bypass is pre-set to 0.25 bar and the max is 0.35 bar. Does it need to be increased, or indeed could it not be closing properly? It does look that a small amount of sludge has leaked out of the bypass valve screw at some point in the past.

I'm also not sure whether the CH has all floors of the house in one loop, or 3 different loops. If the latter there will be a lot of resistance on the top floor if the only radiator on that floor that's part of the CH circuit is manually turned off - which I often do as it gets too hot on that floor. The only other thing I could also try is telling the Wiser controller that I have an oil rather than gas system, as then it will only call for heat 3 times a hour rather than 6.

Thanks in advance if anyone has any other thoughts!

 

[ShaunCorbs]

Can I ask why you want smart trvs on all your main rads ?

 

[Risingson]

Can I ask why you want smart trvs on all your main rads ?

Hi, a variety of reasons including that the living area is only on the ground floor of the house where we also have a wood burner, or I may be working in my study in the attic all day and not using the living area much.

Essentially, saving money where possible as we can boost the heat in a room when we're actually using it - rather than all the time.

As mentioned there are some rads always on when the CH zone valve is on - in one or both of the bathrooms - these also spread heat out into the hallway/landing where there are no rads.

 

[ShaunCorbs]

Just making you aware that it won’t save you heating the other rooms it will just make the rooms that are on
Heat the whole house as normal uk walls arnt insulated internally and internal doors arnt insulated etc so you will heating the house as normal etc

It’s because the boiler can’t modulate down to 0.5 kw eg one rad calling the lowest it could go is around 6kw etc

 

[John.g]

Hi, a variety of reasons including that the living area is only on the ground floor of the house where we also have a wood burner, or I may be working in my study in the attic all day and not using the living area much.

Essentially, saving money where possible as we can boost the heat in a room when we're actually using it - rather than all the time.

As mentioned there are some rads always on when the CH zone valve is on - in one or both of the bathrooms - these also spread heat out into the hallway/landing where there are no rads.

As stated above, the min putput of your boiler is 5.5/6.0kw and its a bit of a challenge to get these boilers to cycle easily as they are like their cousins? the Vaillants which hold the ignition conditions for 1 minute after fire up before modulation, I don't think theyt fire at 100% more like ~ 55%65% maybe, so once the combustion chamber heats up after say 20 secs or less then the boiler temperature will rise very rapidly as its absorbing, in your case, ~ 18kw for another 40 secs or whatever before modulating down so it needs to have a relatively high boiler SP and fairly high flowrate, if the flowrate is say 10LPM which two small rads or towel rails if valves fully open may flow then the dT through the boiler is ~ 26C so the boiler temp on fire up must be only the the SP-26C , realistically the anticycle won't get the temperature much below 30C which means that the boiler SP (at 10LPM flow) must be 56C or highe, of course if you could double the flowrate then the dT is only 13C and SP temp could theoretically be as low as 43C, a auto by pass can achieve this but won't dissipate much heat so the temperature will still rise rapidly, the internal by pass IMO should be "shut" but don't know if you need to be gas registered to do this if it involves removing the boiler casing.
You might also range rate the output to say 8/10kw, it may help.

Also your suggestion of "I could also try is telling the Wiser controller that I have an oil rather than gas system, as then it will only call for heat 3 times a hour rather than 6." may help.

I have seen this in a number of boiler specs, including yours, don't think it has any effect for the first minute or so where the damage is done but may mean then that it will take 10 minutes to reach its range rated or max output, but hopefully not its minimum.

 

[Risingson]

Apologies for the delay in my reply - something else got in the way. I've also been playing a bit with some settings in the meantime.

Unfortunately I can't post the message I'm trying to post because it says it thinks it is spam(?)

 

[Risingson]

2nd attempt:

John g what you've said seems to confirm something I read elsewhere about it running at a higher rate for the first 1min before modulating down. That's too fast to prevent it going straight into anticycling with only 1 or 2 rads open. I did try ranging it down to 8/10kW - the rads were able to maintain temp but it didn't have much effect on the constant anti-cycling. I think that may be because it's still firing at 55/65% of its maximum rating rather than what I've ranged it down to, for the first minute.

I've tried to play around with the Max burner anti-cycle time at 20C (d2) setting. I had this set to 20 min without fully understanding what this would do to the anti-cycling time at more typical boiler temps (i.e. 50C or above) - I just assumed that anything longer would result in the boiler doing nothing for large periods of time and missing scheduled fireups for HW etc. There was nothing in the manual to say either. But I eventually found a table online and have found that my boiler seems to behave in a similar (not identical) way to that. So I basically have to set the d.2 setting to 50 minutes or so, to get anything like a 10-15 minute gap between fire ups at my set point (SP) of 65 degrees.

Sorry if this was obvious to those in the know, but I wasn't aware of what logic the boiler was applying - and it hadn't even occurred to me to try setting it that high as I thought it would actually leave the house cold for the best part of an hour! I'm surprised they didn't just include this table (or similar) in the manual. Anyway, you live and learn! Although I may need to fine tune it a bit more, I'm hearing a lot less starting and stopping of the boiler now, so I think it might now work OK with the Wiser still set to gas.

 

[Risingson]

Just a comment on the bypass issue though:

I had a leak from the RH hydroblock during lockdown and had to replace that myself (no heating engineer would come!), so I know that's where the internal bypass adjustment screw is. (Behind the outer panelling rather than the gas sealed part of the boiler which I won't go near.) I'm a bit squeamish about adjusting it though, as presumably it is there to avoid damaging the boiler in case someone does shut the valves on the radiators without TRVs - and there is also past evidence of a leak from it.

I assume opening up the valves on the rads without TRVs fully, AND turning the pump to its maximum speed will help lower the temp rise during the first minute, or is that counter-intuitive? Thanks

 

[John.g]

2nd attempt:

John g what you've said seems to confirm something I read elsewhere about it running at a higher rate for the first 1min before modulating down. That's too fast to prevent it going straight into anticycling with only 1 or 2 rads open. I did try ranging it down to 8/10kW - the rads were able to maintain temp but it didn't have much effect on the constant anti-cycling. I think that may be because it's still firing at 55/65% of its maximum rating rather than what I've ranged it down to, for the first minute.

I've tried to play around with the Max burner anti-cycle time at 20C (d2) setting. I had this set to 20 min without fully understanding what this would do to the anti-cycling time at more typical boiler temps (i.e. 50C or above) - I just assumed that anything longer would result in the boiler doing nothing for large periods of time and missing scheduled fireups for HW etc. There was nothing in the manual to say either. But I eventually found a table online and have found that my boiler seems to behave in a similar (not identical) way to that. So I basically have to set the d.2 setting to 50 minutes or so, to get anything like a 10-15 minute gap between fire ups at my set point (SP) of 65 degrees.

Sorry if this was obvious to those in the know, but I wasn't aware of what logic the boiler was applying - and it hadn't even occurred to me to try setting it that high as I thought it would actually leave the house cold for the best part of an hour! I'm surprised they didn't just include this table (or similar) in the manual. Anyway, you live and learn! Although I may need to fine tune it a bit more, I'm hearing a lot less starting and stopping of the boiler now, so I think it might now work OK with the Wiser still set to gas.

Your boiler probably has anti cycle look up tables very similar to a Vaillants, below. You can see that a anti cycling time of 50 minutes combined with a target flow temp of 65C gives a actual anti cycling time of 10.0 minutes which should ensure that the system contents temperatures are as low as possible to give the boiler time to modulate down before its flow temperature reaches target temp + 5C and burner cut out. SO
"I assume opening up the valves on the rads without TRVs fully, AND turning the pump to its maximum speed will help lower the temp rise during the first minute." will certainly help

 

[Risingson]

Thanks John - yes that was the table I found. I'm not sure I'm getting exactly the same reported remaining anti-cycling time when it first kicks in, but they are fairly similar - so I've adjusted it accordingly. Thanks also for confirming that it seems to cut out when it goes 5C over the SP - I had guessed that must be what it's doing. (If it hasn't anti-cycled enough it can quite easily shoot from the SP to +5C in a matter of seconds anyway).

I've increased the pump speed although as it's got too hot in the attic again I've closed the second towel rail - but one is fully open. I think in the past both were partially closed to stop them stealing too much heat from the other rads. I think the higher anti-cycling time is making a lot of difference already!

Is it best to ask an engineer to assess and set the bypass valve to the right cutoff pressure when I next need one for something? Thanks again.

 

[John.g]

I would set the internal bypass to its max which is only 0.35bar (3.5M), (default 2.5M). What pump speed setting are you on now as you only want the by pass to open on pump overrun. Is it on one of these settings?

Not quite sure from below which way to turn the adjustment screw.