I am currently renovating (well, procrastinating over options) our utility room and thoughts are focussed on the boiler and whether I should seize the opportunity to replace/move it whilst ceilings are down and other factors mean I've got complete freedom of choice.

The current system setup is as follows:

Ideal Icos HE15 heat-only boiler with a single flow+return leading to...

Range Tribune unvented cylinder in the airing cupboard where there's also an external pump, CH and DHW zone valves connected up in an S-plan configuration.

Honeywell T6R-HW controller

Despite a complete lack of servicing bar me fixing a couple of zone valve issues over the years, the system works well. The boiler easily heats the house and the cylinder seems to be working fine too. But it is 20 years old, and I can't help but feel that I've been fairly lucky and might be on borrowed time. Honeywell's TPI implementation means the boiler is well exercised during steady operation to keep a very stable house temperature, but this must put extra strain on the boiler with its periodic firings. Plus, if something happens which means I need to replace the boiler in the next few years I don't mind replacing it early out of choice when I'd be doing so without the pressure and stress of a cold house.

But the question is: should I stick with a 'heat' boiler or a 'system' boiler?

The question has arisen because going through the likes of the Heatable/Boxt/Warmzilla websites just to get an idea of ballpark costs (~£2k all-in), their selection process goes as follows:

No cylinder at all -> Combi boiler

Vented cylinder + Feeder tank in the loft -> Heat boiler

Unvented cylinder -> System boiler

They don't seem to recognise the setup I've got where we have a heat boiler connected to a sealed system with an unvented cylinder. Why not? There must be thousands of houses just like this. Perhaps they are just dumbing down the ordering process to cater for zero-knowledge customers, and if I want another heat boiler then I could just force the ordering process and pretend I've got a vented cylinder and feeder tank, but this still brings me back to the question as to whether I do want to stick with a heat boiler or go with a system boiler instead given the choice?

Here are my thoughts on pros and cons of a heat boiler:

Pros of a Heat Boiler:

• Slightly cheaper due to fewer internal parts e.g. no internal expansion vessel, pump or pressure gauge

• Slightly simpler due to the few internal parts

• Whilst some of the complexity now sits outside the boiler, this makes it far easier/safer for me to perform DIY repairs on those parts

• I can just get the installer to rip the old boiler out and fit the new one and not require them to do anything else (like removing the external pump, squeezing in a bigger boiler etc). To be frank, I dread getting tradesmen in as it seems to nearly always end up with me being unhappy about something. I'm fussy, and whilst everyone can be a bit sloppy with workmanship and/or make mistakes - and I include myself in this - I really don't like paying someone else for this to happen.

Cons of a Heat Boiler:

• I don't know! What are the downsides? What does a system boiler give me that a heat boiler doesn't?

For what it's worth I've been looking at Ideal boilers again (maybe the Logic Plus line at the budget end), partly because our Icos has lasted so well but also because various teardown/servicing videos on Youtube seem to suggest they are reasonable well made and respected, and also because I know they can support a PDHW configuration with separate CH and DHW triggers and temperatures which is a setup I am keen to have. I know this isn't unique to Ideal but it has been far from obvious exactly which other manufacturers and their models support it. At least Ideal are explicitly talking about it and make it clear how to set up and operate in the installation instructions. I do still need to check whether they support Opentherm to allow my Honeywell T6 to control flow temperatures, but even if not I'd be content to just stick with the CH and DHW temperature controls on the boiler, perhaps with weather compensation influencing the former. Ideal boilers made in Britain which feels like a nice bonus too.

I would welcome your thoughts!

Edited 3 hours ago3 hr by MJNewton

As a previous gas installer, now only heat pumps, I would always go down the line of a system boiler over a heat only. With a system boiler you also get a modulating pump matched to demand, and if you go a little higher in price, you can get 12 years warranty, the benefit of which covers your expansion vessel and pump. If you go with a heat only, nothing outside the boiler is covered by the warranty.

I honestly can't remember to situation with Ideal, but even Viessmann don't provide Opentherm connections with the heat only boiler, you need to go system for that. In addition, the Ideal pdhw setup using a separate SL in the boiler is technically only for when you use weather compensation and their field engineers are generally not happy if it's used with load compensating controls. Ideal boilers are a bit like marmite in the heating game.

18 minutes ago, MJNewton said:

I can just get the installer to rip the old boiler out and fit the new one and not require them to do anything else

That's pretty much exactly what you'll get with the online companies you mention. They arrive and a couple of hours later they've left you with a new boiler....... you don't get to require them to do anything else....

Thanks for your thoughts Simon; really appreciated.

I did wonder if the internal pump on a system boiler would allow for it to have greater control over it, and you make a good point about the scope of any warranty. That said, I am somewhat reluctant to be too swayed by warranty scope/length as I am mindful that a warranty doesn't always make for plain sailing. It is why I tend to be so into DIY servicing and repairs when it comes to pretty much everything.

Could you elaborate on what you meant about the 'their field engineers are generally not happy if it's used with load compensating controls' when it comes to making use of the second SL input? It sounds important, not least in case it means it not quite operate in the way that I'd be expecting it to.

Re Opentherm, the MIs do include it on the wiring schamatic but how much / how well it is supported I don't know. It seems you have to rely on anecdotal evidence for that, for all manufacturers. I suppose they'd rather push their own proprietary control systems. That said, I am only seeking flow temperature control and if it doesn't support that basic functionality then I do wonder what exactly they do support in making their claim for compliance. I wouldn't consider it a deal breaker either way as I am already satisfied the performance of on/off demand, particularly as part of a TPI control algorithm, and was as much interested to try OT out of curiosity than anything else.

In writing this reply I think my position could be summed up that I just want what we've got now, just newer, on the assumption that newer should mean longer remaining life even without any extra features/functionality. This is probably why I am (or at least was - I remain open minded) leaning towards another heat-only boiler. Your preference for a system boiler does mean something to me so I will certainly be continuing to look at that option too.

Edited 2 hours ago2 hr by MJNewton

On the Opentherm support front, I've found this write-up which suggest the Ideal Logic heat-only does support CH flow temp control over OT (with DHW flow being set by the boiler but that's fine as I'd be wanting that fixed anyway) and to good effect in terms of comfort and efficiency.

Edited 2 hours ago2 hr by MJNewton

For me system with UVC. But if doing any form of cylinder get a heat pump cylinder for the bigger coil. That means super quick reheats and or better efficiency while heating the cylinder.

Would only do a boiler that allowed 2 flow temperatures one for heating and one for DHW heating. So Priory demand hot water as you state.

All above is predicted on understanding house heat loss and boiler modulation. No point sticking a 20kW boiler on the wall and it modulation is limited to 10kW and house heat demand is 6kW on the coldest day as the boiler will spend it's days cycling on and off.

Atag (great boilers) and Intergas do pdhw out the box. Atag allow you to adjust the rate of temp rise of boiler delivery, so even an oversized boiler has a long run time.

2 hours ago, MJNewton said:

Cons of a Heat Boiler:

• I don't know! What are the downsides? What does a system boiler give me that a heat boiler doesn't?

The biggest downside of a "heat only" boiler is it doesn't have true control of the circulation pump and this control is a significant parameter in the boiler optimising it's output to suit the emitters

I'd go system boiler if I had my time again although I would have no hesitancy in going for another Viessmann gas boiler

Thanks John.

My working assumption is that we'd keep the current cylinder but, yes, if (when) we do ever replace it I'd make sure it was suited for a heat pump (which I don't want, but that's another story!).

Re sizing, the current boiler output range is 8kW-15kW which is way higher than we need and so the boiler can rarely (maybe never; I don't monitor it that closely) operate with continuous burn without having to cycle. The 15kW models in the Ideal Logic and Vogue range can modulate down to 5kW so there should be some improvement there, and whilst I know other options exist that can go even lower it is not one of my primary goals when it comes to selection.

I had previously considered both Atag and Intergas but had some concerns about them seemingly being something of a niche choice which might be an issue when it comes to spares and engineer familiarity. With an Ideal or another mainstream brand I am assuming I'd be spoilt for choice when it comes to finding someone to come and repair it.

Just now, marshian said:

I'd go system boiler if I had my time again although I would have no hesitancy in going for another Viessmann gas boiler

It feels like we have a recurring theme here, and whilst I was hoping you'd all be reinforcing my leaning towards a heat only boiler but if that's not the case then I really should be listening to that (otherwise why ask in the first place?!).

Edited 1 hour ago1 hr by MJNewton

Oh and I'd go DHWP (X plan) for HW and weather compensation for the CH flow temp

I'd also find the gas boiler that had the highest level of modulation

My Viessmann 100-W is 16kW but capable of modulating down to 4kW - not many "Heat Only" boilers can run down that low

Yeah, my plan is indeed for X-plan and whilst I have yet to commit to weather compensation I am certainly be up for giving it a try to find out for myself how well it works for us.

1 minute ago, MJNewton said:

It feels like we have a recurring theme here, and whilst I was hoping you'd be reinforcing my leaning towards a heat only boiler but if that's not the case then I really should be listening to that (otherwise why ask in the first place?!).

I think the point of forums is to share opinions and knowledge gained

In my case I'm sharing my regret going for Heat Only as I did not have the knowledge that understood the benefit of giving the boiler control of the circulation pump - I'd always had HO boilers - simple units where all they had to do was light, burn gas and heat water going thro them.

The Viessmann technical team however when I raised the issue of "modulation variation" (which to be honest I've dialed out a heck of a lot of it now but it's still there) during cycles effectively said - the boiler doesn't have control of the pump speed so it's always going to be a compromise based on your circulation pump running at a fixed speed.

7 minutes ago, MJNewton said:

Yeah, my plan is indeed for X-plan and whilst I have yet to commit to weather compensation I am certainly be up for giving it a try to find out for myself how well it works for us.

WC is excellent and X-Plan or however you manage a boiler than can do two flow temps is so much better than having to run a fixed flow temp that satisfies the HW cyl but is too high for the CH circuit in the shoulder seasons

Typically at 10 Deg OAT my boiler is running at under 30 Deg C flow temp - the house internal temps is constant at 19 Deg

HW is called for an the boiler target flow temp is 50 Deg C and in 30 mins I'll have 180 litres of HW at 45 Deg C or if I run it 60 Mins at 55 Deg C flow temp I'll have 180 litres of 55 Deg Water (Hystersis in the boiler program will allow 4 Deg of overshoot)

1 minute ago, marshian said:

I think the point of forums is to share opinions and knowledge gained

Yes, absolutely, and I find this forum being particularly good on that front what with knowledge and constructive(!) opinions being in abundance.

3 minutes ago, marshian said:

In my case I'm sharing my regret going for Heat Only as I did not have the knowledge that understood the benefit of giving the boiler control of the circulation pump - I'd always had HO boilers - simple units where all they had to do was light, burn gas and heat water going thro them.

The Viessmann technical team however when I raised the issue of "modulation variation" (which to be honest I've dialed out a heck of a lot of it now but it's still there) during cycles effectively said - the boiler doesn't have control of the pump speed so it's always going to be a compromise based on your circulation pump running at a fixed speed.

Ah, okay. I was going to ask what was behind your use of the word 'significant' in the context of the benefits of a boiler being able to modulate the pump speed. What were the 'modulation variation' issues you had?

From a general perspective, I asked Gemini AI for a list of benefits of boiler-controlled pumps and it offered up the following which do sound like things I'd appreciate:

Key Benefits

• Improved Boiler Efficiency: Slowing the pump down when heat demand is low increases the temperature differential (Δ T) across the system and lowers the return water temperature. For condensing boilers, cooler return water keeps the unit in its high-efficiency condensing mode for longer. [1, 2, 3]

• Lower Electrical Consumption: Electronically controlled pumps draw significantly less power than old-style fixed-speed asynchronous motors, cutting pump electrical usage dramatically during part-load periods. [1, 2, 3]

• Quieter Operation: As thermostatic radiator valves (TRVs) close off rooms, fixed-speed pumps push the same volume of water through restricted pipes, causing whistling, hissing, and high-velocity noise. Variable-speed pumps dial back pressure to prevent these noises. [1, 2]

• Better Comfort and Temperature Control: Modulating the flow rate prevents rooms from overheating or under-heating, maintaining a steady, balanced warmth across all zones. [1, 2]

• Reduced Wear and Tear: Running at lower speeds for most of the time reduces mechanical stress and friction on the pump bearings and motor, extending the lifespan of the equipment. [1, 2]

Edited 36 minutes ago36 min by MJNewton

4 minutes ago, marshian said:

WC is excellent and X-Plan or however you manage a boiler than can do two flow temps is so much better than having to run a fixed flow temp that satisfies the HW cyl but is too high for the CH circuit in the shoulder seasons

Yeah I'm having to compromise at present with a flow temp of ~55°C for the HW to function but I can tell this is way higher than what we need right now with outside temperatures around the 8-10°C mark as the flow never gets up that high when only the CH is running:

(Ignore the wiggly outside temperature for today - the sensor sits inside the MVHR intake and that system is being turned off every 10 minutes because the air quality sensor is picking up that the outside air quality isn't great!)

The fact the boiler is firing so often (every 30-60 mins), but only briefly, is really just down to the Honeywell's aggressive TPI algorithm. I'd prefer it if we could go low and slow instead though.

Edited 39 minutes ago39 min by MJNewton

Just now, MJNewton said:

Ah, okay. I was going to ask what was behind your use of the word 'significant' in the context of the benefits of a boiler being able to modulate the pump speed. What were the 'modulation variation' issues you had?

Because the HO Boiler doesn't have control of the pump, doesn't measure return temp all it's programming is based on flue gas temp and flow temp - because it's got a fixed pump speed driving the flow thro it on the initial start up the flow temp over shoots slightly - this causes a corresponding rise in flue gas temp - the flue gas temp sensor is actually quite slow to respond to changes - the boiler sees a higher flue temp and drives the flow temp up to compensate as it expects flue temp to be at least 2 degs below flow temp.

Basically the boiler gets into self driven short cycling issue - exactly the opposite of what I want running low and slow 23.5/7

Like I said I've dialed it out (almost) now and I'll live with the odd occasion

Just now, MJNewton said:

Ah, okay. I was going to ask what was behind your use of the word 'significant' in the context of the benefits of a boiler being able to modulate the pump speed. What were the 'modulation variation' issues you had?

From a general perspective, I asked Gemini AI for a list of benefits of boiler-controlled pumps and it offered up the following which do like things I'd appreciate:

Key Benefits

• Improved Boiler Efficiency: Slowing the pump down when heat demand is low increases the temperature differential (Δ T) across the system and lowers the return water temperature. For condensing boilers, cooler return water keeps the unit in its high-efficiency condensing mode for longer. [1, 2, 3]

• Lower Electrical Consumption: Electronically controlled pumps draw significantly less power than old-style fixed-speed asynchronous motors, cutting pump electrical usage dramatically during part-load periods. [1, 2, 3]

• Quieter Operation: As thermostatic radiator valves (TRVs) close off rooms, fixed-speed pumps push the same volume of water through restricted pipes, causing whistling, hissing, and high-velocity noise. Variable-speed pumps dial back pressure to prevent these noises. [1, 2]

• Better Comfort and Temperature Control: Modulating the flow rate prevents rooms from overheating or under-heating, maintaining a steady, balanced warmth across all zones. [1, 2]

• Reduced Wear and Tear: Running at lower speeds for most of the time reduces mechanical stress and friction on the pump bearings and motor, extending the lifespan of the equipment. [1, 2]

Controlling the pump speed is the main one - to be able to speed up or slow down the flow to match the boiler modulation is very useful

My pump uses 15W to move circulating water at 0.6m3/hr with a 3.1 m head

On WC none of my TRV's shut except for solar gain - I'm using PICV's and every rad is sized for the room and has the correct flow thro it to maintain the target room temps

Last two points I agree with although I would say that running WC also means lower flow temps are even better for reducing system noise

11 minutes ago, MJNewton said:

Yeah I'm having to compromise at present with a flow temp of ~55°C for the HW to function but I can tell is way higher than what we need right now with outside temperatures around the 10°C mark as the flow never gets up that high when only the CH is running:

(Ignore the wiggly outside temperature for today - the sensor sits inside the MVHR intake and that system is being turned off every 10 minutes because the air quality sensor is picking up that the outside air quality isn't great!)

How many cycles is that in the scheduled heating periods?

That looks F' horrible to me

3 minutes ago, marshian said:

How many cycles is that in the scheduled heating periods?

Yesterday there were 15 firings in 16 hours (0630-2230). Including the CH cylinder reheat at the beginning that amounted to 30kWh of gas used.

6 minutes ago, marshian said:

That looks F' horrible to me

Yeah it's not ideal. Honeywell's TPI control algorithm on the T6 is very limited in how much it can be tweaked, and appears to be the very same implementation as that on the CM927 that I had back in 2005, and it might be even older than that, and so doesn't really have modern-day expected efficiencies and boiler capabilities fully in mind. Basically, if there's a non-zero heating demand then it'll insist on firing at least once within its 30 min control window cycle. It does a great job of maintaining a steady state temperature though, with minimal hysteresis (a flat line with <0.5°C variation), but does give the system a good workout as the graphs show! This is why I am keen to switch it over to fully proportional Opentherm control (or, like you suggest, weather compensation) instead.

Edited just now1 min by MJNewton