I need to replace my ageing combi boiler, and had planned to get a modulating one to complement renewable sources I plan to add in the future. That is, a boiler that adjusts its heat input according to whether the feed water has been pre-heated.

I'm surprised to discover very few models showing up on an online search, and only two, old, hits for "modulating boiler" in this forum. The main domestic manufacturers (Worcester Bosch, Baxi, etc) don't seem to have added a modulating offering. A few use modulating pumps, but only Alpha and Atlantic (the latter being ugly sods you wouldn't want in your house) seem to have true heat modulation.

Are modulating combi boilers an idea that just never caught on? (if so why?), or has it been replaced with some magic new technology I've missed? Or if it's still a goer, what are the good, robust products?

Thanks,
Andy

I think all the Worcester Bosch range modulate. the spec sheets give the powers as 4-15kw or the like.

I have a WB system boiler (18i) and it def modulates, you can hear the fan speed changing but the pump is fixed speed.

I think Andys talking about a combi that can handle preheated water going into the DHW heat exchange , modulating gas flow relative to cold feed input temp. rather than standard modulating of the gas valve relative to return temperature/heat demand on the heating side.
few will reconise pre heated cold feed,
also few combi get in condensing mode or modulate gas valve for DHW anyway as far as I'm aware.

I believe generally people use a diverter valve if connect Solar HW / to a DHW combi , so no preheat , just solar HW when you've got it up to temp. and combi otherwise.
there's the ones you mentioned , I think Vokera may do one also, though this may be instant DHW only,

think i post same time as CW ,see above re. preheated cold feed to combi's
How about Andrew fastflo or similar , do they do one with an additonal heat exchange for heating primary circuit ?

Fair enough , but I dont think many manifacturers , advise or design combi's for preheated cold feed ,
What I think Andys talking about is one that will monitor input water temp. and adjust gas valve to suit desired output temp.
like the Andrews fastflo instant water heater , but in a combi .

HSE doesn't like this set up, because it doesn't give the preheated water long enough at a high enough temperature to kill off any nasties that might be lurking in the solar tank.

So you're unlikely to find manufacturers giving info on these features.

<blockquote><cite>Posted By: jamesingram</cite>What I think Andys talking about is one that will monitor input water temp. and adjust gas valve to suit desired output temp.</blockquote>

Exactly. Unless someone can offer a better approach. Monitoring output temp seems to me to leave scope for scalding temperature spikes in DHW - i.e. you heat the water full blast for a few moments, realise that made it too hot, so you rein in the heat, damage done.

To add further complexity, my ideal dream vision would be to incorporate a wood burner and back boiler, and pre-heat CH water as well. Even remotely a possibility, what with the CH being closed loop? If it is, what's the accepted wisdom as to how to do it? Or do I just need to curb my expectations and stick to DHW? I realise to do the whole job would need a BIIIG heat store.

Maybe someone at Grand Designs will be able to tell me. Unless someone here knows.

@wookey - never found any when I was looking. Must have another look as I was planning on the Andrews Fastflo and a separate (cheap) boiler if the tank needed a boost in the winter for the UFH.

(edit) Had a quick look at the Atmos and the one advantage the FastFlow has is that it will power more than one shower at a time as it has a greater output.

I'll look into the others.

<blockquote><cite>Posted By: CWatters</cite>

There is a problem with _just_ measuring the input water temperature... The electronics controling the boiler power would also have to know the flow rate to work out how much additional power is required to achieve a set output temperature. Would also have to correct for any scaling or other loss of efficiency of the heat exchanger over time.

Why heat the water at full blast from the outset? Could just ramp up the burner over a few seconds and monitor the output temperature.</blockquote>

I don't know how other combi boilers work but do know that the Vaillant works by monitoring output temperature and modulating the burner pretty much like this. It copes with warm water input, because the cold feed to mine runs for around 14 m across the loft and gets fairly warm in summer (around 30 to 35 deg C until the incoming cold displaces the water in the pipes). Scald prevention is by two methods; a buffer tank inside the boiler that allows the boiler to overshoot slightly without pushing the temperature at the taps up too much and an over-temperature shut-down that kills the burner in the event of a major over-temperature event.

I may try an experiment this summer and see if I can feed it a constant supply of warm water, as I can't (from having studied the way the boiler works in depth when trying to fix a design flaw) see any reason why it wouldn't work with a constant feed of preheated water, as long as the preheated water was below the demand temperature that is set.

why have a cylinder and a combi... the combi flow rate will have to be limited, so you get no real benefit...


The other issue is that a combi has to work to a dhw set point, and its minimum output will make that impossible to acheive..say if the set point is 50c and the solar water preheated to 48c how many watts are required for the 2c difference at 10l/m

I know mixing valves for combis exist, but even they are a bit if a compromise if you want a consistent supply of hot water...

<blockquote><cite>Posted By: alec</cite>vaillant say that their combis wont take preheated water...no design flaw there then
</blockquote>

The design flaw in question is very real, I can assure you (as can many others with this boiler in single storey houses). It's well documented on the web and even has an (unofficial) fix from Vaillant (check F.75 mod kit with a web search if you think I'm making this up). My EcoTEC has been a pain for three years, failing to start from cold seemingly randomly, despite umpteen call outs from the installer and two other heating engineers, which is what caused me to effectively reverse engineer the control system, to find out how the boiler responds to different conditions and how the control loops operate. Eventually after lot of web searching (and some help from Colin) I tracked down the unofficial mod kit (that's actually made by Vaillant and sold by them for around £100), that repositions the pressure sensor to a foot above the pump, which fixes the design fault.

As for not working with pre-heated water, all I can say is that in practice it does, at least with modest amounts of pre-heat. I guess if you started stuffing water into it that was only a few degrees cooler than the target output temperature you'd run into a problem with the lowest possible flame modulation power, which may well be why Vaillant say it can't be done. The problem there is just simple physics; if it takes less heat to raise the maximum water flow rate from input to target output than the minimum that the boiler can run at then the boiler can't work properly. This would be the case for any combi. The way to determine if modest pre-heating would be OK is to check the flow rate you get, check the lowest modulation power of the boiler and do a quick check calculation to see if the required delta T and mass flow results in a power requirement that is greater than the boiler minimum power. If it is you should be OK, if it isn't then it probably won't work.

<blockquote><cite>Posted By: alec</cite>yes sorry the pressure sensor was moved that is true...exactly why they needed to work through the control algorithms I have no idea... are you using vaillants ebus controls?

</blockquote>

The snag with this sorry tale is that Vaillant won't tell you the pressure sensor needs to be moved to fix this designed-in fault, neither will they tell their approved installers about the mod kit, unless the installer specifically asks for it by part number.

Me reverse engineering the control system was a desperate attempt to try and understand what the problem was and discover why a brand new boiler kept shutting down and throwing up F.75s, despite apparently, being in perfect working order (according to the installer and the other firms that I called in to check it). When working the boiler is fine, but after this experience I'm not at all sure I'd buy another Vaillant. If they'd had the decency to own up, and admit there was a design problem, perhaps even supplying the mod kit for free to fix one of their brand new boilers, then I'd have been happier. As it was I've had three years of having to get up at 5 am some mornings to drain a bit of water out, open the fill valves to push the pressure up to the red line and then restart the boiler (running it at the red line eases the problem slightly).

Since fitting the "we deny there is a design flaw" mod kit the boiler is now fine, and reliably detects the small start-up pressure rise across the pump, even when cold, and starts first time, every time. Prior to fitting the mod kit the boiler rarely fired on the first start attempt, even when warm, and would randomly fail to fire after three attempts when cold, so invoking the F-75 "no pressure rise detected" shut down.

<blockquote><cite>Posted By: alec</cite>I have installed around 100 vaillants over 7 years and my customers have never had an issue... the only thing I do different is install ebus controls...how that avoids this issue (longer pump running times?) I don't know but it does rather point to the fact that if you install things the way they do in germany then you don't get so many/any issues...</blockquote>

From what I can gather, the problem mainly (perhaps only?) manifests itself in single storey installations where the flow and return flow resistance is relatively low. The design flaw is that the sensor is trying to detect a very low pressure change as a positive way of determining that the pump is running and that it is safe to fire the burner. It doesn't make any difference whether the external control is via conventional, wireless or eBUS, as this particular control loop is low level and safety critical, in as much as it shuts the boiler down after three failed attempts to detect the pump pressure rise.

In two or more storey installations there is often enough inertia and flow resistance in the mass of water in the longer flow and return pipes to provide the momentary pressure rise that needs to be detected, but if the boiler is the highest point in a single storey installation, and if the pipe runs are such that there is only a modest volume and little resistance to flow then the sensor often fails to detect the pressure increase.

To give an idea of what the control loop is up against, it is trying to detect a momentary pressure increase of at least 0.2 bar with a system static pressure of between 1 and 2 bar to see if the pump has started turning. If there is very little resistance to flow (as can happen with a cold system and all the TRVs open) the pressure pulse from pump turn on at the sensor just doesn't reach 0.2 bar. When the system is hot the problem tends to go away, because as soon as a few TRVs shut off the flow resistance increases and the required pressure rise pulse can *just* be detected. In my case the thing was always marginal, and rarely fired at the first start attempt even when hot.

To add to my frustration, Vaillant originally fitted a pressure sensor to this model that was a bit sluggish in response ( the one with the orange body) and thought that fitting a faster response sensor (the one with the black body) was the fix, so they knew right from the start that they had a problem with the design. My boiler was new enough to have the black body sensor when supplied, and along with a significant number of other people I still had the problem from the day the boiler was installed.

Further evidence that Vaillant know the design is flawed, is the fact they've produced the mod kit to fix the problem. If the design was right it wouldn't need a modification (at the customers expense!) to make the boiler run reliably. Repositioning the sensor up near the primary heat exchanger, at the end of a long pipe, gives just enough of a margin to allow the pressure pulse to be detected. In effect, they are now detecting the pressure pulse caused by the flow restriction in the primary heat exchanger, rather than at the main hydroblock where the flow resistance is low.

If you always install this boiler (pre-F.75 mod, as I understand they now supply this boiler with the rigid primary heat exchanger tubes etc as standard) to a system with a bit of flow restriction, then it works perfectly. I proved this with mine by shutting most of the LSVs off, whereupon it fires first time, every time.

<blockquote><cite>Posted By: borpin</cite>Small Claims court? Sale of Goods Act - not fit for purpose? Chances are they would not defend it or settle up with some compensation first. Right up your street I would have thought!!</blockquote>

I did think about it, but the claim would, as I understand it, have to be made first against the installers, and to be honest they've been tearing their hair out over this as much as I have. They came back around a dozen times during the warranty period, tried their best, but because the fault only really occurs when the system is cold, with all the TRVs wide open, it's hard to get it to fault when testing.

What did annoy me was that neither the installer, or the other two companies I had in to try and fix it, had heard of the "F.75 mod kit" from Vaillant, until I found out about via another forum and let them know. Vaillant make the kit, but apparently don't let on that it's to fix a design flaw that affects EcoTECs when used in some installations. I decided in the end that it wasn't worth the hassle of pursuing Vaillant for £100 worth of kit, so I bought one on line and had it fitted (free, out of good will) by the original installer (who was as grateful as I was to find a solution). Since then the boiler has been near faultless, and starts reliably at the first firing attempt, every time. The big test will be when the heating comes on again next winter, though, when the system does an early morning start from cold.

<blockquote><cite>Posted By: alec</cite>Gosh the bunkum these companies come out with...I installed a vaillant 937 3 years on the top floor of a three storey building which according the the theory above should not be working, but it does and it has for all that time...with no issue...the only difference is the ebu controls I use and around which the boiler is designed...I suspect....</blockquote>

It's fine on multi-storey buildings AFAIK, the problem arises with systems where the flow resistance is low, due to short pipe runs and several rads in a single storey house.

It's not the eBUS control system either, I can say that with absolute certainty having reverse engineered the thing.The basic loop that controls the fire-up process is very low level (in process terms) within the controller. When the control system (be it eBUS, wireless stat, controller or whatever) calls for CH the boiler moves the diverter valve to CH, starts the pump, tries to determine that the pump is really running by measuring this tiny pressure pulse of 0.2 bar across the pump and enters the blower on/gas valve on/ignition/flame check process if all is OK. If no pressure pulse is detected the controller tries to fire again by turning the pump off, returning the diverter valve to DHW and then starting the same process as before. After three attempts with no pressure pulse being detected the controller shuts the boiler down and flags F.75 on the display.

It's a safety thing, where the boiler won't fire unless it has checked that water is actually circulating in the primary heat exchanger. They use the pump-induced pressure pulse as a way of checking that the pump really is operating correctly, which is fine as long as the pressure sensor is positioned in a place where it can actually see the pressure rise.

What Vaillant got wrong was the pressure sensor positioning; they didn't allow enough of a margin for low flow resistance systems. Had they done development testing with a system like mine (13 rads, all on one floor, with a relatively low flow resistance when all the TRVs are open) then they'd have found the design flaw before going into production. They clearly know of the problem, as they've issued one fix (the pressure sensor change from orange body to black) that didn't sort the problem out for everyone (although it did for some) and eventually have come out with another fix which is repositioning the pressure sensor.

I have no problem with the idea that a boiler manufacturer can make an design error like this, I do have a problem with them trying to pretend that the problem isn't there and making their customers pay for the fix.