What controls needed for running two boilers in parallel?

[Phil Addison]

Thanks for the earlier advice on subject "Chaffoteaux FlexiFlame 140 boiler periodically cutting out". The decision has now been made to replace it, and the recommendation we have, which I think is reasonable, is to replace the ancient 41kW beastie with a pair of 24kW Worcester-Bosch system boilers (model 24i) running in parallel.

This existing system is sealed, and the Chaffoteaux has an internal pump circulating primary water through a low-loss header* (LLH), and there is a separate pump circulating the LLH contents around the radiators which are spread over 4 floors, see ASCII art below. This is heating only, no domestic hot water.

Currently one radiator is open valved and the others have TRVs.

The proposal is that the two new boilers will each have their own pump and check valve, and that combination will be connected in parallel and tee'd into the existing low-loss header. A magnetic strainer will be included.

So far that seems straightforward. My concern is the control system to ensure the pair of boilers work at maximum efficiency. When it is mild we want one boiler only to be running, and the second cutting in only when more that 24kW is required. Clearly one boiler running at full power will be much more efficient than two running at half power. and for most of the year we should see the benefit of full condensing operation of one boiler with the other hardly ever firing.

First question is, is there a standard 'box of tricks' that does this? I have asked W-B who insist they "don't do system design just make boilers" and could offer no help.

It shouldn't be too difficult to make a controller to do this... my early thoughts are that starting from a cold system, I want to stop #2 boiler from firing for say 1/2 hour to establish if #1 is coping or not. That can be decided by checking the temperature rise across the boiler; once its stabilized (after the 1/2 hr) I can say that if it is greater that the spec figure of (say) 11 degrees then it is dumping its full

24kW into the system and #2 needs to be turned on to supplement it.

So far so good, but with two boilers now running, how do I control things so that #1 stays at full power and #2 modulates as required until conditions require #2 to be turned off entirely and then #1 modulates down. I can see there is scope for instability and hunting here if I'm not careful.

I'm sure this has all been done before but not sure where to start looking, having no experience of industrial systems.

Any clues please?

*Chaffoteaux refer to the LLH as a "monotube", which appears to be simply a 600mm length of 54mm tube with the boiler flow and return teed in near each end, and the radiator system connected at each end with 28/54mm reducers as below. [Fixed font ASCII art]

To Boiler || || (P) || __||________||__ ====________________==== || 54mm || || || 28mm || (P ) || || To Radiators

Phil

[BigWallop]

Any system I've seen with parallel boilers has been controlled by just one timer like any other standard system. Both boilers just heat the water when demanded and the thermostats Etc control the pump and zone isolators and things, like normal systems do.

If one boiler needs to replenish the heating, it does. If you don't need two boilers during the warmer months, turn the thermostat right down on one of them.

Not being familiar with the boiler model, I should ask if they have their own internal pumps Etc? If the system is to be controlled by an external pump, then the boilers can be connected in parallel to the feed and return for the original heating circuit. One external pump will push water through both boilers like it would with just one.

One fill loop is also enough to flood the closed heating circuit and both boilers just the same as filling a system with one boiler. There should be no differences between a system with parallel boilers or just one.

Having two boilers which can supply separate demands to two separate domestic hot water taps might be a great thing. The bathroom can be supplied from one boiler, and kitchen utility room from the other.

But I don't see any problem or major control system changes that you will need. But then again, someone will probably be along in a minute to tell me I've shot myself in the foot again.

[Tony Bryer]

The boxes that do this are called sequencers and don't come cheap.

I had to do this when I replaced our church boiler with a pair of Kestons back in 2001 and needed a budget solution. What we ended up with is this:

The ordinary controller goes into the common of a two-way switch (select lead boiler) with L1 and L2 going to the call for heat on each boiler. Between L1 and L2 there is a connection which goes through a room stat (a CM67 on our system) set 1/2 degree or so lower than the main one, and a pipe stat (normally closed, but set to open at about 60C) on the return from the heating.

So from cold, both these stats are closed so L1 and L2 are connected and both boilers run. If the desired temperature is all but there, the second boiler drops out, and this also happens if the return temp reaches 60 (i.e the rads are not getting rid of the heat fast enough).

I did try one or two other refinements but took them out so as not to make things too complicated for my successor. One was to use an extra stat to delay pulling in the second one until the first boiler was fully up to temp (which it never gets to in the mild spring/autumn days), but without adding yet more complication this then means that if the first boiler fails, the second doesn't either, and one of the reasons for having a

2-boiler system is the backup.

In terms of a simple solution, what I really needed (Andrew could no doubt have designed something), was a delay controller that would close a relay (say) 15 minutes after power was supplied, so that in spring/autumn when the boiler was being cycled it never closed, and if the lead boiler failed the second one was pulled in after a short delay.

[David Hansen]

On Wed, 19 Nov 2008 00:25:12 +0000 someone who may be Phil Addison wrote this:-

As has been said you need a sequencer. Search engines will pull up a whole variety from bare boards to finished products in enclosures. One example of the former is

Such a gadget will allow you to set which boiler leads. That can be done with a switch and thermostats, but an electronic version allows for things like automatically changing which boiler leads. It may offer other features like weather compensation.

[YAPH]

Not answering your question, I know, but my supplier charges more for the

24i system boiler than the equivalent combi (24i Junior). So if you've got a use for instantaneous hot water (times two) from your setup you could have it effectively free.

[Andrew Gabriel]

Kestons very handily (and as far as I know, uniquely in domestic boiler sizes) have a lockout output signal, so providing the failure is of a type which generates a lockout, you could use this to effectively switch the lead boiler. Of course, something like a failed mains supply won't generate lockout, but you could also generate your own lockout signal based on loss of supply. This is exactly what I do with my system -- lockout, or failure of the boiler supply, triggers an input to my house alarm, which will then phone me up and tell me the boiler's failed. That still doesn't cover quite all failures modes. I've had the condensate trap block and the combustion chamber start filling with condensate. It still ran for a few hours with almost no heat output, before the condensate blocked off the flue outlet enough to go into lockout, and the alarm call me up.

To be honest, the simplicity of your system would be hard to beat. As control systems get more complex, they normally get less relible and also more subject to design oversights, and it's not clear to me that any further gain you could make would be balanced by the inevitable risk of increased complexity.

[YAPH]

Thought for the day: if you managed to even out the wear and tear between the boilers perfectly then you'd get the closest possible to them failing through wear and tear at the same time :-)

[Tony Bryer]

If you read the tech sheet on this particular unit, the switching in or out of the second boiler is done by a pipe stat on the return as per my DIY system. The only refinement is that the lead boiler is automatically alternated every six hours - as already pointed out, taken to its logical conclusion this would lead to both boilers expiring together.

[Doctor Drivel]

Buy a Broag Boiler (Dutch). They are top quality (better than Vaillant) and are now freely available...and cheaper too, as they are making headway into the UK market. Their boiler control systems are Opentherm and quite advanced. They are the best buy by far of any boiler at the mo'. They are well established in the UK under Remeha, doing excellent commercial stuff and decided to import their domestic Broag Avantaplus range.

don't know what your DHW requirements are. But a good option is use two

24kW combis and split the DHW outputs across the DHW taps - divide and rule. Combine the outlets using two check valves and a shock arrestor to fill a bath at over 20 litres/min.

To do the sequencing of the two boilers try these below, which are cheap and OK. You will have to put the controls into a plastic control box.

The 0-10v output of the weather compensator will input into the boiler sequencer, thus making one control unit. The boilers will be basic units with no external controls on them, only being controlled by the weather compensator via the sequencer.

The weather compensator lowers the flow temperature of the boilers to promote condensing efficiency and the sequencer decides if one or two boilers needs to be brought in. Also, boiler cycling is at a minium.

Boiler Sequencer:

compensator: the combis will be simple with no provision for DHW diverter valves or the likes. The whole setup is very cheap and highly effective.

Ring Remeha, they may have an off-the-shelf solution incorporating integral boiler weather compensation and boiler sequencing for their range.

[Ed Sirett]

This is something I've yet to do. I'm wondering how bad a really simple approach would be.

Common both demands directly to both boilers and set one boiler to a reasonable primary temperature (say 65-75C) and set the other boiler 1-3C below the other. As the "lower" boiler reaches its target temp it will fairly soon cut-out and (if poss) you can set it's anti-cycle time to a longish value. It will only test the temperature every so often and if the upper boiler is coping will again leave things alone.

I am not familiar enough with W-Bs to know if you can tweak the anti cycle time. You can certinaly do this for Vaillants and Kestons.

[<me9]

A simpler approach could be having an outside stat set to (say) 8-12deg, which holds off one boiler in the warmer weather. A changeover switch, possibly on a timer could be set to change over the preferred boiler at a suitable time interval, and perhaps a half hour timer to use both when the system starts up. A thermostat and a couple of timers would do the trick.

[Phil Addison]

Thanks Ed, good point, so we could start off with no controls and have a system that at least 'works'. Then experiment with added sophistication later. That's an attractive proposition because we're keen to get started with the colder weather imminent.

I'm disillusioned with W-B website and technical support, do you find Vaillant and Keston satisfactory? I'll check out their data/prices.

I have a quote of £3860+vat for the whole job including 2x W-B 24i system boilers, replacing flues to roof, condensate pump to gutter, and wireless stat on a common landing. No controls offered - this firm doesn't know anything about the subject. How does price that sound?

Phil

[Phil Addison]

As mentioned, there is no DHW requirement.

data than found on their site.

stat on combined return pipe. Or possibly a "0-10V modulating output" from the boiler which I think is a measure of what the boiler thinks is the required output power. I'm told W-B boilers have this but their site is hopeless so difficult to check.

Anyone know any more about this output signal and if W-B does provide it?

See my reply to John Stumbles

Anyone else heard of them?

Phil

[Phil Addison]

Good tip. I don't need hot water, but if they are cheaper... OTOH, presumably they are less reliable...

Phil

[Phil Addison]

This one?

a wierd site, went rouund it 3 times before I found that.

The second paragraph under Fuel Economy" saying about "the boiler will oscillate on and off" worries me. That sounds like the instability I mentioned in my initial post.

Seemingly done by a large analogue timeswitch bolted to the PCB!

I wonder how much they want for it; the syxthsence one is £94+vat, though as yet I'm not at all sure exactly what it does.

Phil

[Phil Addison]

True, but at least it's some 2n years into the future rather than n.

Phil

[Phil Addison]

Thanks David, I am wading through google returns. These guys

are helpful on the phone, and I'm awaiting some data from them.

Phil

[Phil Addison]

I don't follow this. You mean if the 2nd boiler fails (generates lockout) you use this to fire up the lead boiler? But won't the lead boiler be firing anyway?

but with no mains neither would fire up whatever you do, unless you have an aux supply?

Crikey, that's worrying, with our reliance on a condensate pump too. Any ideas on failure detection in that area? The thought of the boiler innards filling up with weak acid isn't too comforting.

That's a good point. However, as this system is over 4 floors I'm not too happy with it being controlled by a room stat, which inevitably will finish up in the common draughty stair way. I suspect this would just be turned up high and the system then run on the TRVs. If this is the case I don't see that TB's solution with a secondary room stat would work.

Phil

[Phil Addison]

Thanks Tony, "sequencer" is the magic word I was missing, got several google hits now,

loooks interesting.

By "ordinary controller" do you mean the main room stat? Is the secondary room stst (your CM67) co-located with it?

I like it, certainly seems to meet the basic requirements.

I'm interested in what these were.

Agreed re backup, plus these smaller domestic ones are readilly available and may well be cheaper than a single biggie.

Sounds like a re-triggerable delay; the delay is restarted everytime it gets a trigger signal so it won't timeout till a "delay time" after triggering stops. Very easy to implement based on a 555 IC.

On a slightly different point, the installer is proposing to pump the condensate up through the cieling and out into the gutter, the boiler being on the top floor of a 4-story building and no easy route to a drain. I can't see any real objection, but is this ok?

Phil

[Doctor Drivel]

Try this:

>Weather compensator:

You need weather compensation and a sequence controller.

0-10v output? Do you mean input? 0-10v is in commercial systems. I know of no domestic boiler that has a 0-10v input. I think MAN "may" have this.

No integrated weather compensation/sequencing controller in the OpenTherm protocol.....yet. Once they do it will shine.