I have just got a smart CH controller with room thermostat which will probably be used to switch both the CH pump and an old oil boiler.
However, I notice that the boiler much prefers to run until its own thermostat is satisfied and seems to be a bit recalcitrant if it is switched on and off by external forces too often.
What is the accepted way around this?
The old system was CH controlled only pump and HW controlled only boiler which meant that the boiler was live most of the day (or twice daily) with only one or two potentially brutal power offs mid-flame.
The new thermostatic control has the potential to do this sort of unfriendly cut to the power feed several times a day. Apart from setting minimum cycle time to 15 minutes is there anything else I can do? I suppose it wouldn't bother a gas boiler so much.
Obvious fix being a relay to hold mains on until the boiler fan stops drawing current (but that requires a modification to the boiler).
The advantages of smart start to match target temperature is largely eliminated if I have to set the boiler to come on at some God forsaken hour of the morning just in case the CH pump thinks it needs to be on.
Incidentally why don't these smart controllers provide decent manuals in English with more sensible boiler and pump control outputs?
I don't quite understand what type of overall control system you now have. I assume that the previous system had gravity HW circulation and pumped CH?
Is it now fully pumped? What have you got in the way of motorised zone valves - presumably either 2 x 2-port valves (S-Plan) or 1 x 3-port mid-position valve (Y-Plan)? What controls the pump? In a fully pumped system it would normally be connected in parallel with the boiler's switched live - unless the boiler required pump over-run, which seems unlikely in your case.
By 'smart controller' do you mean a programmable room stat? What make and model? The instructions are often a bit cryptic, but you can usually fathom them if you read them a few times.
I don't know a lot about oil-fired boilers, but my Honeywell programmable stat does have a mode with a longer minimum 'on' time for use with oil boilers, but it still isn't *very* long. You must surely be able to turn the boiler off when it's firing, when all demands are satisfied? If not, there's little point in have a 'smart' control system!
HW is still gravity feed and has to be as a heat dump for the back boiler on the wood burning stove (previously for a Baxi grate).
Yes. With provision to override the pump and have it switched on with or without the boiler if either the wood burner thermostat or a manual switch requests it. This means you can have a fire and run the CH without the oil boiler running at all sometimes. The wood burner with its back boiler is powerful enough to more or less shut down the oil burner except on the coldest winters days.
The layout remains the same hot water is gravity fed (pump not on) and CH is actively pumped. Boiler and hot tank are close together. No zone valves entire house is either heated or not depending on the pump.
Hot water tank insulation isn't the greatest so leaving the boiler live results in occasional wasted firings keeping the hot water hot. More importantly being a fairly chunky oil boiler it is fairly noisy and the rural house very quiet so it is annoying to have the thing come on any earlier than is absolutely necessary in the morning.
The oil burner is a Worcester Danesmoor 15/19 I'd guess from about the mid 1970's from the thickened computer style font on the control panel. The only manual I can find online is for the model with the same name which looks internally nothing like mine although might well have the same quirks as Bosch recommend their own controller and setting it to "G" so that hot water (boiler on) is always demanded when CH pump is on.
Unfortunately with no wiring diagrams anywhere and zero documentation for the actual vintage model trial and error isn't very appealing. Although less daunting now that spring has sprung than in mid winter.
I mean one that calibrates itself to the thermal inertia of the house and fires the boiler at the appropriate time(s) to hit the specified way points on the temperature profile at its thermostat.
The model installed is a Heatmiser RC1-WTS rev 1.0 zone 1 = CH (room thermostat) and zone 2 = HW.
It functions but it seems to be abusing the boiler...
You can but it doesn't shut down at all gracefully since it should switch off properly with a purge of exhaust gasses rather than just stopping dead as all mains is suddenly removed. It didn't seem to be a bother it with the old once or twice a day basic controller but then a fair proportion of the time it wouldn't be flame active when dropped off power since it would happen at the end of an hour or so with the house or hot water probably full satisfied and the boiler up to temperature already in its standby state rather than active.
The new controller is pretty much forever switching the boiler off when the flame is lit and the boiler isn't at all happy with it's lot.
Mention of the permanent live and switched live in an adjacent response has given me something to look for, but I think this unit may be so old as not to make the distinction.
What do you mean by that? You say "old oil boiler" is it a gert lump of cast iron with a burner? If so they can overshoot their own stat quite a bit when that switches the boiler off so it won't run again until it's cooled down.
As far as the burner is concerned what is the difference between the boilers stat yanking the power or an external one? There might be some form of over run for the pump and/or fan but in that case there will be a "boiler on" input to control the actual fireing.
From what you say having the boiler properly controlled by the demand will save you a fair bit of oil.
I have a heatmiser (PRT-HW-TS-WIFI but no difference) driving an old gas fired cast iron lump, gravity H/W and pumped C/H, I had to use a relay when wiring it so that when the H/W output of the heatmiser was on it fired boiler only, and when the C/H output was on it fired boiler and ran the pump, obviously couldn't just wire both outputs direct to the boiler, or they'd both also be wired to the pump.
My boiler just has L/N/E connections to turn it on, no permanent live or PCB required as it has a pilot light, I just set the boiler's internal stat to maximum so it's never satisfied and therefore it fires whenever the heatmiser asks for it (unless H/W only and the tank stat is satisfied).
My Honeywell stat has an 'Optimum Start' function, where it can decide when to start the heating to achieve the set point at a specific time - based on the temperature difference and its assessment of the house heat-up rate. I've disabled that because I found that the heating was coming on ridiculously early. I now start it at a fixed time - and it might not always be *quite* up to temperature when I get up, but it's near enough and I'm using a *lot* less gas.
In your case, it's even more complicated. How is the controller supposed to know whether or not the wood burner is running?
I'm still at a bit of a loss to know what the difference is between having the boiler's stat cut power to the burner on the one hand, or having an external stat do it on the other.
If there *is* a difference - as per my previous paragraph - it must be due to something like that. It sounds as if the boiler's internal stat is cutting power to the burner whilst maintaining power to the fan and to whatever does the purge. With the use of a permanent live and a switched live you might be able to make your controller do the same thing. You may find that there are two live inputs - currently strapped together - and that you need to unstrap them and connect a permanent live feed to one, and a switched live from your controller to the other.
Yes. It is exactly that. The great lump of cast iron is practically indestructible and has outlived the original burner by some margin.
There might be but that is definitely not how it has ever been wired. If it shuts down on its own thermostat it does the right
The old timer system did sometimes go down with a crunch if the flame happened to be lit when the timer suddenly switched it off. But the new smart room thermostat is doing it almost every time with the burner running and unburnt kerosene fumes result (noticeable inside and out).
That was what I thought too and the idea behind fitting one. And also with it having a 5+2 world model a chance of a lie in at weekends.
So far it is probably going to destroy another burner jet with soot before I get to the bottom of how it should have been wired up.
My more modern one is much the same. I can't see why youe would want a fan over run. All it would do is blow cold air through the boiler to the outside and remove some of the residual heat.
There might be. I will have to take a detailed look at the weekend. Our boiler like the master phone socket is conveniently located in the loft (very close to the hot water tank which is a bonus I suppose).
Mine is like yours, but I think you will find there is a solenoid valve that cuts the fuel supply off leaving just the fan running for about 5s at the end of a controlled flame burn to tidy up and vent the flue.
It is lack of this action that seems to be the problem with mine.
Mine is also brutal floor standing cast iron job and physically much wider than the slimline 15/19 shown in the modern brochure. The scifi computer writing on the front adds a touch of surrealism.
If "like yours" is refering to my oil boiler with a single switched LNE feed how does the fan run on for 5s after the oil solenoid has closed as there is no power? It'll be spinning down but that isn't the same as being driven. Yes my boiler does have an oil solenoid valve but that'll close the same time as the fan is depowered.
It's main use is start up, power arrives, fan and pump runs, a few seconds later the valve opens and the spark is started, if no flame is detected within about 10 seconds it locks out.
Only to clear the flue gases, my boiler is a open flue one with a 5+ m 5" flue so has a hefty draw so clearing the gases isn't a problem. For a smaller balanced flue boiler it might be but even then just a few seconds will be enough.
It doesn't need to. The wood burner shows as reduced demand for heat, ie the room stat won't call for heat as often or will be satisfied earlier due to the extra heat supplied by wood burner.
It's not ideal, a thermal store to buffer the heat from all sources would be better. With above there is the chance that the boiler may only fire for a short time, a thermal store ensures that the boiler will get a decent length burn as it heats the store from luke warm to hot.
On mine there is a magic control box which has an LDR to detect flame is good and timer to fire the ignition coil, the same timer on shutdown also keeps the fan running after the thermostat signals at temperature.
ISTR There is a bunch of 3 DPCO 10A mains relays inside doing the actual power switching based on what the control box tells them to do.
Pretty much the same here or start up.
But at end of flame it closes off the fuel solenoid and runs the fan for about 5s to clear the balanced flue of combustion products. This obviously doesn't happen if you just cut the mains feed off externally.
Be careful what conclusions your draw from that though.
I ran some experiments over several weeks with my CM927 and a time-lapse we bcam automatic detecting calls for heat. I found that it was surprisingly a ccurate at reaching the target temperature (0.5C resolution) just on time - nearly always no sooner/later than 5-10 minutes. The environmental conditi ons were quite varied too and so it really got a good chance to exercise it s adapting capability and highlight any weaknesses.
Given this accuracy I would have to put any potential gas savings obtained from disabling optimum start down to me accepting that the target temperatu re might not be reached on time or, to put it another way, that my target t ime could slip.
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