Re: Central Heating - Controls



The above is the way, except have a CM67 do upstairs and one do downstairs. Two totally separate CH zones and independent DHW zone. Do not have a TRV in the same room as the CM67s
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The Ravenheat CSi system boiler has a DHW mode. This means wiring the cylinder stat into the boiler and having a priority system using a 3-way diverter valve. When the cyl stat switches, the boiler runs up to max heat, then modulates when back on CH mode.
The MAN, also badged as a ECO-Homtec, has all the control you want. It is not cheap but if you buy all the controls as separate items you would pay more. Andy gives a good breakdown on his.
The Celsius will sense the heat load of the cylinder and ramp up the burner to suit.

Assess an open vented "integrated" heat bank eliminating the cold water storage tank. The heat bank can supply the CH and of course instant DHW. The upstairs and downstairs CH circuits can be taken off the lower part of the cylinder. Just have a pump and non-return valve on each. You have to specify the two separate CH take-offs to the maker.
A great advantage is that a cheap condensing boiler may be used. I suggest the Ravenheat heating condensing boiler. It is only 399 inc VAT from B&Q and can be open vented. It is a reasonably reliable boiler unlike the old Ravenheat combi's. Or the Baxi condensing boiler, which can have its load compensation control switched off by removing a jumper. Again it can be open vented. The condensing boilers will be efficient enough and depending on the return temp will condense gining good efficiencies.
A simple, highly effective with many superb advantages and also cheap solution if your mains supply is adequate.
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I just looked at the B&Q web site. The Ravenheat appears not to be there anymore. The Celsius is now cheaper than the Baxi. The Glow Worm is well priced. http://www.discountedheating.co.uk/shop/acatalog/Glow_Worm_HXi_Condensing_Co mbi_Boiler.html
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Is this a proper quick recovery or a stardard Part L?
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Hi,
Andy Hall ( snipped-for-privacy@hall.nospam) wrote: : On Fri, 29 Aug 2003 23:26:54 GMT, snipped-for-privacy@giskard.demon.co.uk (Dale : Shuttleworth) wrote:
[...]
: >1) I want hot water at 60 degrees, what flow temperature does the : >boiler need to be set to to achieve this in a reasonable time? : >The Typhoon heat exchanger claims to be rated at 25kW but neglects : >little details like the temperature differential necessary to : >achieve this. : : You could contact the manufacturer for this but it would be 10 degrees : approx for a conventional boiler, normally 20 for a condensing model.
Sorry, I was talking about the differential between the HW and the boiler flow. Obviously, a given flow/return temp differential at a given flow rate will deliver a given amount of power to the water, I was hoping that I could deliver a reasonable amount of power with a small flow / HW differential.
[...]
: >2) I want to run the radiators at as low a temperature as possible : >- for most of the year I suspect they will be sufficiently over-rated : >to operate with a 60 degree or even 50 degree flow. : : Almost certainly yes. I assume from what you've said that you have : done the radiator derating calculations. Don't forget to derate : further if you plan to do anything that obscures radiators like : covers.
Yep, I've done the necessary derating calculations, looked at radiators, got confused when there was a significant difference in radiator size between Myson and Barlo for the same output, scratched my head, read the data sheets again, realised that they each quote based on different temp differentials, did my calcs again and (I hope!) got it right. (Too late now - I got the rads today!).
: >3) If I go for a simple boiler like the Keston Celsius then I suspect : >that the two requirements above may be somewhat incompatible. : : The Celsius will still do a respectable job, because it will attempt : to supply heat as required by the load. For the radiators, when the : weather is warmer, the TRVs will tend to close down and reduce the : heat demand. The pump speed will drop, as will the boiler firing : rate. Since the boiler is monitoring all the temperatures, it will : try to get the flow and return to as low values as possible, while : still delivering the required load.
This is the bit I don't understand. What is the boiler control attempting to control? In the case of the Celsius, I assume that it is attempting to control the flow to a temperature set by that little knob on the front of the boiler. In addition, I assume that once the temperature set point is reached, the boiler reduces its output to maintain the minimum possible return temperature whilst still maintaining the set flow temp.
The upshot of this is that I would think that the flow temperature will remain constant but the return temp will fluctuate with demand.
In the absence of other inputs, the boiler cannot possibly know the real heating load since the load is proportional to the flow temp. If the boiler backed off the flow temperature when the return temperature was high for example then it would effectively back off the load leading to a very economical but ineffective system.
Of course, in the case of a clever boiler like the Micromat then there are other inputs and the boiler can play games with the flow temperature to reach maximum efficiency.
: When there is demand to heat the DHW, the boiler will "see" a fairly : heavy heat load. This is because by the time the cylinder thermostat : operates, there is already going to be a fair amount of cold water in : the bottom of the cylinder. Therefore, the return water temperature : will be fairly cool as well and the boiler will view it as a hefty : heat load and fire up accordingly. : : As the DHW temperature approaches the set point, the boiler may start : to back off a little as it sees the return temperature rising.
Yes, but I assume it won't back off the flow temperature, only the power input, maintaining the flow but dropping the return temp.
The real question I have is that assuming I want hot water at 60 degrees then how low can I set the flow temperature on the boiler? I'm hoping for something like 65 degrees.
[...]
: >4) I'm planning on a system with two zone valves, one for CH and one : >for HW ("S-plan" in Honeywell speak). I'd probably be looking at : >a programmable room stat (e.g. the Honywell CM-67 or similar) for : >controlling the main room temp downstairs and thermostatic valves : >everywhere else. Any comments on the best place to site the room : >stat (i.e. would the landing be better - the hall is not really a : >sensible option since it is very small and hence prone to drafts : >when the front door is open)? Any thoughts on the best choice of : >stat? : : Remember that if you use the living room as your controlled room, you : can't really have any other form of heating in there like a gas fire : because it will have the effect of backing the heating off to the rest : of the house.
My concern at the moment is that I know the radiators upstairs will be grossly overrated so I'm reluctant to put the thermostat on the landing, even though all the rads will have thermostatic valves. The radiator for the hall (and hence landing) is slightly underrated due to space constraints. Last year with no radiator in the hall, there was a very discernable temperature gradient as you went down the stairs - there is a risk that I will have the same thing again but to a lesser extent this year.
The downstairs radiators are fairly close to the mark for the rooms they are in but at the moment downstairs as a whole is 1500W short since I haven't planned anything for the kitchen - nowhere to put anything (see question below). I have no other heating downstairs at present (but I do have a hearth for a gas fire - if I do install something it will be decorative rather than for heating though).
The hall or main room downstairs seem like the best bets under these circumstances. (Someone suggested the kitchen but I think thats cheating!).
[...]
: >6) I have noticed things like the Honeywell "Smartfit" system. This : >looks like it has everything conveniently integrated with control of : >both the CH and HW readily to hand. Does anyone have any views on : >things like this? I can see the convenience, I can also see the : >nightmare getting spares in ten years time... : : It is potentially quite a nice solution and can also be integrated : with some home automation systems like www.comfort.org.uk : : Since it is Honeywell,who enjoy a good reputation, I would not be : overly bothered, but the valves are proprietary.
I've decided to give it a try.
[...]
The only remaining issue now is the kitchen. At the moment I'm probably going to survive this winter with it being heated incidentally from elsewhere downstairs. There isn't space for a radiator and I'm wary of the reliability of these fanned under unit things - anyone have any experience of them?
Dale.
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Dale Shuttleworth wrote:

I should really have snipped the above... I think the MO of the Keston is to control the return temperature and then set the boiler power and pump speed so that the flow temperture is about 20C hotter. This method of control seems to acheive to lowish return temperatures.
I am reassured that whilst doing HW only the boiler can still make condensate.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Hi,
Ed Sirett ( snipped-for-privacy@makewrite.demon.co.uk) wrote:
[...]
: I thought that I might have a problem with the single control on my own : Keston C. 25 : : I have experimented quite a bit to see what is the lowest setting I can : run the boiler on and still get HW. : : The cylinder (Standard modern copper with fast coil) is set to 60C. If I : run the boiler at 50% setting the : behaviour is stunningly good. 30 seconds full power, then a glide down : to minumum over 10-15minute, then off because the cylinder has been : reheated. : : If I had longer flow return circuit to the cylinder the inital flat out : might be longer. : : When I run the boiler at about 45% then the glide down is faster and the : boiler will spend a bit longer on its lowest setting. : : When I run the boiler at 40% then the minimum setting goes on for about : twenty minutes. The boiler then indicates 'can't go low enough' (fast : red flashing) but still heats the tank. : : I moved it to 50% and am totally happy with the operation. I guess if I : wanted the 'standard' 65C HW then I'd have to move the control up a bit. : Because the reheat time is so short it permits having the HW cylinder a : bit less hot.
Thats extremely useful info - thanks Ed.
Am I right that the little knob effectively controls the flow temp by the way? - None of the Keston literature actually says anything useful about what it does.
Dale.
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Are teenagers not human?
Suzanne "You can't scare me, I've got children."
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ago. In the end I chose a Keston Celsius 25 boiler and a huge Range Superduty cylinder .

25kW) for most of the reheat time then dropping to about half power for the last few minutes as the differential drops. I played with it a little lower but (as expected) it took ages on the lowest modulation to reach final heat (stat at 60deg)

comfortable and sort of self regulating as the temp diff between rads and room reduces.

with it's proportional control, DHW setting and very low minimum modulation, walks all over the Keston for this kind of job, but in the end I just couldn't justify the extra cost. The lack of a separate DHW demand on the Keston is a major flaw, but I plumped for it knowing that I would be attempting a bodge to correct the problem. The solution relies on pulling the output of the boiler stat control when DHW demand is on. This is a straightforward job as the control panel board is separate from the main electronics and can be achieved with a mod to the loom comprising a relay and a few resistors. If discovered, it would of course in validate your warranty so is not for everyone. I will publish the mod here when it is fully tested.

(small remote electronic affairs) are being added to the rooms as I refurbish. In the mean time I just rely on the TRVs and have constructed a long duration lockout to stop the boiler short cycling. I'm not a fan of the single stat master control type system but I realise that is what most people have to work with. From what I gather, don't put it in a room with auxiliary heat sources, kitchen or lounge. Hall sounds bad (cold), landing sounds bad (rising heat), how about half way up the stairs . . . . no really.

differential on my cyl stat, I haven't found the boiler cycling to maintain 60.

heat wise, single convectors to be huge but best for comfort as the heat comes from a large area. I compromised with plenty of Stelrad P+ rads which are double panels with a single convector, just a bit slimmer and less obtrusive than double panels. I used mainly 600mm high units as these seem to blend in quite well, plus taller ones where more heat density was required and smaller single panels near large windows.

Good luck
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fred

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"pulling the output"? What do you mean?

Are you forcing the boiler to full unmodulated power when DHW calls? BTW, the Ravenheat CSI system boiler has this function. Why any load compensation modulating boiler does not have this simple function is beyond me. It can be selectable, by moving jumpers, if need be.
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The control is set by a potentiometer, by judicious use of a few resistors and a relay, the control board can be fooled into thinking the demand on the pot is set to a higher level when DHW demand is on.

power. Quite a failing I think, from speaking to them they didn't seem to think so tho', "the 40 has got it", er yes. Perhaps that makes the Ravenheat a better choice for this app then, I have no experience of them.
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An old trick, switch out the pot, with the relay, and bring in a resistance that equals the desired setpoint on the pot.
With older fixed rate boilers I found it best to keep the boiler stat set to max for DHW, and switch in a pipe stat on the flow for CH. With load compensation control on modern condensers, that is impossible.

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in message (Dale Shuttleworth) writes:

Multi-core cable is easily available and much neater than 2 T&E
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