Thermostat Central Heating

[pwdrive]

Hello Mark, the reason for gravity flow in solid fuel/wood fired boilers is for safety reasons, there must be nothing in the primary HW flow system than in any remotely possible way can impede the thermal flow and thus circulation of the hot back down to cool flow.
If for instance there was installed a solenoid operated Y valve and it jammed open in the CH position and the CH pump had stopped working ie: prolonged power failure (bad scenario) then that would mean that the primary HW flow would cease to flow, the water temp. in the boiler would quickly rise until it turned to steam, the laws of thermo dynamics take over and bang, possible shrapnell through any soft objects in the way, they have been know to blow the back wall of the fireplace reccess right out.
With gas or oil fired once the supply is shut off this will not happen, these types of boilers have a permanent temp. sensor possibly capillary that shut the fuel supply and/or burner off instantly, the same as the "fire" valve in an oil feed supply line between the oil tank and the boiler.
With solid fuel/ wood you cannot shut the heat off instantly, there might be systems now that do, I don't know, but personaly i would only have a full gravity primary flow system as it is pretty much tried and tested.
PS I am not a plumber, any propposed diy heating then need to satisfy regs/safety yourselves.

Lawrence.

[Michaelr]

Just to add , I presume this scenario would not occur with say a modern combi or gravity boiler sytem because you can switch the heat source on and off unlike an open fired system.

Mark the link to the low cost sensor works for me , It is possible this may fall into the same problem area as is prompting the need to change at the moment. The delay in the sensor reaching temperature is too long, no figures are quoted but I suspect it probably is not much different to what he is using already.At least the lm335 should be seconds as opposed to minutes if the sensor is pressed up against the copper pipe and secured there.

Mike

[pwdrive]

Hello Mike, yes, in an oil fired or gas fired boiler the oil/gas supply can be shut off more or less straight away in the case of a boiler overheat situation, so far as I am aware the onus is on the oil/gas fired boiler manufactures to include fail safe systems in their products to that effect, if the boiler uses coke/coal/wood then the heat source ie: the fire, can not be shut down / extinguished straight away, this can lead to thermal runaway for the water now trapped in the boiler if the primary flow route has been blocked ie: jammed solenoid valve, manual shut off valve that the kids might have played around with, power failure etc it can turn to steam in the boiler resulting in pressure being built up until something gives and that's where the danger arises.
With my own installations using wood/solidfuel boilers I have always followed the manufactures installation instructions and in all the ones I have done they all say no valve of any type or reducers whatsoever in the primary flow and expansion circuits including the heatleak radiator/towel rail flow.
Again to anyone that might read this, satisfy yourself as to suitabilty/safety when fiddling with such systems. So far as gas goes, if I had a gas installation I would not attempt to service any part of it whatsoever myself except user recommendations in the user operating manual, this stems from the fact that my Mum used to work for the Gas Board and used to come home with some real horror stories sometimes.

Lawrence.

[Mark Hennessy]

Thanks for the replies - I did wonder if it was that simple. As you say, it's much easier to switch off the gas than put out the fire...

This is all very different to my new boiler, which can output anything between 4.8 and 19kW, and varies the flow temperature according to the outside temperature. There is a scary amount of software in there, but so far so good - the weather compensation seems to work very well indeed. When heating water, it does HW priority so that it can up the flow temperature to 70C, meaning the water can be heated to a safe temperature. It would have been possible to retain my Y-plan system, but I would have been mad to do so as the flow temperatures would have been too hot for the house (and for efficient condensing operation) if I wanted the hot water to actually get hot! Of course, the risk of Legionnaires disease is a problem for under-heated stored water...

Regarding the comments about that thermal switch suffering from delay, I have to say that in my experience I haven't come across this. After all, whatever you bolt it to will have a much higher thermal inertia than the switch itself, and providing it's in good thermal contact (silicon grease, etc), then it should work well. I've used them countless times in the past. They are frequently seen inside cookers and boilers (to name but two examples) doing similar things.

However, they might not be the ideal solution for you as they are not adjustable unless you buy a few. An electronic solution would obviously be more flexible as you can vary the hysteresis as well as the set point, but it must be designed with high reliability built in - and I would still include a bi-metallic switch as a fail-safe. The LM35 and its derivatives (of which the LM335 is one) are fairly vice-free, but need a fair bit of external circuitry. I'd consider an NTC thermistor (available in a heat-sink mounting package) and a couple of transistors driving a relay. Again, I've done that lots of times, and it works well. Simple is always best.

Good luck,

Mark

[pwdrive]

Thanks Mark, i am confident that I can sort it out when I have time that is. I hear that a lot of the new boilers are combined heat and power types that feed by genereation surplus heat back in to the grid as it were, from what I read a while back the cost savings amount to around 5% when everything is taken into consideration, I guess they will be standard at some point.
The system I have is not what you would call convienient heat, it never is burning wood, we only have a small HW tank due to space restrictions where it is located but it heats the water and all the radiators no problem, if a bath is run and the fire is still going then tank recovery time for anothe bath run is usually about 30 minutes, once the tank hits temp. then the radiators kick in and the house soon warms up so it's not to bad really. No room stats but individual stats on all the radiators except the heatleak one of course.

Lawrence.

[Michaelr]

Hi Mark,

Acccording to Lawrence the pipe feeding the tank has a relatively fast change in temperature and that is his problem. Once the sytem stabilises and has heated up and gravity feed is fully established the thermal inertia of the sytem could be considered high. It is the startup condition before gravity feed is fully established where he has a potential problem.

Maybe I have misinterpreted this

Mike

[pwdrive]

No Mike you are spot on, that is exactly the effect I am getting. Once full gravity circulation is underway the HW tank is well on the way to reaching it's set stat temp. and the metal interface to the pipe on the cheapo stat is well up near the cheapo stats setting (85 C) and thus that stat will then cut in if 85 C is reached as it's body has already been preheated as it were, this then dumps excsesive boiler temperature water into the CH system and by that very nature the flow return is cooler than the boilers exit temperature thus lowering the temp. of the water in the boiler to below 85 C. All the water contained in the boiler is common to both the primary HW flow and the central heating flow, the cross flow arrangement is designed to effect a more even temperature gradient thoughout the boiler thus reducing possible large temperature stresses within the boilers metal plate construction.....I think!!

Lawrence.

[Mark Hennessy]

Hi Mike,

Yes, that's my reading of the situation too. But, it is my belief - and I could of course be quite wrong about this - that a small bi-metallic thermal switch would be considered to have very low thermal inertia compared to the rest of the system. Certainly, just a section of 28mm copper pipe is pretty "hefty" compared to one of these switches - they are only about 12mm or so in diameter. Sure, an LM35 or equivalent probably responds faster, but what is "fast" in this context? I sense that the difference being discussed between these different sensors is seconds or perhaps 10s of seconds rather than minutes, and that this sort of time is probably short enough. Lawrence? I'm probably wrong - but interested nonetheless...

In my old boiler, one of these thermal switches was used as a safety cutout on the flow pipe - in a boiler with a rather pathetic and fragile aluminium heat exchanger. Another was used to cut out the boiler if the flue gas got too hot. A third detected the dry-fire condition. Not that anything found in this dreadful old thing is necessarily a recommendation in itself, but I've noted very similar things in the German replacement.

All the best,

Mark

[pwdrive]

Hi Mark I am not a guru on boilers but I have installed about 4/5 woodfired jobs in various properties we have had, I know a lot of the new gas/oil boilers have all soughts of saftey systems in them. I did replace the previous one I did with an oil fired one on a "Y" plan but only because we were selling up (not every one likes chopping up wood). As regards the thermal transfer time from the actual pipe surface to what ever the sensor device is it needs to be 30 seconds or under, I think this is now achievable if I go down the electronics route with a decent thermal interface. Had we had the hieght to put the HW tank a lot higher in relation to the boiler then I doubt if this problem, for what it is, would have happened as we would then have a more vigorous gravity flow and return in relation to the boilers thermal gradient when initially fired up using dry wood.
It's just one of those niggling things, you know what us engineers can be like.

Lawrence.

[Paula A]

You know you can cheat by running the "up" pipe as high as you can immediately out of the boiler?.. When I was in my teens father designed a solid fuel system where because of the house layout the hot water tank and 3 of the 8 radiators were below the level of the boiler. His solution was to run 2 "up"pipes right up the chimney and then back down again giving a cracking head on the riser. Clever bloke my father.. one of the designers of the Concorde fuel balance/trim system.

just to note:: we were "off the grid" at that house due to being in the middle of nowhere. He also built a wood fired expansion engine/turbine to generate electricity as it was cheaper and more efficient than running a noisy diesel generator. We had big valve inverters all over the place turning the 60v dc into an approximation of 250v/60Hz.. Lighting everywhere was 12v from large storage batteries. This meant a lot of our tellys had to be "tweaked" as our mains was off frequency (ex US/RAF aircraft inverters)