Delay timer for heating pump

[SJB060685]

Good point about the boiling water, and of course with the slightly increased pressure in a sealed system, it will as you say instantly boil if there was a leak. Hmm, scary!

I have wanted an excuse to deploy a PLC or even something like a Raspberry Pie to control the heating for some time, this might push me over the edge, although I don't suppose the management will approve

I'm glad of your advice and you're quite entitled to ask a heating amateur obvious questions!

We're here to offer free, impartial and safe advice.
I'm glad you understand the connection between pressure and boiling point. In case you didn't know water which flashes to steam expands some 1600 times, so if your system consists of 100 litres volume (quite a large system and used as an example) then you would have 160,000 litres of steam contained within the boiler and system pipework, this would obviously be causing extreme pressure and something will rupture in a flash, most likely taking out a large portion of the dwelling, I've seen it on unvented cylinders a couple times.
I can't comment on the PLC or Raspberry Pi control systems as I have zero knowledge or experience with them, although I will have a read up later.
If you do decide on controls like that then make sure its compatible with your system.

 

[John.g]

I didn't actually get as far as wiring the pipe stat in, I put another thermometer on the pipe, temporarily wired the pump to come on and then watched the thermometer on the pipe and the one I already had on the boiler. Having watched the temperature readings I then put the pipe stat back in its box as I realised it wasn't going to do the job Maybe it would do if there was somewhere for the heat to go to...

So given that I'm sure you're both right and I'm going to need to find somewhere to lose the residual heat to, I thought I'd be better monitoring the "problem" at source, which is why I've ordered the stat with a capillary bulb so I'm looking directly at the boiler rise. I told you was a sparks not a heating engineer!!!

Once I've proved to myself that I can't lose enough heat just circulating round the pipework I'll add something more complicated. It maybe that I can lose just enough heat to keep the boiler sufficiently below 110C that the overheat won't trip which will be OK for now. I'm sure you're going to tell me that this isn't going to do the boiler any good in the long term though!

To answer an earlier question, there's about 7.5metres of uninsulated 22mm copper in the loop from the boiler through the ABV and pump and back to the boiler. I know it should be insulated, but SWMBO likes her airing cupboard to be nice and warm...

Thanks both, Graham

7.5M of uninsulated 22mm copper will emit ~ 450 watts (0.45kw) at only 75C so I would be (very) surprised if this isn't sufficient to dissipate the heat in a relatively short period and well before the temperature reaches 110C, you don't want to be wasting energy either so if the stat is, as you said, set to ~ 90C then I think (hope) you will be in business, I also think you have already proven this anyway as the temp was only 55C with the pump running in overrun, the stat/pump may come in a few times but that should be it.

 

[GrahamPye]

7.5M of uninsulated 22mm copper will emit ~ 450 watts (0.45kw) at only 75C so I would be (very) surprised if this isn't sufficient to dissipate the heat in a relatively short period and well before the temperature reaches 110C, you don't want to be wasting energy either so if the stat is, as you said, set to ~ 90C then I think (hope) you will be in business, I also think you have already proven this anyway as the temp was only 55C with the pump running in overrun, the stat/pump may come in a few times but that should be it.

Excellent - I'll report back once I've wired the new stat into circuit next week!
Thanks, Graham

 

[John.g]

We're here to offer free, impartial and safe advice.
I'm glad you understand the connection between pressure and boiling point. In case you didn't know water which flashes to steam expands some 1600 times, so if your system consists of 100 litres volume (quite a large system and used as an example) then you would have 160,000 litres of steam contained within the boiler and system pipework, this would obviously be causing extreme pressure and something will rupture in a flash, most likely taking out a large portion of the dwelling, I've seen it on unvented cylinders a couple times.
I can't comment on the PLC or Raspberry Pi control systems as I have zero knowledge or experience with them, although I will have a read up later.
If you do decide on controls like that then make sure its compatible with your system.

If you have 100 litres of water at 110C and a massive leak/rupture occurs then this water will fall to 100C instantly but 100 litres of water will not flash off as steam, ~ 2 litres will, and will expand as stated above, the remaining 98 litres will remain as water at 100C, still not pleasant if it rains down on you.

 

[SJB060685]

If you have 100 litres of water at 110C and a massive leak/rupture occurs then this water will fall to 100C instantly but 100 litres of water will not flash off as steam, ~ 2 litres will, and will expand as stated above, the remaining 98 litres will remain as water at 100C, still not pleasant if it rains down on you.

My understanding is any water at or over 100°c will flash if there is a rupture, although not all the system volume will be at those temperatures. At the moment the pressure drops water will retain that heat but be at or below atmospheric pressure quickly flashing to steam. That's the problem with unvented cylinders wrongly installed and YouTube videos will show the damage possible.
If I'm misunderstanding something then please correct me.

 

[John.g]

If you get a pot of water holding 1 kg (~1 litre) and heat it to 100C it will be at atmospheric pressure, 0 bar, but no more, if you put a (tight) lid on the pot and boil it until it reaches 110C it will be at 0.42 bar.
The heat content at 0 bar, 100C, is 418.55 Kj and at 0.42 bar is 460.86 Kj. remove the lid from the pot and what happens?, some of the water flashes into steam, the water in falling from 110C to 100c will give up (460.86-418.55), 42.31 Kj and because it requires 2256.47 Kj to convert 1 kg of water into steam at 100c then, 42.31/2256.47, 1.88% flashes off as steam in falling to 100C.
I won't repeat the dreary calcs but if you had a electric water heater that was heated to 184C then the pressure is 10 bar and 16% of its contents will be released as steam if it ruptured with resultant great propulsive force in expanding while falling from 10 bar to 0 bar which is why you occasionally see a 10 litre water heater heading for Mars. The boilers I serviced operated at 44.8 bar so if one of those ruptured then over 30% of the contents (of 16 tons) would be released as steam and so on.

 

[SJB060685]

If you get a pot of water holding 1 kg (~1 litre) and heat it to 100C it will be at atmospheric pressure, 0 bar, but no more, if you put a (tight) lid on the pot and boil it until it reaches 110C it will be at 0.42 bar.
The heat content at 0 bar, 100C, is 418.55 Kj and at 0.42 bar is 460.86 Kj. remove the lid from the pot and what happens?, some of the water flashes into steam, the water in falling from 110C to 100c will give up (460.86-418.55), 42.31 Kj and because it requires 2256.47 Kj to convert 1 kg of water into steam at 100c then, 42.31/2256.47, 1.88% flashes off as steam in falling to 100C.
I won't repeat the dreary calcs but if you had a electric water heater that was heated to 184C then the pressure is 10 bar and 16% of its contents will be released as steam if it ruptured with resultant great propulsive force in expanding while falling from 10 bar to 0 bar which is why you occasionally see a 10 litre water heater heading for Mars. The boilers I serviced operated at 44.8 bar so if one of those ruptured then over 30% of the contents (of 16 tons) would be released as steam and so on.

I'm drunk at the moment and awaiting the grand national. I will go over this later mate.

 

[SJB060685]

I like your maths John but do you really mean 418.55kj for example? That's is over 400kw.

 

[John.g]

418.55/3600 = 0.1162 kw.

 

[SJB060685]

If you could mate please explain your comment 4 above in extra detail. I'm sort of with you but missing crucial information. Again thank you, you're one of only a few with this level of knowledge.

 

[John.g]

What is comment 4, SJB?

 

[SJB060685]

4 above mate. I'd like to know the maths as to how you arrived at that in depth

 

[John.g]

Sorry, still not with you, do you mean where I got the figures/numbers from or what?

 

[SJB060685]

Yes where you got the figures from. I'm almost with you but not quite 100%

 

[John.g]

Ok. Will do shortly after counting my G.national winnings.

 

[SJB060685]

Haha. Thanks to a friend I won a little myself.

 

[John.g]

The first three are the only numbers that interest us just now.
"Saturation Temperature" is the temperature of the water.
"Specific Enthalpy of Water" is the energy content of the water. (Sensible heat)
"Specific Enthalpy of Evaporation" is the energy required to turn 1 kg of water into 1 kg of steam at the same temperature. (Latent Heat)
They all come from.....

Steam Table Calculator| Dry Saturated Steam Line | Spirax Sarco

Use our Dry Saturated Steam Line steam table calculator to help you plan and design your steam system.

www.spiraxsarco.com

If you want me to explain it further, fine.

 

[SJB060685]

I will get my thermodynamics book out again. What is the website you found that calculator on?

 

[ShaunCorbs]

I will get my thermodynamics book out again. What is the website you found that calculator on?

Steam Table Calculator| Dry Saturated Steam Line | Spirax Sarco

Use our Dry Saturated Steam Line steam table calculator to help you plan and design your steam system.

www.spiraxsarco.com

Steam Table Calculator | Wet Steam Region | Spirax Sarco

Use our Wet Steam Region steam table calculator designed by steam industry experts to help you plan and design your steam system.

www.spiraxsarco.com

 

[Ric2013]

Analogue multifunction timer relays Selec 600XU-A-1-CU

Multifunction timer relays, analog, 13 time functions, 10 time ranges, 20-240 V AC/12-240 V DC, 1 x changeover contact, DIN rail mounting, IP20 protection, Screw-type terminals, Functions: On-delay/interval/flashing with off…

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GEYA On Off Time Delay Relay ACDC12-240V 1xSPDT Multifunction Timer Relay 1SPDT | eBay

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Something like this is what you need

This was what I wanted years ago to give my own bathroom fan an over-ride function. Unfortunately Rapid Electronics had nothing of this sort and I didn't know where to look. I suspect this plus a separate non-timer fan would prove more reliable than rely on the cheap internal circuit of an all-in model, and at least when one unit breaks you don't need to change the whole lot. So thanks, Shaun. Next time...

 

[SJB060685]

So John, I've taken on board what you've said. IF I have a 200 litre cylinder at roughly 110°c how much of that volume flashes to steam if the pressure drops below saturation point?

 

[John.g]

If the pressure/temperature falls from 0.42bar&110C to 0.0bar&100C then the % of flash steam (using steam tables, above) will be, ((460.86-418.55)/2256.47)*100, 1.88%. So 200 litres will flash off 200*1.88%, 3.76 litres.

 

[John.g]

Suppose should/must mention the qualities of steam for heating purposes where it is almost universally used in plants/factories normally at 8 bar which gives a temperature of 175.4C necessary for some processes.
As you are aware steam gives up its heat at constant temperature (isothermally) so if you compare a heating requirement of say 20Kw with steam vs hot water then only steam at a rate of 0.59 kgs/min (~0.6 LPM) is required as it gives up 2030.25 Kj/hr/kg (0.564 kw), if water heating is used then ~ 29 LPM at a deltaT of 10C will be required. Also steam heating reacts very rapidly to changing heating demands as its controlled on pressure, hence lower/higher steam temperatures instantly , the coil outlet is normally fitted with a float type steam trap which (traps the steam) will only pass water and ensures all the steam's latent heat is extracted.

 

[SJB060685]

Suppose should/must mention the qualities of steam for heating purposes where it is almost universally used in plants/factories normally at 8 bar which gives a temperature of 175.4C necessary for some processes.
As you are aware steam gives up its heat at constant temperature (isothermally) so if you compare a heating requirement of say 20Kw with steam vs hot water then only steam at a rate of 0.59 kgs/min (~0.6 LPM) is required as it gives up 2030.25 Kj/hr/kg (0.564 kw), if water heating is used then ~ 29 LPM at a deltaT of 10C will be required. Also steam heating reacts very rapidly to changing heating demands as its controlled on pressure, hence lower/higher steam temperatures instantly , the coil outlet is normally fitted with a float type steam trap which (traps the steam) will only pass water and ensures all the steam's latent heat is extracted.

Yes I can wrap my head around this but will read over what you've mentioned above a few times. I thought what I knew above was enough, clearly I was wrong

 

[plumb_know]

Hi everyone,

got to the bottom of the wiring for this relay should anyone else do one.

perm live goes to A1 with link to 15
neutral = A2
switch live to S
pump feed on 18

thanks for everyones help

 

[GrahamPye]

Excellent - I'll report back once I've wired the new stat into circuit next week!
Thanks, Graham

I've wired the capillary stat in now, and as predicted it seems to be doing the trick just pumping round the circuit formed by the ABV.

I've put the extra stat inside the boiler cabinet and brought an extra permanent live across from the main junction box to power the pump bypass. In fact I haven't actually wired it in permanently yet as it occurred to me that I now needed to isolate both my new permanent live to the boiler as well as the existing switched live. I'm using a three pole isolator switch for this (the sort that are used for timed fans) in place of the current switched, fused spur. The whole installation is already fused elsewhere in the room, so I don't think that losing this fuse local to the boiler is a problem - unless you advise otherwise! I could put a simple fused spur outlet in as well if thought to be needed.

Thanks again for all your help, and the interesting diversion about steam thermodynamics - I haven't seen some of those terms since I was studying Physics at school which was a very long time ago

Graham

 

[SJB060685]

So the new stat wired into keeping the pump circulating after boiler shut down is pumping around that shirt 7m section of pipe and keeping the limit stat from tripping? If so that's handy to know that short length of run is enough for future reference.
Is the whole central heating circuit on its own 5 amp fuse and all wires come off that incoming supply?

 

[GrahamPye]

Is the whole central heating circuit on its own 5 amp fuse and all wires come off that incoming supply?

Yes. Actually, I have a feeling that it's also on its own circuit from the consumer unit (probably with a 15A MCB but I'm not at home so would have to check that) as well as the local switched spur which powers all the heating circuitry.

 

[SJB060685]

It could be on its own circuit and more often than not people will spur off a ring main. If its got a switched fused spur then it's fine and will protect the system componentsin the event of a problem. Oil should be on 5 amps but could be a 3 amp fuse in place.

 

[GrahamPye]

It could be on its own circuit and more often than not people will spur off a ring main. If its got a switched fused spur then it's fine and will protect the system componentsin the event of a problem. Oil should be on 5 amps but could be a 3 amp fuse in place.

I wouldn't have been surprised if it was spurred off the ring, but I don't think it is. I'll check the fuse rating in the fused spur tonight. I presume that 5A for oil rather than 3A is due to the starting current of the burner motor.
Thanks, Graham

 

[SJB060685]

I wouldn't have been surprised if it was spurred off the ring, but I don't think it is. I'll check the fuse rating in the fused spur tonight. I presume that 5A for oil rather than 3A is due to the starting current of the burner motor.
Thanks, Graham

The inrush current of the motor will be more than steady state yes. If its got a 3 amp in already and it's been fine until now then it should be fine, after all a 3 amp fuse should melt before a 5 amp in the event of a problem. I was just told on my OFTEC courses oil should always be 5 amp. If you really wanted to do the maths you could work out what current was being pulled but I've never bothered.

 

[John.g]

I've wired the capillary stat in now, and as predicted it seems to be doing the trick just pumping round the circuit formed by the ABV.

I've put the extra stat inside the boiler cabinet and brought an extra permanent live across from the main junction box to power the pump bypass. In fact I haven't actually wired it in permanently yet as it occurred to me that I now needed to isolate both my new permanent live to the boiler as well as the existing switched live. I'm using a three pole isolator switch for this (the sort that are used for timed fans) in place of the current switched, fused spur. The whole installation is already fused elsewhere in the room, so I don't think that losing this fuse local to the boiler is a problem - unless you advise otherwise! I could put a simple fused spur outlet in as well if thought to be needed.

Thanks again for all your help, and the interesting diversion about steam thermodynamics - I haven't seen some of those terms since I was studying Physics at school which was a very long time ago

Graham

What is the stat setting and does it cut in/out a number of times before the boiler temperature stabilizes?

 

[ShaunCorbs]

Hi everyone,

got to the bottom of the wiring for this relay should anyone else do one.

perm live goes to A1 with link to 15
neutral = A2
switch live to S
pump feed on 18

thanks for everyones help

that’s an odd setup how did you figure it out ?

 

[plumb_know]

spoke to couple of electricians and they both said they thought perm L should be on A1, then found these on geya website:

GRT8-M1 Multifunction Electronic Time Relay - GEYA

Multifunction Time Relay GRT8-M1 - At GEYA offers a multifunction time relay which fulfills the high-quality standard of the product...

www.geya.net

Single Function Time Relay GRT8-B2 - GEYA

Single Function Time Relay GRT8-B2 - GEYA is the best supplier and manufacturer in China when it comes to Single Function Time Relays...

www.geya.net

tested it on leaving sat morning and all seemed good & customer not contacted me since so seems to have resolved the issues

 

[GrahamPye]

What is the stat setting and does it cut in/out a number of times before the boiler temperature stabilizes?

The stat is set to about 90C (I'll explain the about in a bit) - I couldn't decide whether it was better to have it come on sooner and "waste" electric running the pump, but nip the overheat in the bud so to speak, or set it high so the pump only comes on when there's a danger of the overheat trip going off. So 90 was a bit of a compromise for now.

My criteria for success was that the overheat trip didn't trip and it hasn't so far so that seems to be OK.

I can't totally answer your question as I haven't (yet) studied it closely. What seems to happen is that the worst time is first thing in the day when the house is cold and the boiler is firing for a long time to get everywhere warm. That's when I have seen/heard the pump running from my new thermostat. What then happens is that after a while the room stat calls for heat, which then opens the motorised valve and that obviously cools the boiler down much quicker until it fires again.

I wanted a stat that was in an enclosure, and also one that had a range over 110 so that it wouldn't suffer if the boiler got so hot that the overheat trip went. So I got this one from BES: Capillary Control Thermostat - https://www.bes.co.uk/capillary-control-thermostat-9781/ which has a range of 50 to 300C which is why I said the setting was about 90! This was the best one I could find - I expect you'll now point me to a much better one

Once I've studied how it's working I'll post here. I might also be asking you why my room stat appears to be short cycling the boiler - but not now...

Graham