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Author Topic: Short cycling oil boiler. Commercial size  (Read 7500 times)
martin W
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what do you mean my snoring is too loud!


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« Reply #15 on: December 18, 2017, 06:17:20 PM »

Guy,

are you saying that when the boiler heats just the DHW it just runs full throttle (or modulated) until the cylinder temperature is satisfied and does not short cycle?

How on earth do you heat a cylinder with a 64kW boiler without it short cycling? Is it a large thermal store and heated direct?

If this is the case you must have a problem on the heating side to do with the water flow around the heating...
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knighty
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« Reply #16 on: December 18, 2017, 11:27:55 PM »

Guy, feed and return temperatures at the boiler will tell you a lot, you need to check them first to narrow things down


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guydewdney
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« Reply #17 on: December 19, 2017, 07:42:30 AM »

I have done the survey and there's 81kw worth of radiators! So in theory it should run flat out comfortably.

When I run the dhw the temps get much higher on the return and stay there. So it doesn't short cycle on the off part. When it does fire, yes, it runs for a short time.

On the heating side it runs for a short time but also stops for a short time.

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guydewdney
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« Reply #18 on: December 19, 2017, 07:45:31 AM »

I bought half a dozen pipe temp gauges. Waste of time. They are saying that the pipes are at like 50 degrees when the essebe device has gauges in it which are showing a flow temp of 75 on shutdown which is correct


If anyone can give me a link to temp gauges that actually works it would help. Admittedly I bought 4 quid ones on eBay.
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bxman
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« Reply #19 on: December 19, 2017, 09:39:00 AM »

Good Morning Guy

may be you spent to much on your temp gauges.

https://www.ebay.co.uk/itm/LCD-Digital-Refrigerator-Fridge-Freezer-Temperature-Thermometer-FREE-DELIVERY-UK/322463987916?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49129%26meid%3D9c26d5c92c76440e80749c4f6d8328aa%26pid%3D100005%26rk%3D5%26rkt%3D6%26mehot%3Dpp%26sd%3D282459600059&_trksid=p2047675.c100005.m1851



I have several of these,  all remarkably reliable and consistent in their readings

If you strap the sensor tight to the pipe and then cove it with insulation so that the heat from the pipe and sensor does not escape I suspect you will achieve a more realistic value of the water within the pipe .

Good luck with your problem I am looking forward to learning of the solution that  achieves a successful resolution  of this particular situation .
cheers Patrick
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JohnS
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« Reply #20 on: December 19, 2017, 09:50:17 AM »

I agree with Bxman. 

I use the same on my boiler return and have shoved the sensor about 10cm under the insulation.  Works well.

John
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supremetwo
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« Reply #21 on: December 19, 2017, 04:22:47 PM »

On the heating side it runs for a short time but also stops for a short time.

Similar symptoms here and traced to a partial blockage in part of the heating circuit.

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guydewdney
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« Reply #22 on: December 21, 2017, 08:07:53 PM »

Thanks lads,

Have bought two of these types https://www.ebay.co.uk/itm/4-Channel-K-Type-Digital-Thermometer-Thermocouple-Sensor-200-1372-C-2501-F/171488884355?epid=1863804396&hash=item27ed88b683:g:pJEAAOSwKfVXIf62

So can monitor 8 channels.

Interestingly, they are quite sensitive to location and they also go a bit mad if they interconnect via the copper pipe. So bit of tape to insulate the pipe, and tape on under thermal insulation.

All within 0.5 degree

So, placed 4 of them, boiler flow and return, heating flow and return.

Initially, boiler flow = heating flow but boiler return =/ heating return, it was hotter. The best explanation for this is that the boiler flow was too high, causing water to be short circuited back to return from boiler flow.

Adjusted pump, it turns out this is also more sensitive than expected. Have an adjustment of head set, from 0 to 11m of head. Itís now set to 3.2. 3.4 is too much, 3.0 is too little. Now the return temp of the heating is within a degree of the return to the boiler. It was 10 degrees out!

Then hospitals had to be gone to. I now have a new hole in my mouth :eek:
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guydewdney
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« Reply #23 on: December 22, 2017, 06:59:55 PM »

OK - survey done. 45 radiators. Largest is a 1800 x 60 type 22 at 4kw. Smallest is a towel rad at 800w. Total is 81.8Kw
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knighty
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« Reply #24 on: December 23, 2017, 01:24:25 AM »

can you increase the flow around the central heating system ?

my thinking being, as the house is warmer/cooler, the weather is warmer/cooler, and probably a million other factors, the boiler flow pump setting might not match the rest of the system anymore
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dimengineer
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« Reply #25 on: December 23, 2017, 04:02:29 PM »

Thanks lads,

Have bought two of these types https://www.ebay.co.uk/itm/4-Channel-K-Type-Digital-Thermometer-Thermocouple-Sensor-200-1372-C-2501-F/171488884355?epid=1863804396&hash=item27ed88b683:g:pJEAAOSwKfVXIf62

So can monitor 8 channels.

Interestingly, they are quite sensitive to location and they also go a bit mad if they interconnect via the copper pipe. So bit of tape to insulate the pipe, and tape on under thermal insulation.

All within 0.5 degree

So, placed 4 of them, boiler flow and return, heating flow and return.

Initially, boiler flow = heating flow but boiler return =/ heating return, it was hotter. The best explanation for this is that the boiler flow was too high, causing water to be short circuited back to return from boiler flow.

Adjusted pump, it turns out this is also more sensitive than expected. Have an adjustment of head set, from 0 to 11m of head. Itís now set to 3.2. 3.4 is too much, 3.0 is too little. Now the return temp of the heating is within a degree of the return to the boiler. It was 10 degrees out!

Then hospitals had to be gone to. I now have a new hole in my mouth :eek:

Hi Guy, another tip is to use some heat transfer paste (RS does it) when you attach the temp probes. Works a treat. I use it for temperature mapping of process vessels at work.
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dimengineer
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« Reply #26 on: December 23, 2017, 04:17:06 PM »

Guy, I've been looking at the basic numbers & I think your F&R pipework to the house is too small.

In these systems, small bore, you really want your flow to be down at less than 1.5m/s (or thereabouts). For a 64kW boiler you need a flow of 1.53 kg/s at a delta T of 10C. That sort of flow needs 3m/s in the 28mm pipe - which is a pretty high velocity for these type of system. So I dont think you are getting the flow round the house that you need to move the 64kW of hot water. So it short cirtuits and short cycles.
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guydewdney
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« Reply #27 on: December 24, 2017, 04:41:21 PM »

Getting bigger pipes would be a nightmare, due to contamination.

However, we have two 28mm pipes going round, one does ground floor, then the front as a zone, and splits off to the back as the second zone. So the ground is on either way, which works for us.

Thereís a second pipe which does the hot water only, again 28mm. The route it takes means it goes past the rear zone valve.

How could I use this and still have hot water?

Or should I just put another pump on the second zone to help the flow down there, as thatís where the problem shows itself?

Happy Xmas Smiley
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supremetwo
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« Reply #28 on: December 24, 2017, 05:48:05 PM »

Or should I just put another pump on the second zone to help the flow down there, as thatís where the problem shows itself?
Happy Xmas Smiley
That is what I have and a much smaller installation.

My second pump is controlled via a relay and the relay is controlled via a time clock and thermostat in the main living area.

To prevent back pumping, I fitted a non-return valve in the flow just after the pump of the circuit with the lowest head.
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dimengineer
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« Reply #29 on: December 25, 2017, 12:31:30 PM »

The more I think about it, the more it points to insufficient flow through your radiator circuit. You say when the DHW is on, the problem vanishes. I dont know about your system, but my 250 litre cylinder has a 17kW coil so I suspect that yours is bigger, so it takes a LOT of heat away from your boiler. And so the problem goes away.

So, yes, I'd try putting a 2nd pump at the start of the radiator circuit to increase the pressure and therefore the flow through that circuit. Should help. Make sure its near the start of the circuit - you want it to pump, not suck.
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