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Author Topic: Short cycling oil boiler. Commercial size  (Read 7501 times)
guydewdney
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« on: December 13, 2017, 05:35:49 PM »

This is my latest project. Large house, 15 bedrooms. Approximately the right amount of radiators to boiler ratio - but I will check

Boiler is running at 1 minute off two minutes burn. Boiler is 70kw oil boiler running at 64kw. Pipe is 35mm to low loss header. 28mm from header to house. Flow calculation shows adequate flow with grundfos pump on I of III to keep boiler happy, just. So trying II and III settings.

CH pump is the one on the right, thats an 11m head pump, with electronic flow control. I can adjust the head set point to 2M to 11M. It seems about right at the 6M area. Too much and the flow temp drops (presumably short circuiting around the LLH?) ditto too low and the same happens

I've tried every possible combination of flow rates and I can't get it to stay on.

If the system is off for say half an hour, then when it is switched back on it burns for 20 minutes.

Flows seem correct in that the flow to the house is the same temperature as the flow from the boiler.

What are we doing wrong?

Flow from boiler goes round the back of the flue to the top of the LLH. Return goes via the black box with 3 gauges - this keeps the return temp above 50C so on startup it isnt too cold thus protecting the boiler. Normally seen on wood burners. return continues to the two magna clean things, and via the pump back to the boiler.

Flow for the CH goes from the top left of the LLH around the top, down to the right side pump, down and out via the silver foil wrapped pipe. Return via the same type of pipe, across the bottom, and back in the bottom left.

Theres a bypass 'shorting' the CH which is set to about 0.5 bar, which appears to be open even though the TRVs are mostly open

« Last Edit: December 13, 2017, 06:48:03 PM by guydewdney » Logged

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brackwell
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« Reply #1 on: December 13, 2017, 10:35:24 PM »

Without a modulating boiler then it has to cycle shurly?   The boiler will/should be chosen for the worst case scenario and this oversizing also helps with the warm up time. I am also guessing you have insulated ie reduced heating requirement from original design.  This is behaving exactly how my old non modulating gas boiler behaved, my new one just modulates down and stays on all the time.

Ken
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Tinbum
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« Reply #2 on: December 14, 2017, 09:18:20 AM »

Are the flow and return the correct way round on the boiler?  Huh
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Iain
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« Reply #3 on: December 14, 2017, 11:31:19 AM »

Hi

Quote
.Theres a bypass 'shorting' the CH which is set to about 0.5 bar, which appears to be open even though the TRVs are mostly open

If this is happening surely all the heat produced is not being taken by the central heating, some is recircing straight around the low loss header.
Try shutting the bypass and see if there is any change. Just as a trial or do a test with a higher bypass pressure.
Can you downrate the boiler at all, rejetting? To allow a longer run time.
Are the radiators working correctly, i.e. Heating up adequately? Normally with TRV's the lock shields are normally fully open unless you have to adjust others for a slow heating rad.
Iain
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guydewdney
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« Reply #4 on: December 14, 2017, 05:40:09 PM »

Thanks guys

Tinbum - yes - definitely. The return is contrainstinctivly on the top, the flow from the side. It has a sticker on it an' evryfingk Wink Pump is definitely flowing down into the return. You arnt the first person to ask.

Iain
~Ive closed off the bypass as much as I can - even removed the little cap to get another turn out of it. Didnt help.

All TRVs working, as a general rule - theres about 40 radiators, with many different TRVs - some old some new. A couple of them are weird - one in bedroom M for example only really works if its set to 4+ wheres the one in the library (in fact the same type I think) is open even on * mode! (maybe I should swap them over?). Some of the old ones are a bit sticky, ut we are working on that with a hammer and silicone spray Wink

Yes - we can derate the boiler. Its 64kw at the moment and we can do to 58kw possibly less.

My next 'trick' is to interrupt the boiler power with a timer, set to 15 minutes on 15 off - maybe 30 on 15 off so at least it has a chance to do a proper burn. Leaving the circulating pumps running to extract / spread out as much heat as possible.
« Last Edit: December 14, 2017, 07:55:17 PM by guydewdney » Logged

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desperate
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« Reply #5 on: December 14, 2017, 08:23:26 PM »

40 rads unless they are one and a half kW each ain't gonna soak up all the heat that the boiler can produce so it is bound to cycle quite a bit, I bet quite a few of those rads are considerably less than 1.5kW too. Get a bit of sludge and muck in them as well and the boiler thinks it's way too big.



Desp
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rogeriko
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« Reply #6 on: December 14, 2017, 08:38:46 PM »

I would say the 2 magna clean things are restricting the return flow to the boiler. They have pretty small holes inside. The reason they are plumbed like that is probably because there was originally 1 but it restricted the flow so much that they put 2. Remove both of them and put a straight piece of pipe and the improved flow will allow more heat to be removed from the boiler quicker. The magna cleans do not have to be on the main flow, put them somewhere else where some water will flow through them slowly, eventually all the water will pass through them.
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guydewdney
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« Reply #7 on: December 14, 2017, 09:06:14 PM »

No - the magnas are 28mm each - and the manufacturer says 2 is ok. When we first built this up we had one, and its did the same thing. The pump is an 11M head pump - it isnt a problem. The boiler requires 3.0M3/Hr to work. Its pumping 3.5M3/Hr and is on less than half power. Ive tried at full chat, it didnt help.

Might be 44 radiators - whos counting Wink 8 are quite small - 600mm twin type. Biggest is probaly 1.8M twin. Plus 150M at least of 28mm pipe. Ill do a survey. Hey - at least its better than teh previous owner who had 17 rads totalling 22kw - and mostly singles...... on the same size boiler.
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desperate
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« Reply #8 on: December 14, 2017, 10:16:46 PM »

Yeah, I still reckon it's way too big for the system, if it is running approx two thirds of the time you are drawing 45ish kW from it which seems about right for the rads you mentioned. Can you measure the flow and return temps for the first 20 minutes after firing up from cold? that'll help tell us what's going on.

Desp
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knighty
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« Reply #9 on: December 15, 2017, 09:33:20 AM »

you haven't said what the flow and return temps are at the boiler... and what temp the boiler is set to ?

can you add a thermal store?  - any tank would do really, the bigger the better tho

can you adjust the on/off temp setting for the boiler?   - off at 90' but waits for 60' before it kicks on again?

all radiators heating up ok ?  all about the same temp ?
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brackwell
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« Reply #10 on: December 15, 2017, 09:33:40 AM »

Desp +1

But whats the problem with a recycling boiler anyway particularly if its not condensing. I say again ,my old gas boiler did it all the time unless it was -10C outside as by then it had met its output. Also the boiler is oversized to give DHW and start up.

Ken
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martin W
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« Reply #11 on: December 15, 2017, 04:59:50 PM »

Can you survey all the rads you have and estimate the output of each rad...

I would be surprised if it is 30kW... even with 30-40 rads... Also are all the bedrooms heated???

If all radiators are hot (65 deg C plus), and the boiler is short cycling, it is not a flow issue, it is the boiler is generating more heat than the radiators are dumping..

I suspect the boiler is well oversized. Biggest issue is when a room is up to temperature the trv (if fitted) will shut down. Then the boiler is serving many less rads..


I've got a factory with a 70kW boiler serving 22kW of rads for the offices, its about 3 year old and replaced a very old boiler.... why on earth no one survey the system is beyond me. On start up in the morning runs on full burn for about 5 mins then modulated down to around 12kW. Rest of day it is cycling on and off all the time with probably 6-8kW load.

The only thing I believe you could do is re-jet the current boiler to reduce the output. Or fit a thermal store and run the rads of that, with the boiler controlled via a differential thermostat so it comes on when below 60 and off when about 75 or so.....

It might be worth speaking to the boiler manufacturer and see what the absolute minimum output you could run the boiler at with a very small jet (i doubt lower than 40-50kW).

I would think short cycling will not be very good long term for the boiler. If you think a normal system would start and stop say 20 times a day and yours is doing 100... the boiler lifespan 'could' be shortened by 75% or more..
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TT
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« Reply #12 on: December 15, 2017, 05:35:07 PM »

I think I would get a small controls company to build a panel, a small BMS and  terminals for all input and output devices for the system.

I would wait until summer and strip everything out, ply line the wall and plan a new install.
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Philip R
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« Reply #13 on: December 15, 2017, 10:35:53 PM »

Guy,
The minimum power limit will more likely be determined by the burner rather than the boiler. For any given range of oil burners, there are usually a few variant blast tubes which suit different output ranges. Certainly the nozzle could be reduced in size. Are you using a 1.00 or 1.2 US Gallon/hr nozzle.?

You are protecting the boiler from going too cold by using the Laddomat typr mixer/ bypass valve. So Firing at a lower power can be done with burner tweaking: nozzle, air damper and blast tube (turbulator plate to nozzle tip clearances). The grant manual and the accompanying Riello manuals should explain this procedure.

Any burner changes will require it to be set up and commissioned by a competant oil technician.

The boiler looks like a condensing model, judging by the condensate bottle trap on the LHS. The instruction manual may define a minimum return temperature, to limit corrosion.. ( I have read it is about 40 degC for the vortex boiler range.( When used with underfloor heating). You might find the return temperature controller, might be flow limiting and making the boiler short cycle. Do you actually know what the flow rate is or is it inferred from the pump data? IE is there a flow indicator?

Try and get that bypass valve fixed. It is too light and short circuiting the radiator flow. Especially as some of the rad valves are passing even when shut down. At lest these are being heated.

I recently have taken over servicing an old potterton BOA (1960s) with a 1.75USG/hr nozzle fitted. Even sooted. it was short cycling.
Reduced nozzle down to 1.5USG/hr, less short cycling, lower oil consumption too and a cleaner boiler, albeit inefficient.

In the longer term would you consider going down the wood chip or pellet route?

Philip R
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guydewdney
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« Reply #14 on: December 16, 2017, 09:38:21 PM »

Iíve looked up a quote for electric radiators we had done, and there were 30kw worth. We kept 5 of those of various sizes. Rest we used wet system. Plus existing of 22kw. Plus 10% losses? 30-5?=25    25+22+10%= 52kw or so. Itís running at 64kw iirc. I can get it modulated down to about 42 or 44kw minimum, below the published stats.

So at mid fifties, plus the dhw which is unknown as a load, doesnít seem toooooo far off?

Weirdly, if I run just dhw only, it behaves. As in the temp gets up to top temp, 75c. Plus no short cycling. Which implies that the issue isnít in the boiler room but in the house iyswim
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