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Good evening Gentlemen (and ladies?).

I have a large holiday let, in rural Somerset, and commercial heating engineers are rare out here. I’m a design engineer by education, so have taken on the task of getting this system running better than it is at the moment.

Long story short, the heat isn’t going where I expect and want it to go.

System comprises of a Grant oil boiler - 58/70 feeding a LLH six feet away. The primary is pumped with a digital variable speed DAB pump. The secondaries are pumped / zoned with similar DAB pumps.

Surely, when the heat load is large, eg on startup, the LLH should almost ‘not be there’ from a heat point of view? IE the hot from the boiler flow (at 74C) should pretty much be the flow to the rads? Plus or minus a degree or two? Mines at 53...... which indicates unwanted circulation in the LLH probably from heating return.... but HR is 46..... and boiler return is 49....measured with installed thermocouples (my plumber wouldn’t know what a thermocouple is....)


Where the hell is the hot water going?

Boiler is pumping at 2M/hr and each of the two zones are pumping at 1m/hr

LLH is vertical. Yes, it’s definitely plumbed correctly WRT flows etc.

Pic to follow. P155 taking of the pipework is not allowed - it was done in a hurry in a listed building with asbestos preventing access to pipes etc. And we put in 14 bathrooms in 5 months (half into new spaces) at the same time. And CH....


Oh - and hello, my name is Guy and I’m not a plumber. ;-)
 
IMG_20181116_120847.jpg
 
Where's / is there non returns on the return pipework?
 
No non returns, there are zone valves

So there's zones on all 3 sets

If you can set up a thermo on the cylinder feeds

Flow and return
 
Yes, two are visible in the picture, white box to left of LLH, white box to right of multimeter thing (actually 4 zone temperature display). HW cylinder is tiny, its 150L or so, and has its own zone valve beside the cylinder (I’m sure, but will double check). It’s one of 7 cylinders in the bulding
 
Think you have a problem with gravity circulation either by a port valve not totally shut or no
 
What's open when heating up mode is on the diverter valve egvtc ?
 
The VTC500 is a temperature controlled mechanical ‘short circuit’ valve. This allows the heated water from the boiler when first firing up to short circuit back to the boiler, allowing it to heat up the mass of the boiler as fast as possible, a bit like a wood burner (which is where its usually used).

So - when the boiler first fires, on a cold start, the VTC is fully ‘open’ and nothing passes from the LLH to boiler return. All from boiler flow goes straight to return. ......... As flow to VTC approaches 50C it started opening valve from LLH regulating the return temp to 50 as best it can (until 100% of the water is from the LLH and none is short circuiting). After this return temp follows LLH return.
 
So on a ‘warm’ start, eg in the middle of a heating cycle, the VTC is mostly or completely irrelevant, as its fully closed to the boiler flow and fully open to LLH return (after a couple of minutes to be fair). It has temp gauges to show this
 
Yes that's because it's an oil boiler

That's one you said there's two what's the other one
 
Sorry, I’m not understanding.

2 x zone valves, 2 port, 28mm versions of domestic things you find on yer cylinder. Control flow to heating zones.

1 VTC.. reduces heat up time for boiler preventing rot. Grant recommended.
 
If you are drawing out more flow rate on the secondary side than you are supplying the LLH with the the primary pump you will get return water being drawn back up to the secondary flow.

Is the drop in temperature across all 3 secondary circuits ?
 
To be fair, I’m measuring just one of the flow to one of the zones. I have looked in the past and both flows were similar.

Both zone pumps are, as stated above, set to 1M3/hr and the boiler is set to 2M3/Hr - so the same in as out....
 
What is controlling the mixing valve that prevents the back end corrosion ?
When this is open the flow rate to the header will drop & there will be mixing in the header dropping the temp.
 
The VTC is mechanical, and internally controlled.

Yes, for the few minutes that its open things will go a bit haywire. But once up to temperature, whic is where I’m measuring, its shut and all the flow is going to / from the LLH from the boiler.
 
Hmmm - doesnt help that the thermocouples were lying.... 10C difference between TCs as I swapped them about agh! New ones ordered from ebay with rings on, Im going to clamp with jubilee clips onto the pipes.

Would putting a thermocouple on the end of a long wire that runs alongside mains wires (in a YY type control cable) work?
 
some explanation might be a plan here.... We had existing pipes running through an asbestos laden area, so we used these to get the water to a LLH to then allow the 3 zones (2 x heat 1 x dhw) to use both of the sets of pipes. Yes its crude, yes it isnt ideal, yes its a bodge, but its all I have. The plumber isnt joyful about clambering through there again.....

I was wondering if I should have a 'short circuit' deliberately on the end of the long runs - the house is a T shape, and each leg can be about 25M long. Or shuld the last radiator just be fixed open?

How do you stop muppets fiddling with lockshields? Ive tried to find a tamper / vandal resistant solution, that is removable for me to adjust, and it doesnt seem to exist - maybe it should?


Answering my own question - yes a thermocouple on the end of a long wire in a YY cable is OK - there may be a 2C difference, but that might be different measuring devices reporting different temperatures
 
Best Practice: Low Loss Headers

is a similar document you might like too. Thank you for your time, I will ensure the pump speeds are set in a cascade speed style.

My pumps:-
https://www.pumpsalesdirect.co.uk/media/wysiwyg/datasheets/evoplusdatasheet.pdf

I have 1 of the multifunction modules, which can be used on any of the pumps, Im not quite sure it will work the right way if I add a NTC thermistor to it - I cant quite get my head around the graph / flow / temperature, and the behaviour of a LLH.

in the mean time:-
The primary boiler pump should be set on constant speed mode (constant curve control mode?) and Ill adjust to say 3m^3 / hr (the boiler requires a minimum of 2)

The LLH-LLH pump should be set to 1.5M - again on constant curve?

The hybrid pump, the one that does the 2 pipe and 1 pipe and LLH set to constant pressure, and adjusted to about 1.2M^3/hr when all TRVs are open (75 litres per hour per radiator, rule of thumb?)

Then the two CH pumps from the upper LLH set to 1M^3/Hr on pressure mode?
 
Update...

Turns out the two LLH to LLH pipes were plumbed wrong - one pair went to the top one pair (F&R) went to the bottom - so one side looked like

F
R


F
R

instead it should be

F
F

R
R

lets hope this fixes it!
 

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