Hi I have a similar situation to a previous post but did not want to jump on the back of that so here goes....

I have been trying to review my plumbing setup since last winter but need some advice on my Solar/UFH/ Oil boiler set up as it is difficult to get advice from the various contractors who installed my system.

My set up (as best that I can draw it) is attached where I have a buffer store supplying DHW and heating demands controlled by a single cylinder stat. This cyl stat fires the boiler frequently with DHW or heating demand when the temp drops below the preset 60deg. A picture of my buffer store is attached.

I know from reading various posts here that I need a 2 stat setup and plan to replace the analogue thermometer at top of tank with another cylinder pocket probe stat but have the following questions

1. For my plumbing setup e.g boiler flow/return tank positions on wiring diagram should I put a NC cyl stat down further to middle of tank to increase buffer volume? I assume the current stat I have is a NO type so I could replace the analogue thermometer at the top of the tank with this and then I would need to get a NC type for the bottom stat?
2. My solar guy is trying to get me to buy another solar controller to act as a differential controller but I believe I just need another cylinder stat yet he says this would not work as stats he has only have a temp differential of only 3deg? Assuming I can get a NC type cylinder pocket type stat what brand/model should I get which will work in my application? I see a lot of clip on stats on net but few pocket sensor types and local plumb centre guy is not familiar with any Normally Open /normally closed type stats?
3. I read here also about use of a relay in the wiring setup. Do I need this in optimising the set up?
4. Would I need a mixer on the return flow to boiler from the buffer store as if bottom stat is below the
boiler return then return temp could be too high/low for efficiency?

Appreciate any comments to above with info/diagrams I can go back to plumb centre and electrician with to finally get the set up I need.

bar

Re 2) I believe with a dual stat arrangement the stats shouldn't need any hysteresis/differential. Thats effectively provided by their seperation and setting.

Re 3) I think a latching relay is required. The stats are set up so that the top one calls for heat when it's cold BUT the boiler must keep running until the bottom stat is satisfied. The problem is the the top stat will quickly be satisfied and that mustn't turn the boiler off.

Hi, yes the twin stats are set to differenet temps can be anywhere between 10-20 deg at least. The absolute values do not matter so much except that the upper is set cooler ie a Normally Open contact closes on cooler water to fire the boiler, conversly the lower stat Normally Closed conatct opens on rising temp to switch the boiler off ie full of hot water. Alternativelt there are controllors that are the similar as solar differntial units to do this based on 2 temp probe inputs
Cheers Mike up North

I think I would build it without a mixer and see what return temperature you get. If it's too hot then add a mixer.

I think you need two normal stats (eg types that go OC when hot) and a latching relay... The top stat is normally hot so will normally be OC. When the top stat cools it will go SC and will turn the relay ON. The bottom stat is normally cold so will normally be SC. The relay stays ON until the bottom stat gets hot and goes OC. The bottom stat removes power to the relay turning it OFF...or something like that.

Hi Mike
the acaso system looks good but on contacting them they do not supply to public so not sure on budget price?

Think I may go for another probe stat same as i already have and get electrician to wire them up NC/NO with a relay?
Is there a wiring diagram for this setup available or is it standard stuff -excuse my ignorance

How easy is it to de-pressurise an unvented system? i.e to fit the new thermostat in the tank?

seems I may need to empty it as need to fit pocket :-(

came across the Honeywell L6188 imersion stat and thought this would do the job for top stat but it is a high limit stat. Would ideally like a low limit stat which closes on drop of temperature so not have to do relay setup?
Anyone know of any?

Hi,

two stats control the boiler on and off. Top stat = boiler on set to say 50 deg so boiler does not come on untill water ant less than 50deg is present at this top stat.
the lower stat is boiler off and is set to open at say 70deg so when the hot water has come down this fat the tank is full and the boiler is off.

cheers
mike up north

Hi, sorry for the quick answer - doing too much at once. I would check with grant about the boiler stat. I suspect it is switching at a lower than expected temp. Usually for most boilers the hottest setting is around 80-82 deg. Hence it should always be set to max and the store bottom stat at least a few degrees less but probably best at say 75 deg so it will always switch off on the store stat. These stats are not that accurate and have a couple of degrees of hysteresis so don’t get the settings too close. However, I suspect (anecdotally) that the max on the grant is 70 ish. I think this is so that they maintain condensing and meet the very high eff they always quote. Check with them what it is set at. It may be that the boiler stat is over ruling the store stats.
Cheers Mike up North

Hi well that sorted one point, you may just have a low stat just a bad un, can you do a sketch, and what tank is it? any diagrams?

cheers

mike up north

Hi,
Make sure the bottom stat is 5 deg less than the boiler max, so if it is only 62deg at the boiler try top =45, bottom = 55 deg to see what happens. But just as a test. With the grant set to 70 max the realistic top store temp is 65 deg.
The TMV into the boiler RAISES the temp to 45 deg so it must be less than this. The boiler then adds say 20deg and it comes out at 65 deg. You could run 55-75deg excepting the top limit on the grant and the dew point of oil is 47deg so running 45-65 will always ensure condensing. Try what grant suggested and defeat the boiler stat. This is a little dodgy as all control is with the store. If this works fit a pipe stat on the out let set to say 80+ and feed the boiler supply through it so you have some high temp protection.
However i still suspect the upper stat is overuling the lower stat and thus running till the boiler stat operates.

When the lower stat operates it removes the boiler supply.
When the upper stat closes to operate the boiler the lower stat must also be closed (other wise there would be nothing to open when the hot water comes down to it).

cheers
mike up north

Bit late to this thread but it's Interesting. My Grant will go upto 75-80C flow and I've not had to remove a limit spring. Perhaps this is a newish addition.

When yours shuts off at 76C what's the return temperature?

If you are getting a large temperature rise across the boiler (eg 40C return and 76C flow) then I'd have a look at the pump speed. Basically an oil burner has a fixed output power so the faster the water goes through the boiler the lower will be the temperature rise.

You shouldn't lower the return temperature below 40C. In fact most systems have a mixer to prevent the boiler return going below 40C. (aside: This mixer has come up twice on the forum thie week already. Funny how that happens).

I am now looking at plumbing a circluating loop pipe from bottom of tank to top controlled by a slow setting pump

Deliberately stiring the tank shouldn't be needed.

Is it possible the boiler is cycling so fast that the average power is too low to heat the store on a cold day. That's what happened to me first winter.

My boiler is the 36/46 now fitted with the smaller 1mm nozel so 36kW. The original pump supplied with my DPS store was a grundfos UPS 25-60-130 and I changed it to a UPS 25-55-180 which has about twice the flow of the original. Note this wasn't a drop in replacement - the last three digits are the width. The original was 130 and the new one 180mm.

I can't be sure pump speed is your issue but if you see a large temperature rise across the boiler when it's all at the peak of a cycle that would be my first suggestion.

PS What part of the country are you in? My UPS 25-60-130 is going spare. Low milage!

temp rise is 15-20 rise from return but as in other thread my TMV rises after a period upto 60°. Pump is -130 'width' with 3 settings, mine is on setting 3. Condenser Boiler power I believe is OK at 31kw as told better having boiler working for heat rise rather than too powerful.

By the way based in Eire!

Posted By: bartemp rise is 15-20 rise from return but as in other thread my TMV rises after a period upto 60°. Pump is -130 'width' with 3 settings, mine is on setting 3. Condenser Boiler power I believe is OK at 31kw as told better having boiler working for heat rise rather than too powerful.

Ok so probably not a slow pump. I'd try dropping the store temperatures by 5C to see if that drops the peak boiler return temperature and allows the peak boiler flow to remain below 76C

Hi
I got a small pump installed yesterday to help circulate the heat to bottom of store as heat not gettin to bottom of store and not helped with location of probe stats on oppisite side of tank in tank and now the boiler never cycles but can run for 1hr plus with a flow of 60-67deg as depending on demand for UFH etc the tank stats are set to 50 top 'on' and 60 bottom 'off'. After adjusting the boiler thermostat to increaseflow upto ~80 and the circulating pump the system is doing what is should via the differential solar controller program. The tmv valve hasn't climbed above 52deg since the circ pump installed. Pumps and boiler are all controlled by the tank stats.
Although the boiler is running in condensing mode practicly all the time do I need more power or is it normal for bad setup to have it run to 1-2hrs constant during peak demand times?
Have boiler installed 3yrs now Mike never considered Warmflow as Grant condenser was recomended to me back then.

Hi, me not why that is helping although the figures seem to be what would be expected.

Although that seems to be working it does seem to defeat logic. The original low temperature at the bottom suggests the layer of hot incoming water was not getting down that far (in order to switch off the boiler) hence the boilers own stat taking over on high temp. I think what you are doing in stirring it up is simply slowing things down. Its quite a high powered boiler so with a lowish flow the temp rise within the boiler would have been quite rapid forcing the boiler to cut out before the store water got that far down, or settled into the period where the hot/cold interface layer is progressing down the cylinder.
Looking at the diagram of the buffer the inner T in T does extend down quite a long way inside. Often they are fatter at the top with a much thinner section dropping down to near the bottom. The cold feed dip tube goes right the way down so you introduce the potable cold water in as low as possible. This help maintain the balance of hotter at the top and cooler at the bottom, also if you have a solar coil this would be wrapped around this lower section to place it close to the cold potable water.

I’m still interested to see where you have the flow ins and outs and the temp probe points relative to each other measuring up from the floor level.

Cheers

Mike up North