HRM Wallstar Oil Boiler - Advice please

[Simonvr46]

Never did get to the bottom of this issue.

Decided rather than spend/waste money on trying to fix it.

We’ve gone down the route of having a new boiler, I purchased Worcester Greenstar heatslave II through staff discount scheme we have where I work and then paid an accredited Worcester installer to install and commission boiler.

It’s been installed 11 days and no problems at all. Starts every time and we now have constant hot water.

 

[SJB060685]

Thanks for updating us. Im sorry to hear you made that decision, although it does suggest the problem was never fuel related and was within the burner, unless the installer rectified a proper when installing.
Not a fan of combi boilers personally but if I remember correctly there's just the two of you and they are decent boilers.

Again thanks for updating us.

Many thanks, Simon

 

[Simonvr46]

Thanks for updating us. Im sorry to hear you made that decision, although it does suggest the problem was never fuel related and was within the burner, unless the installer rectified a proper when installing.
Not a fan of combi boilers personally but if I remember correctly there's just the two of you and they are decent boilers.

Again thanks for updating us.

Many thanks, Simon

Hiya, only things that have changed regarding the fueling are that the boiler is now sat on the floor rather than it being in the wall so that the pump doesn’t have to lift the oil up and the fire valve has been replaced with new one instead of the HRM version with non return valve. The installer let me take off all the parts that had been replaced, which I thought was decent of him as he could have kept them and used them.

I think we will use less oil especially during the summer as both of us are at work and we have boiler set to only come on in morning and evening to give us hot water When we need it, instead of it being on all the time and keeping the heatslave tank at constant temp.
Thanks for all your help.

 

[John.g]

Thanks for updating us. Im sorry to hear you made that decision, although it does suggest the problem was never fuel related and was within the burner, unless the installer rectified a proper when installing.
Not a fan of combi boilers personally but if I remember correctly there's just the two of you and they are decent boilers.

Again thanks for updating us.

Many thanks, Simon

SJB, you may have a few comments to make re Stored combi boilers output and flow rates, there are numerous queries on here re gas combi flow rates as well.

I am looking at a brochure for a Firebird C35 combi boiler and to quote "D.H.W. Guide Performance in litres/min (120 litre draw-off at 40C deltaT) 20". It has a primary store of 40 litres heated to 78C.
Do you know if this guide performance is typical for oil fired combis? and also how is this number of 120 litres arrived at?
My simple calc shows that if one assumes cold water at 10C, then with a deltaT of 40C the stored cylinder will give a volume of 40*(78-10)/(50-10), 68 litres at 50C (which should give 20 LPM but only for 68/20, 3.4 mins, the burner should kick in at 73C and then the 35kw boiler will give a continuous flowrate of (35*860)/(40*60), 12.54 LPM.
My main query is this 120 litre figure?, the flow rates are simple to understand IMO.

 

[SimonG]

120 litres to fill a bath. They want to give the idea that you can fill a bath before the store gives out. It doesn't last though.

 

[SJB060685]

Its to do with the amount of stored energy in the thermal store. 120 litres is what can be provided at that delta until the store depletes.
Gas is different, they ramp up to full output and can modulate down for the lower load of CH.
Because up until recently domestic oil burners are fixed rate they have to use a store to aid in HW production.
If my understanding and maths is correct then 20 litres per minute at a delta t of 40°c would require an input of roughly 50kw but as said above if there was no thermal store and you had a nozzle size and pressure to input 50kw for HW then the CH side will also have the same input, obviously being massively oversized.
Perhaps when fully modulating oil burners take off there will be no need for a thermal store.
Thats my understanding, if I'm wrong then please correct me.

 

[John.g]

My calcs show that a 70 litre store would be required to give that 120 litres, the store is definitely required as you say because of the fixed boiler output and if you only needed a flow rate of 4 or 5 LPM then the boiler would be constantly cycling on/off. A continuous 20 LPM at a deltaT of 40 requires 56kw, the more normal quoted deltaT of 35C would require 49kw.

 

[SJB060685]

Can you break down your calculation method.
Bare in mind as the store temp decreases the burner will engage and supply additional energy, until a certain point where the input cant match the output.

 

[John.g]

OK, you are mixing cold water (at say 10C, but you can use your own number) with hot water at 78C (stored water temp) to give a mixed temp of 50C so every 1 litre of water at 78C will give 1*(78-10)/(50-10), 1.7 litres at 50C, so a store of 40 litres at 78C will give 68 litres at 50C. Additional energy will be supplied when the store reaches 73C but store will just about be depleted then and a 35kw boiler will then have a flow temp (to the store) at 20 LPM, of ((35*860)/(20*60))+10, 35C. The only way to get a sustained flow rate at 50C after the store is depleted is to reduce the flow rate to 12.54 LPM (35*860)/(60*(50-10).

 

[SJB060685]

How are you reaching your mixed temperature?

 

[John.g]

I presume the Firebird has a mixer to reduce the temp to ~ 65c and then the shower or whatever will reduce it to 50C but it doesn't matter as the stated conditions are water from a stored temp of 78 to water at 50C (from 10C) so the ratio is 1.7:1.

Thinking back again to the burner re firing at a store temp of 73C, if the probe picks this up very early in the draw off then you could be adding in energy early on so that will have the effect of increasing the thermal store so the 120 litres may actually be not too far off the mark as it would only need a additional ~ 2.5 kwh to achieve this but needs a bit more thought.

 

[SJB060685]

So have I done the maths correct.
Say we have a store of 50 litres with a temperature of 80°c and we want a delta t of 35°c and an incoming temperature of say 12°c
So 1 litre will provide 80 ÷ 35 = 2.28 litres, so 50 × 2.28 = 114 litres?

 

[John.g]

Not quite, the calc IMO is 1 litre will provide (80-12)/(47-12) or 1.942 litres @ 47C so 50x1.942= 97 litres. I will show the actual "long" calc when i return in ~ 2 hrs.

 

[SJB060685]

yes I slipped up. Your figure is correct.
Yes please show me later and also explain where you got this method from and how accurate it is mate please.

 

[John.g]

The following is the heat balance method, I just use a shortened (my own) version above but it is 100% accurate.

L Litres = volume of cold water that must be added to 1 litre of hot water to give you the required mixed water temperature.
So: (1*80)+(L*12) = (1+L)*47
80+12L = 47+47L
33 = 35L
L = 33/35 or 0.942 litres
So 1 litre of hot water at 80C will give 1.942 litres of mixed water at 47C from cold water at 12C.

 

[SJB060685]

Thanks John. Yet again you've provided me with an equation or method for calculating something I didnt know before.

 

[John.g]

I presume the Firebird has a mixer to reduce the temp to ~ 65c and then the shower or whatever will reduce it to 50C but it doesn't matter as the stated conditions are water from a stored temp of 78 to water at 50C (from 10C) so the ratio is 1.7:1.

Thinking back again to the burner re firing at a store temp of 73C, if the probe picks this up very early in the draw off then you could be adding in energy early on so that will have the effect of increasing the thermal store so the 120 litres may actually be not too far off the mark as it would only need a additional ~ 2.5 kwh to achieve this but needs a bit more thought.

Think I may have finally come up with a reasonable calc for the above but is based on the boiler firing up almost immediately. The 78C high temp flow rate from the store will only require a flow rate of 20/1.7, 11.76 LPM from the store until it has fully depleted its 40 litres, the 35 kw boiler will also be heating the water to almost 53C at this flowrate so by the time the 40 litres at 75C is depleted then the store will be full of water at 53C = 43 litres at 50C, so 68 litres at 50C after mixing from the 78C store plus 43 litres at 50C with a little mixing gives a total of at least 111 litres at 50C?.

 

[SJB060685]

I'm going to trust your maths and judgement and say you could be correct.
Most oil combis state a maximum draw off at 40°c delta t, so there will come a time I assume and as I said above where the HW output temperature will start declining until the store is recharged.

 

[John.g]

It certainly will fall off because if the demand is kept at 20 lpm then once the first store is drawn off then the second store flow rate will rise to 20 LPM (practically no mixing) and once this is depleted (at 50C) the 35 kw boiler will only give a steady temp of 35C (35*850/20/60)+10), the flowrate must be reduced to 12.54 LPM, (35*860/40/60), to give a steady temp of 50C. In other words a flowrate of 20 LPM @ 50C will only be maintained for 5.5 minutes, 111/20, and the temp will then fall to 35C if not throttled in., I suppose that really isn't too bad as it will (almost) fill a bath or give a super power shower of 20 LPM for over 5 minutes. The key to the increased (temporary) flow rate is the burner cutting in almost as soon as there is a flow demand.

 

[SJB060685]

When there is a demand at a HW outlet the flow switch and PCB do their thing and the burner and pump should engage straight away anyway.