HRM Wallstar Oil Boiler - Advice please

[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.

 

[John.g]

I think I saw somewhere that the Firebird store temp has a hysteresis of 5C so the burner may not cut in instantly on flow demand? Also one would think that the very nature of a store would allow a draw off of maybe 5 litres or so depending on where the probe is positioned. It would be interesting to get some feed from someone with a stored combi oil boiler.

 

[SJB060685]

All stats have a hysteresis but I couldn't tell you how wide it is on the thermistors or stats used. If you look at the flowchart for combi boilers you'll see a series of relays and stats and other components on PCB's. Of all my manuals stored show flow switches bringing on the boiler and pump when a hot outlet is opened. The store stat can also bring on the boiler to heat when no outlet is open to top up.
Another thing I will say is the newer firebird combis don't use a flow switch but instead another thermistor on the cold inletto PHX, which detects a change in temperature and brings the boiler on that way.