Repurpose gas combi for hot tub (analysis/shortlist stage)?

[NIMAN]

First post.

 

Background:

Retired so all projects are mostly/fully DIY on a relative shoestring.

Previously, little in the way of psychcrometric/hydraulics/heat loss calculation background (but been reading lately)

 

Full Project:

Renovation/flat roof extension to my 1958 three bed bungalow.

 

So, a year ago I decided to prepare for suspended wooden floor inter-joist insulation. Having removed all radiators and CH/DHW underfloor plumbing pipes I am left with a Gas Combi Vaillant Ecotec Pro 28 (2011 vintage) hanging on a wall connected to a single radiator (flow/return via attic void and rf thermostat sat on top of the radiator so that 5C frost protection doesn't switch boiler on too much) and single hot tap DHW connection in bathroom. To stay warm over Winter I built a little internal room insulated on all faces with 0.25U insulation and space heated with a little thermostatically controlled electric heater. Pretty happy at 500kWh in 12 months to stay cushty (thermostat at 21C continuously).

 

I recently stumbled upon Air to Air Heat pumps - i've pretty well decided to use that for future space heating and keep the gas combi for DHW & A.N.other (maybe).

 

Mini Project:

As per title.

 

Having had a "Jacuzzi" type bath many years ago (ultimately very seldom used as anything other than a bath) I was appalled when the missus hinted at her wish for an outdoor hot tub - I immediately said "no bubbles though".

 

Anyway, she seems serious so it's less than worth it for me to ignore the "simple outside bath" idea. To plan for that (and the A2A space heatpump/s) i've started to build a 30 year hourly weather database (1994 to present at 1000 records/day) to aid with heat loss calculations/insulation/heating strategies (including possible setback/zone analysis). Got to admit that the engineering toolbox equation I came across for evaporation currently has no tangible meaning for me other than for the loss calculation purpose.

 

Anyway, if the old gas combi was to be used as a hot tub heater I need to make some mention of the layout/services in my Building Control Full Plan Application (I've pretty well completed the drawing part of that now.

 

Problem is, the more I read about pump diagrams and heat exchangers the more confused I get so any comment would be appreciated in attempt to straighten the theory in my head a bit.

 

The initial modelling will be based on a 1 metre cube (1000 litres) of water with all 6 faces having 0.125U insulation at all times other than a 3 hour open top on 2 weekend evenings per week (all year). It'd be a big cost but i'd probably wish to refill every week ... spreadsheet and programming analysis are not imagined a problem for any of this. 

 

Please also find generic gas combi functional block diagram image attached - I don't imagine mine is much different.  

 

With a Spaghetti Western level of wonderment/questions here's a start...

 

Question(s) (these relate to initial transient heating, as quickly as possible, of hot tub water from cold to 40C)  - there could be a lot of "impossible" / "it depends" / "needs clarification" associated with this:

 

Comparing 3 cases...

 

a/ Using combi boiler primary gas to water heat exchanger direct connection to hot tub

Combi CH flow connected directly to highest point of hot tub water, Combi CH return connected directly to lowest point (to aid condensing) of hot tub water (for now, please imagine pipe connection friction losses as zero).  

 

The combi instruction manual says "Your appliance has a pressure sensor - if it falls below 0.3 bar the appliance switches off":

 

(i) If the water at the top of the hot tub is 3 metres above the level of the combi pressure sensor should the appliance stay on i.e. now a vented system?

 

The combi installation manual says "Circulation water volume  (delta T = 20K) = 1032l/min" ... at 4200 J/kg°C SHC this is 24.08 kW:

 

(ii) If the water at the top and bottom of the hot tub were both at, say, 5C when heating began could I expect, at least, a 24.08kW transfer rate when the hot tub thermostat initially called for heat?.

 

(iii) Can I assume the Gas combi will be in condensing mode throughout this heating period because the  CH return will always be less than or equal to 40C? 

 

b/ Using combi boiler primary gas to water exchanger connection to external hot tub water to water heat exchanger

Combi CH flow / CH return connected to the primary of a newly purchased water to water heat exchanger (the secondary of this new item being supplied with pumped pool water. For discussion, please imagine  the thermodynamic properties of this new heat exchanger are exactly the same as those of the gas combi existing inbuilt water-to-water (DHW) heat exchanger  (for now, please also imagine pipe connection friction losses as zero).

 

The gas combi instruction manual says "DHW flow rate delta T = 35K rise = 11.5 l/min" ... at 4200 J/kg°C SHC this is 28.175kW:

 

(i) if the hot tub circulation pump pushed 11.5litres through the new heat exchanger's secondary in 1 minute would the hot tub have gained 28.175x60 kJ?

(ii) Since heat transfer is directly proportional to temp difference then, In each subsequent minute, the heat transfer rate would decrease from this 28.175kW as the pool gradually heats? 

(iii) Can the state of the return flow temp,in this case, be easily determined to be above/below condensing level? 

 

c/ Using combi boiler internal DHW Hot Outlet to fill hot tub via open hose (i.e. without recirculation)

 

 

repeating from b/ above:

 

The gas combi instruction manual says "DHW flow rate delta T = 35K rise = 11.5 l/min" ... at 4200 J/kg°C SHC this is 28.175kW:

 

If mains water was supplied at constant 5C at 11.5l/min it would take  1000/11.5 = 87 min = 1.45 hours to fill the tub.

 

(i) Assuming zero heat loss this would require 28.175 x 1.45 = 40.85kWh of gas - is that correct? 

(ii) Again, can the state of the return flow temp be easily determined to be above/below condensing level for DHW operation? 

 

My current observation, with low level confidence is that, for initial heating only, DHW using a hose could be the quickest fill of the 3 options.

 

Must get back to those crazy pump diagrams now - maybe a beer or two first could help straighten them out.

 

Apologies for the long post and hope I haven't started out from a parallel Universe sometimeplace!!      

 

 

 

  

 

 

 

  

 

      

 

[JohnMo]

5 hours ago, NIMAN said:

i've started to build a 30 year hourly weather database (1994 to present at 1000 records/day) to aid with heat loss calculations/insulation/heating strategies (including possible setback/zone analysis).

Think you are over thinking things.

 

5 hours ago, NIMAN said:

Anyway, if the old gas combi was to be used as a hot tub heater I need to make some mention of the layout/services in my Building Control Full Plan Application (I've pretty well completed the drawing part of that now.

Why do you need to tell BC anything about the hot tub? Only inform them on what they need to know - no more, no less.

 

Your post was too long, you are over thinking a lot.

 

Your combi needs to be in an unvented circuit, so it gets a permission to start.  Get yourself a plate heat exchanger - cheap enough on eBay.

 

Plumb your primary circuit to the primary side of the PHE and the hot tub to secondary side with a circulation pump. Use a thermostat and timer on the hot tub to heat as and when you want. Set the heating side flow temp to about 40 to 45 so you don't boil yourself. But a return water pipe stat on as well set to protect you should things get a little hot without you knowing.

 

 

[NIMAN]

20 minutes ago, JohnMo said:

Think you are over thinking things.

Even at 0.125U on the tub and considering convection losses only a setback strategy  versus constant 40C heating over a period of two weeks using hourly data in a spreadsheet model has identified several kWh savings - what bit do you think is overthought?

 

23 minutes ago, JohnMo said:

Why do you need to tell BC anything about the hot tub? Only inform them on what they need to know - no more, nI want o less.

 

 

You might not need to but I do (as in "want to") - have always had a better result when discussing / disclosing information to BC so as not to trip and/or impress them so they never even look at a "borderline" issue - my point was relatively irrelevant though.

 

27 minutes ago, JohnMo said:

Your post was too long, you are over thinking a lot.

 

You already said a half of that.

 

29 minutes ago, JohnMo said:

Your post was too long

 

Patience is a virtue and knowledge is both interesting and useful

 

30 minutes ago, JohnMo said:

Your combi needs to be in an unvented circuit, so it gets a permission to start.

 

 

Do you mean legally or actually?

 

If legally, can you please refer to a BS or Building Control Statutory Instrument?

 

If actually, can you describe why a 3m static head won't permit that?

 

34 minutes ago, JohnMo said:

Get yourself a plate heat exchanger - cheap enough on eBay.

 

 

I already have two and not sure I need another - if I do I want it/them (pumps etc) to be thoughtfully sized to do the job as quickly/efficiently as a tradeoff might allow.

 

38 minutes ago, JohnMo said:

Use a thermostat and timer on the hot tub to heat as and when you want. Set the heating side flow temp to about 40 to 45 so you don't boil yourself. But a return water pipe stat on as well set to protect you should things get a little hot without you knowing.

 

 

As I mentioned, the question was about initial transient heating.

 

But now you mention it I have briefly thought about reheat during the period when the cover is off by latching heating to off until tub drops by 2C before calling for heat again and transferring the required 2.3kWh back in quickly to minimize pump waste/cycling or whatever else.

 

When the tub thermostat stops calling for heat at 40C why do you imagine I need to be concerned about more than 40 to 45 on the combi side flow boiling us?

 

I hope you didn't waste your time reading it all and thereby becoming bored.

 

 Anyway, thanks for your comment and would appreciate more since the more that I know the more I realise how much I don't know (as Mr Rumsfeld might have once said).

[NIMAN]

Just now, NIMAN said:

Even at 0.125U on the tub and considering convection conduction losses only a setback strategy  versus constant 40C heating over a period of two weeks using hourly data in a spreadsheet model has identified several kWh savings - what bit do you think is overthought?

 

 

Oops, error corrected in green above.

[JohnMo]

3 hours ago, NIMAN said:

hope you didn't waste your time reading it all and thereby becoming bored.

I did

[NIMAN]

42 minutes ago, JohnMo said:

I did

 

 

Congratulations.

 

Most people are a waste of spacetime.

[Nickfromwales]

4 hours ago, NIMAN said:

You might not need to but I do (as in "want to") - have always had a better result when discussing / disclosing information to BC so as not to trip and/or impress them so they never even look at a "borderline" issue

They will just shrug their shoulders, and look at you blankly, at best. At worst they'll just tell you it's absolutely nothing whatsoever to do with BRegs and will simply dismiss you.

 

For completeness, you and your questions / data will fly way over their heads / knowledge sets, so you are completely wasting your time engaging with your BCO for this.

 

Apologies if the replies seem blunt, but you are making the most humungous mountain out of a molehill, and 30 years of data is a 1/3 of your lifetime!

 

4 hours ago, NIMAN said:

When the tub thermostat stops calling for heat at 40C why do you imagine I need to be concerned about more than 40 to 45 on the combi side flow boiling us?

Because with a series of unfortunate failures here, you have a scald risk. Those that don't know, don't know, and they are the dangerous ones. A little like those who argue that doing your own gas work is legal, which on a dark day may well be the case, but ONLY if you are the sole occupant of the house and you have the only key to get in to it.

 

5 hours ago, NIMAN said:

a/ Using combi boiler primary gas to water heat exchanger direct connection to hot tub

Combi CH flow connected directly to highest point of hot tub water, Combi CH return connected directly to lowest point (to aid condensing) of hot tub water (for now, please imagine pipe connection friction losses as zero).  

Absolutely do not do this, it is dangerous as feck.

 

5 hours ago, NIMAN said:

b/ Using combi boiler primary gas to water exchanger connection to external hot tub water to water heat exchanger

Combi CH flow / CH return connected to the primary of a newly purchased water to water heat exchanger (the secondary of this new item being supplied with pumped pool water. For discussion, please imagine  the thermodynamic properties of this new heat exchanger are exactly the same as those of the gas combi existing inbuilt water-to-water (DHW) heat exchanger  (for now, please also imagine pipe connection friction losses as zero).

Possible, but expensive and complex.

 

5 hours ago, NIMAN said:

c/ Using combi boiler internal DHW Hot Outlet to fill hot tub via open hose (i.e. without recirculation)

The easiest, as it's what I do to top up the kids outdoor paddling pool to take the edge off the cold water it gets filled with after cleaning.

However, this is done via a thermostatic mixing valve to prevent scalding water getting to the hose. Any instantaneous water heater carries a serious risk of delivering scalding water.

 

OK, time for a reality check methinks!!!!

 

Just go and buy a simple, cheap A2W pool / hot tub heat pump LINK and end this madness. Great CoP and you can do away with the relic of a boiler you are gambling all this cost and complexity on, as it will fail shortly and then you'll be kicking yourself.

[Nickfromwales]

Also the boiler won't be able to cope with the short-cycling of supplying heat via the heat ex and this will further expedite it's demise.  

[Nickfromwales]

1 hour ago, JohnMo said:

I did

 

Maybe, possibly, offering up some slightly more productive comments would help, or perhaps no comments in the absence of? This is a new member and they need to acclimatise a little before we bite their arse, lol. 

 

@NIMAN, you'll get used to the folk here, just we've seen it all, heard it all, and the answers are just frank and honest, but are largely based on fact and knowledge sharing. Just slip your big-boy pantaloons on and take it as it's meant; just 'to the point' sometimes but not 'nasty'.

[Nickfromwales]

Cheaper again :-

 

LINK

[Temp]

A.

i) Not sure.

ii) I would expect it to deliver close to 28kW. Even when the return is 40 and the flow 60C. That's also a 20C uplift. But I think you should check the data. I might be wrong.

iii) yes.

Corrosion inhibitor in the tub water?

 

B.

i) Google suggests heat exchangers are about 90% efficient so you won't quite get 28kW out if you put 28kW into a heat exchanger.

ii) I looked at the 30kW heat exchanger here...

https://heatpumps4pools.com/myfiles/file/Elecro-G2-heat-exchanger-Leaflet.pdf

It has a table for the power transfer vs delta T..

 

If your boiler is 60C and the tub at 40C the delta is 20C. The data for that exchanger says the power transfer is in the range 11kW to 15kW. 

If your boiler is 60C and the tub at 20C the delta is 40C. The data for that exchanger says the power transfer is in the range 20kW to 26kW.

Perhaps use a bigger heat exchanger?

iii) I think once the tub is upto 40C the boiler return will be too hot for condensing. 

 

C.

If we assumed the heat exchanger power transfer was a constant  15kW thats 15kJ per second.

To raise the temperature from 5C to 40C requires 35*1000*4200 = 147MJ.

That would take 147*10^6/15*10^3 = 9800 seconds or about 2h 45 min.

 

147MJ is about 40.8 KWh (units) not including losses due to efficiency of boiler and heat exchanger, pipes etc.

 

I'm not a boiler engineer!  This is just physics to me.

 

Edited May 9 by Temp

[NIMAN]

13 minutes ago, Nickfromwales said:

Apologies if the replies seem blunt, but you are making the most humungous mountain out of a molehill, and 30 years of data is a 1/3 of your lifetime!

 

 

I previously said this:

 

4 hours ago, NIMAN said:

You might not need to but I do (as in "want to") - have always had a better result when discussing / disclosing information to BC so as not to trip and/or impress them so they never even look at a "borderline" issue - my point was relatively irrelevant though.

 

As you say, the humongous mountain is, in reality, a molehill (my fault for first mention of - please forget it).

 

19 minutes ago, Nickfromwales said:

30 years of data is a 1/3 of your lifetime!

 

An automated script to add 1000 records (6 week days) to a database takes milliseconds to run and zero amount of my time. Surely, hourly heat loss calculations and boiler / heat pump sizing based on median / mean hourly over 30 years is more sensible than those for an, idiosyncratic?, 1 year period.

 

 

27 minutes ago, Nickfromwales said:

Because with a series of unfortunate failures here, you have a scald risk. Those that don't know, don't know, and they are the dangerous ones.

 

Appreciate that and enjoyed the Rumsfeld like comment.

 

For the (very) unfamiliar with all the subject matter (me) it seems possible the failure(s) could occur (equally?) on the gas to water flow temp side - could the risk not be managed by precautions (or a preponderence of those) on the hot tub side? 

 

38 minutes ago, Nickfromwales said:

Absolutely do not do this, it is dangerous as feck.

 

Appreciate this again. That's what my original post referred to "impossible". For the unfamiliar can you, again,  please explain Why?

 

44 minutes ago, Nickfromwales said:

Possible, but expensive and complex.

 

OK - i'll consider that more before further comment.

 

46 minutes ago, Nickfromwales said:

Any instantaneous water heater carries a serious risk of delivering scalding water.

 

My comment is directed at initial filling without occupancy but understand your concern - thanks.

 

49 minutes ago, Nickfromwales said:

Just go and buy a simple, cheap A2W pool / hot tub heat pump LINK and end this madness.

 

I've briefly considered A2W and, for use case, do not, currently imagine it is suitable.

 

I wish to consider very fast heat up times (or as quick as possible from weekly empty throughout the year for convenience) and don't, currently, plan to have a large diameter electric cable feeding a big enough A2W heatpump in the vicinity to do (electrically / Capital outlay) that job (the COP of that unit will be relatively awful cf gas in bleak mid winter into the bargain).

 

52 minutes ago, Nickfromwales said:

Also the boiler won't be able to cope with the short-cycling of supplying heat via the heat ex and this will further expedite it's demise.  

 

For 40C maintenance, with cover off,  I'd plan to run the gas combi flat out (probably making set flow = 85Cs) over a short period to raise from 38 to 40C as described in a previous comment - the aims of my analysis are to avoid short cycling and always be condensing so not at all sure what you mean here.

 

The combi would have to run flat (no cycling) for  nearly 10mins at, say, 24 kWh to replace the 2.3kWh lost by 2C cooling - I can't imagine demise by that any time soon - can you?

 

1 hour ago, Nickfromwales said:

@NIMAN, you'll get used to the folk here, just we've seen it all, heard it all, and the answers are just frank and honest, but are largely based on fact and knowledge sharing. Just slip your big-boy pantaloons on and take it as it's meant; just 'to the point' sometimes but not 'nasty'.

 

No problem at all here Nick. I'm just searching for facts based in Physics/Chemistry & Engineering (not so much rules of thumb or that should do nicely).

 

I've done all of 3 those a bit but am trying to get better before I can't ☺️

 

[NIMAN]

1 hour ago, NIMAN said:

The combi would have to run flat (no cycling) for  nearly 10mins at, say, 24 kWh to replace the 2.3kWh lost by 2C cooling - I can't imagine demise by that any time soon - can you?

 

Oops again, error corrected inline above ☹️ 

[NIMAN]

9 hours ago, Temp said:

Corrosion inhibitor in the tub water?

 

Not sure whether that or skin soup is worse ☹️

 

9 hours ago, Temp said:

ii) I looked at the 30kW heat exchanger here...

https://heatpumps4pools.com/myfiles/file/Elecro-G2-heat-exchanger-Leaflet.pdf

It has a table for the power transfer vs delta T..

...[]...

Perhaps use a bigger heat exchanger?

 

Yes - that is definitely the sort of thing that (I imagine) I need to contemplate - thanks for the link.

 

9 hours ago, Temp said:

C.

If we assumed the heat exchanger power transfer was a constant  15kW thats 15kJ per second.

To raise the temperature from 5C to 40C requires 35*1000*4200 = 147MJ.

That would take 147*10^6/15*10^3 = 9800 seconds or about 2h 45 min.

 

147MJ is about 40.8 KWh (units) not including losses due to efficiency of boiler and heat exchanger, pipes etc.

 

I'm not a boiler engineer!  This is just physics to me.

 

 

Could it not be more reasonable to think that the instruction manual delta T 35K RISE AT 11.5 l/min is the max transfer rate (instuction manual Mad DHW output 28kW) from the gas to water exchanger into the water to water exchanger?

 

i.e. to achieve this the gas flame is set at max, all available 28kW is sucked out of the water to water heat exchanger leaving nearly 5C as return water temp to gas to water exchanger but that then supplying output at, or near, max 85C flow to the water to water input (that's all imagination since I haven't considered the combi pump flow at all yet).   

 

On that basis seconds to heat the metre cubed is  147 / (28/1000) = 5250 (1hr 27.5 min).

 

If there is any truth in this it would probably mean that most gas combi's in UK are in condensing mode during DHW most/all of the year?   

 

Hmm, having said that the combi doesn't know the mains water temp. If the DHW setpoint is 40C then is it more likely that the the flame is always modulated to, er, "some" level ? 

[Temp]

19 minutes ago, NIMAN said:

Could it not be more reasonable to think that the instruction manual delta T 35K RISE AT 11.5 l/min is the max transfer rate (instuction manual Mad DHW output 28kW) from the gas to water exchanger into the water to water exchanger?

 

i.e. to achieve this the gas flame is set at max, all available 28kW is sucked out of the water to water heat exchanger leaving nearly 5C as return water temp to gas to water exchanger but that then supplying output at, or near, max 85C flow to the water to water input (that's all imagination since I haven't considered the combi pump flow at all yet).   

 

Yes you can run the boiler flow temperature hotter than 60C.

 

Perhaps test carefully to ensure the system cannot send a pulse of very hot water into the tub when it starts up?

 

 

Edited May 10 by Temp

[NIMAN]

On 10/05/2024 at 00:19, Temp said:

B.

i) Google suggests heat exchangers are about 90% efficient so you won't quite get 28kW out if you put 28kW into a heat exchanger.

ii) I looked at the 30kW heat exchanger here...

https://heatpumps4pools.com/myfiles/file/Elecro-G2-heat-exchanger-Leaflet.pdf

It has a table for the power transfer vs delta T..

 

If your boiler is 60C and the tub at 40C the delta is 20C. The data for that exchanger says the power transfer is in the range 11kW to 15kW. 

If your boiler is 60C and the tub at 20C the delta is 40C. The data for that exchanger says the power transfer is in the range 20kW to 26kW.

Perhaps use a bigger heat exchanger?

iii) I think once the tub is upto 40C the boiler return will be too hot for condensing.

 

Looking at this again, just to be sure I have it straight / not:

 

The gas-combi instruction manual includes, for example "CH heat output range 9.0 - 24.0 kW"

 

For info, on the off-chance it matters - simple thermostat on/off control was used in the combi boiler while it was hiustorically used for space heating (no bus modulation device was ever employed).

 

For case B, is it/could it be correct to imagine that either of the following apply:

 

a/ as the combi return thermistor registers a gradually increasing temperature it then modulates (in combination with the flow thermistor reading) the flame in order to keep the flow temp to set/target value (i.e. load matching)?

b/  the above at a/ is not the case  -  rather, some Vaillant predefined return temp causes the flame to extinguish entirely and the pump to continue until some "other" condition is met at which point reignition occurs (i.e. on/off at max power output)?

 

I'm tentatively assuming that the boilers internal single stage pump supplies a constant flow.

 

Example repercussions:

 

If a/ is true and the boiler (at target 60C) automatically modulates down to meet the Electro-G2 (as per link) load then, for example the hot tub open top/occupied reheat time (from 38 to 40C) will take, rather than nearly 5.75 mins at, say, 24 kW to replace the 2.3kWh will take, say, 2.3kWh/13kW = 10.61 mins?

 

If b/ is true on heating the entire 1000 litres from cold once its temp reaches 30C (delta T 30) the Electro-G2 max dissipation to hot tub falls to 20kW and the boiler will continuously cycle on/off all the way to tub thermostat stopping call for heat at 40C (since the boiler is still trying to produce mismatched 24kW)?

[Temp]

Not sure I follow all that but my understanding is the boiler modulates down to keep the flow temperature below some set limit.

 

They can't modulate down to zero. If they reach the minimum flame power they start cycling on/off.

[NIMAN]

On 15/05/2024 at 15:55, Temp said:

Not sure I follow all that

 

Reading my comment again I know what you mean - neither do I entirely !!

 

On 15/05/2024 at 15:55, Temp said:

my understanding is the boiler modulates down to keep the flow temperature below some set limit.

 

The differential equations associated with transient / dynamic simulation of a water to water heat exchanger look like they will take a while to understand. I also imagine, in my case, that they will need an iterative (discrete) Euler approximation to solve  so spreadsheeting / programming that may take a while and be computationally intense (this is an old Win 7 laptop)

 

In the meantime, therefore, please let me get back to basics to see if I do have some simple, but real, understanding
 of how modulation works or is likely to.

 

To make things as, conceptually, simple as possible I hope the following are reasonable assumptions:

 

a/ let the combi CH flow/return be directly connected to the hot tub (i.e. no new water to water heat exchanger involvement)

b/ assume hot tub  is infinitely insulated (i.e. Zero loss)
c/ assume the hot tub is always perfectly mixed (zero temperature gradient throughout)
d/ assume combi internal combustion chamber pipe and hot tub connecting pipe volume is negligible
e/ 1m3 mains tap water is used to entirely fill hot tub at 5C

 

In an attempt to identify the maximum combustion chamber to water instantaneous available power - the combi installation manual states:

"CH heat output range 40C flow / 30C return 9.8 to 26.0 kW"

Therefore, assumption is that maximum flame /fan can generate 26 kW

 

The integrated pump is single speed (apparently of the non-modulating variety)- the installation manual states:

"Circulation water volume (delta T =20K)" = 1032 l/hour

Analysis based on above (simple as possible line by line):
 

4200    J/kg°C   SPECIFIC HEAT WATER (assume at all temperatures)

1 lite water = 1kg (assume at all temperatures)

1032    l/h    =    0.287    litre/sec    Circulation water volume             
60 C     manually adjust boiler set-point flow temp to this

 

The available 26kW (assuming max flame/fan) provides an uplift to a first 0.287 litres of CH flow of:                                      26000/(4200 x 0.287) =            21.570 K       This first  0.287 litres enters the hot tub at (21.570 + 5) = 26.570C and is instantly mixed to raise its temperature from 5C by:                     26000/(4200 x 1000) =            0.00619 K       At the start of the 2nd second the CH return is therefore 5.00619C                                 The boiler flame / fan is maintained at max (modulation logic only reduces these if flow equal or greater to 60C),   A further 26kJ is added, to this subsequent 0.287 litre, over the following second  and enters the hot tub at:   21.570 + 5.00619 =         26.576 C    (measured at CH flow thermistor, so flame/fan kept at max)   The additional 26kJ is instantly mixed and the hot tub  temperature becomes                                                      5.00619 + 0.00619 =        5.01238C                      This is the CH return temp at start of third second                                          i.e. CH flow temp rises, linearly, by 0.00619K every second until Hot Tub thermostat call for heat ends at 40C. In this entire heating period the flame/fan are kept at max as the CH flow thermistor never rises to 60C.                                          The hot tub requires a 35K uplift to attain it's thermostat 40C cut off. This will take                                  35/0.00619 = 5654.28 sec    (1.57 hours)   Is that all pretty much correct (or fantasy)?   If, instead, the boiler set-point flow temp had been initially set to 30C (hot tub thermostat still calling for heat until 40C) then what does the combi modulating logic do:   e.g. full flame/fan until CH Flow thermistor sees 30C then works it's flame down quickly to lowest output power but then sees a subsequent CH Flow thermistor reading above 30C and then shuts the flame/fan off but keeps the pump going in expectation of a future CH Flow thermistor reading below 30C in which case it would immediately fire again at max fan/flame or have some memory logic to try the lowest fan/flame first (or something)?     Another thought (with CH Flow at set point 60C) - If the gas flame is around 1960C, am I correct to imagine the transfer rate drop slightly from 26kW as the CH return linearly increases from 5 to 40C?