Setting up boiler for optimum efficiency

[rick]

A discussion in a separate thread has prompted me to look further at the set up of my gas boiler. I have a heating system designed & supplied by NuHeat ...... 17 zones, all underfloor heating - no rads.

There is a thermal store (Energymaster) and if the stat on the store (fitted on lower 1/3)is lower than 65C .. it requires heat .. zone valve opens on primaries, pump starts and 220V sent to boiler. Boiler runs until stat reaches 65C and opens.

The heating system us fed from a lower heat exchanger, and the DHW form an a upper heat exchanger via a thermostatic valve. There is no blending valve on loop circuit - design relies of store stat limiting temp. ( I intend fitting a blending valve - but that id not in current design)

Sketch of set up here:

Safety bypass and pressure vessels left out for clarity

The initial advice was to set boiler temp to Maxm .... but appreciate this may not be optimum .... I went back to NuHeat .. they advise # Boiler stat should be set to 75C ..... # Diff between boiler F& return should not exceed 13C

Any suggestions on how to set this up ....... I have only the pump speed and boiler temp setting to adjust.

Do I turn down boiler setting until it is at 75, then adjust pump for

13C difference ........ just not sure how to approach this.

[John Rumm]

It seems like you have a system optimised to get the best efficiency possible from a conventional non condensing non modulating boiler. i.e. Running the boiler at high output temperatures on high load in measured bursts. Traditional boilers were not good at trickling heat into a system at part load to match the heat loss of the building, and so the thermal store made a good buffer. It is also a simple way of using stratification to get a variety of output flow temperatures to suit different applications such as DHW and UFH which have different requirements.

So the first question is, what kind of boiler are you using?

[Martin Brown]

The simplest empirical way is to monitor gas usage and daily min/max outside temperature whilst making adjustments on a weekly basis. I doubt if the boiler stat gets much of a look in compared to the thermal store provided that the pump is actually running.

My instinct is that provided the pump is fast enough to avoid a >13C temperature drop on the loop the overall efficiency won't be all that much different. The only thing that might make a difference is if one of the burner controllers is smart enough to determine the system thermal inertia and loading to avoid overshooting the 65C target.

[rick]

Gas condensing - Baxi Barcelona set for 1000,000 Btu/h (sorry meant to add that in OP) it has been fully upgraded to 100HE spec

[rick]

Just a little unsure of what I'm trying to set .... NuHeat say 75C .... Comments I have read elsewhere state that I have to get return no higher than 55C to remain condensing ... that would be >13C difference

I can happily take temp readings - but not sure what I should be setting to.

[Clive Page]

Can anyone explain to me why it is desirable to avoid a temperature difference of more than 13C ? That means when it starts up from cold, with the inflow around say 15C that the outflow should be no more than

28C - which means the pump must send the same water around several times before the outflow gets to 75C?

I don't understand why a boiler with a flame at several hundred Celsius would be more efficient working like that than it it raised it right away from say 15 to 75C.

[John Rumm]

You have a conflict of interests here really... Thermal stores work well at high temperatures, and condensing boilers get bigger efficiency gains at lower temperatures.

Even at return temps above 54 degrees (the dew point of the flue gasses) the modern boiler will be more efficient than an older style one as a result of its larger heat exchanger. The efficiency graph just gets a bit steeper at that point.

With a condenser there is no real problem with a delta over 13 degrees - that used to be a limitation of older boilers where you wanted to prevent them from condensing (since there was no mechanism for collecting and disposing of the condensate, all it was going to do it accelerate corrosion in the boiler). Modern condensing systems are often run at 20 degree delta or more.

I expect your best situation is to have the boiler flow temperature set so that it can just satisfy the store stat in the coldest of conditions.

[rick]

Did have a follow up email form Nuheat .. they stated manual advice was based on old style boilers, not modern condensing .... but no other advice on setup.

[Andy Wade]

Like this (reposted):

The design seems mad to me. Surely for optimum efficiency you want the UFH running straight off the boiler primary, at the lowest feasible temperature. Weather compensation would help here, by reducing the temperature further in mild weather. Then use the store for DHW only with appropriate controls to heat it to ~65 deg. a couple of times a day, or as required, by running the boiler flat out into it for short periods (i.e. HW priority control).

[John Rumm]

Not mad - but possibly from a different age! Its the kind of setup that a previous regular of this group was fond of promoting. It made sense with old clunker boilers.

Yup I think that is the way I would design it if given the same choice of components (assuming the boiler can modulate low enough to run the UFH directly - you may need a blending valve or two in there).

The boiler would need to be capable of (or be fooled into) split temperature operation so that it could run an appropriate flow temp for heating, but then use a much higher temp for store recharge.

[John Rumm]

Its advice that applied to old non modulating non condensing boilers. If you designed for a delta of about 12 degrees, then you could be safe in the knowledge that even if you dropped the flow temperature to 65, you would still keep the return temperature above the point at which you would get condensation of the flue gasses in the boiler (once it was all up to temperature of course - you would still get some condensation at startup - but that water could be boiled off soon after). This at the time was a good thing since it stopped the boiler with its cast iron lump HX from rusting away.

With old boilers they just ran at a fixed output - the system balancing combined with the boiler's internal stat would set the parameters. So it would condense a bit at startup, and then run normally once up to temperature.

They don't have the power to lift that quantity of water to 75 in one pass.

The efficiency gain in the condenser comes from two factors. Firstly being able to extract more heat from the combustion in the first place (bigger more efficient HX), and secondly by condensing the water produced during the combustion process, you can claw back the energy tied up in the latent heat of vaporisation of the H2O manufactured when you burn a hydrocarbon fuel.

[rick]

A bit of pushing back at NuHeat and they admit the commissioning notes of setting to 75C and a 13c differential were actually based on old style non-condensing boilers.

Even though my system was specifically to be used with a condensing boiler. When asked what to set it to all they will advise is Boiler must be set higher than store stat temp ... anything else is up to me.

So back to question ... do I set the boiler to run the water at 75C, then what do I set the pump speed at.. on old system you would adjust pump speed to to get correct differential ... Or would I be better setting at lower temp ... boiler has option to limit flow temp to 63C by a control board jumper setting.

This is what it is currently set to ... and pump is on max speed.

(If any of you have knowledge of Baxi's .. I have posted separate Post on Baxi control board jumpers - welcome input)

[Clive Page]

Thanks. I hadn't thought about the efficiency of the condensing stage. I guess that is a large part of the explanation.

[Andy Wade]

Er, if the heat demand is less than the minimum modulated output of the boiler the boiler will just cycle on and off (PWM). If you force it not to do that it will be delivering excess energy which has to go somewhere. Either the house will overheat, or a room 'stat will perform the PWM function instead. Unless weather compensation is employed that will be the main control mechanism in any case.

[Re DHW:]

True. Combis do this innately of course if you replace "store recharge" with "provide hot water, but I dont know if there are any system boilers that have two demand inputs like that. There should be though...

[Andy Wade]

If efficiency is your aim I'd ignore that and set to the lowest boiler temperature that gives you sufficiently hot water and adequate space heating, with your system as-is. (Min. 60 deg. for HW to kill Legionella of course.)

Yes, try it and see,

... and pump is on max speed.

Temperature differential is generally not critical over quite a wide range. I'd aim for the lowest speed that gives adequate performance. Preferably keep the return temp down in the low 50s (or less) to keep the boiler in condensing mode.

[John Rumm]

It will indeed.

The purpose of the blending valve in this case is to control the maximum flow temperature into the UFH to prevent it being uncomfortably hot on the feet, or distorting floor coverings etc.

The boiler will still need to cycle to only deliver the power level actually needed.

There are a few - I spent some time researching when looking for a system boiler setup for my place. Some have it built in, others you can add it or get the same effect with the right control system. Anything that has weather compensation will normally include it.

Most of the Vaillant models seem to include separate temperature controls on the boiler itself for the DHW and CH - even on the basic 400 series "heat only" vented range.

The posher weather compensating controls allow for the CH flow temp to be slaved to the external temperature, modified via user selectable response curves to model the thermal properties of the building, and also to introduce an element of PID style control such that the current target temperature in the building will be taken into account (i.e. girl mode - if its well below the current set temperature it will turning it up more for faster warm up ;-). The DHW then runs at a separate preset maximum flow temp (with an optional weekly "anti legionella" cycle where it recharges the DHW at a higher temperature).

Many weather compensators seem to use a NTC thermistor as their temperature sensor. So a control system could obviously switch in additional resistors to trick the system into running a different flow temperature.

[John Rumm]

So long as the flow temp is higher than the set temp on the store you should be ok.

With your boiler I would not worry about differentials particularly. Just go for the lowest setting that does the job adequately (less system pump noise that way)

The lower temp will give better efficiency but possibly at a slight price of longer re-heat times on the store.

How is the store heated? (i.e. direct or indirect coil)

If its an indirect coil, then it might be worth checking what the maximum transfer rate for the coil is specced at. If its an older diesgn it may limit the maximum power to somewhat less than the boiler's full output anyway.

(stores used for system buffers like that are often "direct" - i.e. they are full of primary water heated by the boiler)

Nope, nothing specific, although I probably have collection of manuals for many of the older ones.

[rick]

Direct - whole contents of store circulated around boiler

2 heat exchangers - lower for UFH Upper for DHW (with Thermostatic mixing valve)

as per:

The tank is set to 60 degrees .. to limit max temp of water that can get to UFH circuits. My plan is to install a Thermostatic Mixing Valve set to 50C and run tank at higher temp (probably 70)

The store has a baffle - to help stratification, but as soon as you draw any hot water it energises the pimp on primaries to 'stir' the store and keep recharging heat exchanger.

[John Rumm]

ok, so it can take the full boiler output which makes it a bit simpler.

Yup saw that, was just not sure if there was no coil or you had omitted it for simplicity.

It would store more energy in the tank, at a slight cost in boiler efficiency (but probably only for the last few degrees of the reheat).

[rick]

Not easy trying to get information on how to best set up ........

GasSafe servce guys have no clue at all about Thermal stores, in fact couldn't even advise on how to set up jumpers on control board.

NuHeat who designed & sold system - admit their commissioning notes are based on non-condensing boilers - and when asked about what I should do for a condensing boiler - advise just make sure flow temp is higher than tank stat ... the greater the difference the quicker it will recover - but other than that they say to contact boiler manufacture.

Baxi reply with "with the store needing to maintain a constant store temperature of 60 degrees. You will need to follow the manufacturers guidance, as it is their system that requires these temperatures to run correctly, and unfortunately achieving a target 55 degree return temperature doesn't sound possible."

At the moment the boiler control board is set to 100,00BTU/h and 83C Turned down variable temp control (user front panel knob) to get 75C and reduced pump to speed II I'll try fitting a pair of thermometers and watch the F&R and see if I can reduce speed further. Maybe I can get a reasonable differential on F&R ... but Baxi don't think I will not get 55C