Boiler efficiency

[Tim Lamb]

With the current interest in energy prices, would this be an appropriate time to discuss boiler settings?

I think we all understand the linked concepts of building insulation, ventilation and thermostat settings but what impact do actual boiler settings have on overall consumption?

For example, modern condensing system boiler supplying thermal store and underfloor piped system. Boiler max temperature has to exceed that required for hot tank safety.

60deg. C plus feed pipe losses plus a consideration for thermal transfer from the heating coil.

The underfloor heating requires a maximum of 50 deg. C regulated by valves at the manifolds.

In my case, with 24 hour occupation, the boiler feeds the manifolds when it is not topping up the thermal store.

As a consequence it runs almost continuously on the lowest flame setting at 70C+. When the tuits are gathered, I intend to fit *weather compensation* (exterior thermistor) which should allow lower boiler settings for room heating.

How critical is this and is there any benefit in reducing the *heating on* period to load the boiler more heavily?

[Martin Brown]

Probably not.

As a general rule of thumb running the boiler harder will require a greater volume of air and fuel flow and speed up the exhaust gasses which makes it harder to condense the same fraction of residual energy.

Dropping it to 65+C operating temperature might be worthwhile though and that would probably make the burns more intermittent.

Some of the smart thermostats learn to anticipate the thermal inertia of your house so that they can hold the temperature closer to your ideal daily set profile with minimum use of fuel. The boiler thermostat will always overshoot somewhat since there is considerable thermal energy stored in its heat exchanger when it reaches its off point.

[John Rumm]

While it needs to exceed a sterilising temp, it does not need to do it all the time. A lift to that temp once a week would be adequate. Some boilers offer that option in their controls.

(I run my water reheat to 60 most days, and then to 70 one day a week. The heating will often run on flow temperatures under 50)

Yup, as long as it can support split temperature operation (i.e. that it can know what it is re-heating, and choose an appropriate temperature), you can get better performance that way (and generally more comfort and quieter operation when feeding rads directly.

Modern modulating boilers tend to make some of the older rules of thumb less relevant. There was a time that part load, and short cycling were "bad". Now they should load match and try to keep condensing efficiency high regardless. They all have pump overrun and low water content HEX etc.

[Tim+]

With a thermal store I thought tap/bath/shower water was only heated via a low volume heat-exchanging coil. If this is the case, I would have though that there wouldn?t be any stagnating hot water to worry about.

I could be wrong though. Is a heat store the same as a heat bank?

Tim

[Harry Bloomfield Esq]

Yes, which is why they have two separate settings for boiler temperature, one for when meeting heating demand, the other for when meeting stored hot water demand, plus can only supply one or other at one time.

I have fitted weather compensation and predictive demand for room temperature needs, well worth while too. The boiler then knows exactly how much heat to produce, to exactly match the requirements, rather than simple on/off, need heat/no heat needed of conventional systems.

Room temperatures are maintained precisely, with no over run, pipes don't creak as the cycle heat on and off, just a steady flow of heat from the better able to accurately modulate, boiler.

[Tim Lamb]

Actually thinking more clearly on this, thermal store, so hot supplies are always direct from mains and no storage or air contact. Biggish store and only 2 geriatric users (cold fill dishwasher and Wm/c) might allow knocking back the hot water store setting!

Indeed. The boiler knows whether the load is HW or heating.

Any rule of thumb pointers to best efficiency? There seem to be conflicting considerations. Low flame temperature=good, long periods of operation=bad. There must be a sweet spot somewhere:-)

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[Tim Lamb]

Indeed. Well spotted. The header tank is only concerned with providing space for expansion of the thermal store water.

[Tim Lamb]

Spotted elsewhere is that 60+ for safety is not required. More a matter of total hot water availability.

I feel a tuit approaching:-) Means drilling a hole through the cavity wall, carefully avoiding numerous pipes and gas supplies!

With 10 individual room stats and 2 manifolds, I am finding the boiler runs pretty much continuously on the lowest (1 out of 5) possible flame settings.

[Harry Bloomfield Esq]

on 17/01/2022, Tim Lamb supposed :

Generally recommended as at least 60C, to kill legionella.

Quite necessary, put batteries in unit, fit unit in a suitably screened from the sun location.

[Harry Bloomfield Esq]

Tim Lamb expressed precisely :

Long periods of heavy modulation, means it will be condensing much better. The lower the temperature of the water needed in the heating system, the better, the more efficiently it is running. Which means large enough radiators to work at lower temperatures. On and off firing of the boiler = poor efficiency and lots of wear and tear on the system.

[Harry Bloomfield Esq]

It happens that Tim Lamb formulated :

None the less, you do not want an expansion tank packed with Legion Ella.

[Tim Lamb]

In message <ss3ovn$rl8$ [e-mail], Harry Bloomfield Esq [e-mail]> writes

Not much I can do other than some form of treatment. Must be a similar issue for conventional radiator systems with open expansion tanks.

[Tim Lamb]

In message <ss3ojg$jj1$ [e-mail], Harry Bloomfield Esq [e-mail]> writes

Yes. Also clear of flue gases. No battery in my unit. Thermistor and terminals only.

[John Rumm]

Yup you could - it would also lower the total energy stored which might be a consideration if the store is borderline on heat capacity.

Flow temperature is in some respects not that important from an efficiency PoV - it's the return temperature that will have most effect on condensing efficiency.

A modulating boiler will typically automatically adjust its output power to achieve a better match to the load and also to try and maximise condensing efficiency.

Weather compensation can give it another helping hand in the process by allowing it to run lower flow temperatures when conditions permit. That is "better" in that you get better comfort with less temperature swing, and quieter operation from the rads and pipework (less creaking and clanking from thermal expansion/contraction)

Long periods of operation are also not necessarily bad. If the burn closely matches the actual heating demand, then arguably to have ideal operation - heating no more or less than required.

Historically, long burns at part load were potentially inefficient with old fixed output boilers as it often meant they would frequently hit their internal thermostat set point and cycle frequently.

[Tim Lamb]

OK John. I'll fit the weather compensation and knock the heating flow back. I may reduce the *heating on* time duration and see how that affects boiler modulation. Also dig out the digital thermometer and monitor the return temperature.

The test will be number of complaints from the other occupant balanced against gas used compared to last year:-)

[rick]

You sure you h ave it set right ? I have a UFH heating system, boiler primaries connected to Thermal store. (an no connection to anything else) Once store is up to temp there is no call on the boiler and it shuts down .... the boiler obvioulsy can modulate while there is a call to it for heat.

If no zone call for heat there is no pumped flow to manifolds so again no need for boiler to run. It never idles along on small output.

[Tim Lamb]

Set by installation plumber:-)

The pipe spacing and layout follows that specified by the heating designers. The kitchen diner area has problems because the units, island, fridge and cupboards etc. take up a lot of the otherwise heatable floor space. The other problem is that there is only 30mm of insulation below the pipes (timber frame construction on concrete pad). In consequence that circuit takes a long time to trip the thermostat. I am tempted to cheat on the 50 deg.C max. manifold setting.

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[Andrew]

But hopefully they are not inside the cavity, so you can see where they are ?

[Andrew]

I always thought that athermal store had multiple outlet tappings allowing water at different temperatures to be used. I.e the tapping near the bottom of the thermal store would be used for UFH while# the top would be used for DHW.

[Tim Lamb]

Indeed. It does mean waiting for the domestic dept. to be otherwise occupied in order to work in her laundry! The other issue is an SDS drill bit will blow a chunk off the outside wall..