Combi boiler - Hot water 'heat dump' too hot

[The Natural Philosopher]

I know you do John.

But under Nu Laber you have been released into community care. We do our best with you John.

[John Rumm]

No you are mixing singular and plural here. Also a "Z" on its own does not form a word in English.

Yes, really must.

No, a plumber would have a pipe cutter....

No, your basic English language comprehension seems to be lacking as well. The inclusion of "or" would indicate they where alternative options and not necessarily needing to both be true.

Yes, I think you are agreeing with him.

[IMM]

He doesn't know that.

..and not an axe.

Yep.

[IMM]

Have you given your house a haircut yet?

[John]

This is a recognised problem with some combis which are the tube within tube heat exchanger type. When the heating is running the heat exchanger attains a high temperature and thus the bit of tapwater within this also reaches this temperature. When the hot tap is opened this "slug" of very hot water travels along and passes out of the tap. The boiler runs through its sequence and fires at the hot water rate which is designed to provide hot water at the "normal" temperature. The net result is just as described i.e. a brief pulse of scalding water followed by cooler then normal temperature hot water. A long length of pipe from boiler to tap will mitigate the problem but creates an annoying lag in delivery times. C'est la vie

[Phil Addison]

Thanks Ed and John. So summarizing your replies, Tania should try turning down the C/H temp if the weather is not too cold. Apart from that I'm sure telling her it is 'normal' will be of some comfort, so she can just expect the hot slug and wait for it to drop back to the proper HW temperature.

I appreciate the explanation of what causes it - and wonder if there is a safety regulation covering max temp of DHW. Is there a list, or some easy way of recognising combis that do or don't display this mis-feature?

I'll be passing the advice onto Tania.

Phil The uk.d-i-y FAQ is at

NOSPAM from address to email me

[IMM]

You could put in a secondary circulation loop on the combi and guarantee hot water instantly at the taps. This would for most of the draw-off time not be scalding as the pump would slowly circulate the water in the loop and combi heat exchanger. Have a pipe stat to keep the draw-off temp to around 40C, so it will not be extracting heat from the combi continuously.

[John Rumm]

My ideal isar has a plate heat exchanger for the HW side, and it does not demonstrate this behaviour. It also maintains a small (i.e. couple of litres) store of water that it keeps tempered at the selected HW output temp to try and provide a more instant and stable temp during initial demand. Presumably others of similar design would also be free from the problem.

[John Rumm]

Works well enough with a stored water system, but how are you going to do it on a combi?

[Dave Plowman (News)]

I *think* he means having a heating loop for the water off the central heating side.

Any bodge rather than do the job properly.

[IMM]

The same way as it is implemented on a heat bank or thermal store, using a couple of check valves and a pump. The pump has to be big enough to activate the burner, or better, the combi operates on a very low minimum mains pressure. water. Best have a length of 28mm about 1 foot long, or two in parallel, as the mains enters the combi. This acts as a hot water store preventing the cold spot as the burner proves itself. The secondary loop must be insulated. The pump must be on the return of the loop, but not in the cold mains path. i.e., mains in to combi, check valve, tee (return from loop), check valve in loop as it meets the tee, from tee to combi have a

28mm section, or two in parallel (this acts as a hot store that feeds into the combi to prevent the cold spot), pipe stat, set to 45C after pump but before loop check valve. Voila.

[IMM]

Don't think, you are not up to it.

[Phil Addison]

Hmmm... I thought he meant take a pipe back from the tap to the source and keep pumping it round the loop, as can be done with a stored DHW system. But with mains pressure hot water from a combi??? Errr, nope!

Phil The uk.d-i-y FAQ is at

NOSPAM from address to email me

[IMM]

Err. yep. Similar to how it is done on a thermal store or heat bank. I implemeted on a Microgenus.

[John Rumm]

gosh, an IMM post that is actualy plausible and did not mention politics or insult anyone once - well done!

I presume you need to set the pipe stat to below the minimum stat temperature that can be selected for HW from the front panel of the boiler, otherwise you could end up unable to meet the demand from the stat if someone turns the boiler stat right down?

How do you ensure that you don't enter a deadlock situation where the water temp in the secondary return is close to target, but the pipe stat demand is still on, however the boiler refuses to fire since the delta required is below its minimum modulation setting?

It strikes me you also have a conflicting requirement here: The 28mm pipe "hot store" would have a higher thermal mass than the bulk of the secondary loop, and hence would cool slower unless you reduce its lagging (in comparison with the rest of the loop) such that its temperature decay approximates that of the rest of the loop. However you need to maintain enough hysteresis in this control loop to prevent firing the boiler for HW too often (and hence diverting the boiler from the CH on a regular basis). Perhaps a simple programmer would be of use to disable the loop during the night or other times when HW is unlikely to be needed.

[IMM]

All my posts are 110% plausible. Your low intekingence would not see that though. Sad but tue.

I don't mention politics. When I do it is pure wisdom.

I never insult being the perfect gentleman. I make observations.

Thank you.

yep.

A combi modulates on temperature. When it nears the setpoint is modulates down and switches off on reaching setpoint. Not an issue.

May have a higher mass, depending on the length of the secondary loop.

This 28mm pipe(s) would be in the cold water main line just before the combi. This stored hot water is pushed into the combi by the incoming cold mains. it makes up for the cold lag in the burner proving cycle.

Depending on where the pipe stat is fitted. Too close to the tee with the cold water mains may mean heat is convected off.

A time clock is essential on all secondary loops. The heat loss on this loop would be no more than any other loop on any other type of system.

[John Rumm]

In which case, are you aware there is another poster on this group using the name IMM? Alas his posts frequently make no sense at all. You should track this scoundrel down and remonstrate with him.

You may be right, however you may not. This is something that will vary with boiler types and instalation details.

You have an inherent difficulty in that you have two separate (i.e an outer and an inner) closed loop control systems, but your outer loop is operating with no a priori knowledge of the characteristics of the inner loop.

As you can demonstrate with many boilers, if you run a hot tap slowly, the boiler can turn off the burner because the heating load is too small and it is unable to modulate low enough to not exceed the preset temperature. This is not much of an issue with a tap - you either turn it off or turn it on more. With the feedback loop you describe, I can see that you could arrive at the same situation. Pipe stat set at 45 degrees, water return currently at 43, and boiler stat set at 50 degrees. Boiler unable to fire because at minimum burner power it would cause a temp delta of more than 7 degrees.

In some cases it will work fine, others can be tweeked, some may not work. You still run the risk that the outer loop "hogs" the boiler and deprives the CH. Much depends on the sophistication of the control system implemented in the boilers software. If it is a simple first order "error following" feedback loop, then your system will be fine (and given the simplicity of implementing this type of control, I am sure many do). If it is a more sophisticated second order control system, then you may have a problem.

In fact my terminology above was insufficiently precise... the thermal mass is not so much the issue as the thermal inertia. Yes, the secondary loop may have the greater mass, but its mass to surface area ratio (given consistent insulation) will influence the rate of heat loss.

Yup, follow that, not sure how it relates to the issue I mention above however.

This store of water could also be counter productive if the boiler also implements its own store for exactly the same purpose.

True, but not relevant. In the most typical loop (i.e. the CH) you are not that interested in pipe temperature - only room temp.

[Ed Sirett]

There are regulation covering maximum HW temperature but they don't (yet) apply to normal domestic dwellings.

I doubt there has been a list of suspects made with this problem. Pott. Lynx Saunier Duval Ferroli Fer Vaillant Turbomax+

It can also occur where the 2ndary heater exchanger is also used a CH bypass circuit.

[John]

SNIP

Since many combis operate the hot water function to the exclusion of the heating function any of this ilk will become useless for heating duty whilst the hot water recirculating loop is running

[IMM]

Go away...you don't say....

The loop will run for a matter of a few minutes as it heats up the loop and the pipe stat will cut the pump out when the loop is up top temp. It is not a continuous pumping operation. The pump also charges up the pre-heater

28mm pipes.

Works wonderfully.