Secondary Circulation loop on a combi (2023 Update)

[Mr Fizzion]

Can you have such a thing or is this one of Doctor Evil's fantasies?

If you can...how would you do it?

TIA

Mr F.

[Christian McArdle]

You'd do it the same as any secondary circulation loop. It doesn't really matter what the heating appliance is, provided there are no pressure reducing devices such as TRVs or air breaks (i.e. cold water cisterns) actually in the loop. These may be before the loop, however.

However, not all combis would be suited to a secondary circulation loop. They work better on a storage based system as an instantaneous combi will cycle with much greater rapidity than a system boiler off a storage cylinder.

Christian.

[Aidan]

Assuming youre referring to a secondary circulation loop on the hot water supply system, then;

The combi fires on a draw off from the hot water outlets. The only way I could imagine installing a circulation loop would be to install a heat exchanger (tube-in-shell, preferably) on the secondary return and pipe the primary side of the HX as a zone on heating. You'd need the heating to operate 365 days. You could use an electric heater.

There are also electric trace heating system for hws, which are installed under thermal insulation to minimize the heat loss from the pipes. These don't need a circulation loop.

If you're doing that you'd be better off installing an unvented hot water storage cylinder ( which has got a heat exchanger & has got a large volume of water, so it won't discharge scalding water if the controls overshoot the set-point slightly). If you're doing that, you might as well scrap the combi and put in a conventional hot water storage system.

You should realise that Drivel is a moron and his rantings should be treated accordingly.

[John Rumm]

It ought to work on "simple" combis so long as the manufacturer states it is ok for the "cold" inlet temp to much hotter than it usually would. You also need a combi that can operate on very low mains pressure (so that it will work correctly when the flow is being driven by the pump and not the mains).

However you could potentially run into all sorts of difficulties with a modern combi that uses a more sophisticated control loop, or one that includes a small store of tempered water so as to give "instant" hot water.

Timer, pipe stat, check valves, secondary circulation pump, loads of good insulation on the pipe run etc. You would then need to juggle stat temperatures carefully to get the operation you require without ending up in a deadlock state with the pipe stat demanding heat and the combi HW stat satisfied and hence inhibiting the burner (and the result that the boiler sits in HW mode for extended periods while the house gets cold).

[Ed Sirett]

For me this is just plain nonsense.

1) I doubt very much whether any manufacture would permit the boiler to be used in this way. 2) The boiler is not designed to take luke warm let alone hot water as it's input. 3) All the while the 2ndry pump is going the boiler is in DHW mode. When does the house get heated? 4) The whole system would rely on the boiler shutting down because the primary is at or over its maximum temperature for DHW mode. This will only happen when the DHW temperature is very hot. Ergo the heat losses on the 2ndry loop will be huge. 5) Whilst the burner is shutdown the fan will still be going so that the primary will be losing heat at a rate comparable with the burner rating i.e. tens of kW. This is going straight out the flue! 6) The water bylaws forbid the permanent connection of the mains to a closed circuit.

If a house needs a secondary loop it also needs a grown up heating system that will include a stored HW of some type (vented, unvented or thermal store).

[John Rumm]

I would agree that the concept has serious flaws in many cases.

Those two are likely to be show stoppers anyway...

That is where the pipe stat and the timer comes in I suppose. Once the loop is up to temperature it kills the pump and allows it to return to heating mode. The big problem will be the heat capacity (or more the lack of it) of the pipework - it will get cold quickly.

Where do most boilers sample the DHW output temperature. in the DHW flow on on the return of the primary for example?

Circumventable with a doublecheck valve?

Indeed ;-)

[Doctor Drivel]

Boy are you confused.

[Doctor Drivel]

What difficulties might these be? Or did you make that up?

Can be booster pump to achieve the pressure to operate the combi.

Center also sell them. You also need a time clock to switch off during the night.

Set the pipe stat to 40C

[Christian McArdle]

But I suspect most boilers would cope for the minute or two operation period.

It only needs to be on for about one minute to heat the loop. Then the loop will stay hot for many minutes with a high hysterysis thermostat satisfied. Obviously any secondary circulation system would have to be very well insulated. This applies to all types.

It will still cycle the boiler much more than a storage based system, though. However, I don't see this as a show stopper, just a disadvantage.

I can't see how this applies to secondary circulation loops. They aren't really closed loops.

Whilst I'm no fan of permanently on secondary circulation loops in most cases due to energy efficiency reasons, I can see the benefit of a manually operated (i.e. 2 minute timed) system in a domestic property, as it would be very effective at water conservation. A manually operated system would also actually save energy, as the reheated water will start at a higher temperature than fresh mains.

Christian.

[Doctor Drivel]

On what does he base this wild claim on....we shall see....

The boiler is not being monkied about with.

Some are and most are really. See makers. Many accept very hot water from solar preheated water. Eco-Hometec for e.g. The cold inlet to most combi's is just a pipe to a plate heat exchanger and a flow switch. Nothing there to say 40C water will screw the system up.

You really can't figure this out can you. A highly insulated loop from the outlet to the inlet with pipe stat set to 40C means that the loop will be warm within a minute or so and then be switched off, amd stay off for a long time. There are keep warm boilers around. The same thing but extended to outside the boiler casing.

What are you on about?

Again. What are you on about?

More confusion.

Not so. Thermal store are done this way. Here are The DPS Heat bank makers giving advice on uk.d-i-y on how to connect up a heat bank using "exactly" this method.

If a house needs a secondary loop it also needs a grown up heating system

What tripe. The basin and washroom may be way off from the combi, so looping the DHW makes sense. In fact a secondary circulation loop saves a hell of a lot of water and gives instant DHW at the taps, which is a great convenience.

Have a look at the one box AVC Heatmaster for a system in your view is not grown up.

..and you are a professional? Struth.

[Doctor Drivel]

What serious flaws might these be?

Any evidence of this? Or just making it up again.

Not it heavily lagged.

DHW outlet

A rank DIYer and confused professional agreeing with each other. My, oh, my!

A rank DIYer and confused professional agreeing with each other. My, oh, my! The professional should stick to Vaillants.

[Doctor Drivel]

On what do you base this article of faith?

Yep, you are right.

6 of 1/2 dozen of the other. You save a hell of a lot in water wastage which on a meter you pay for. Then the convenience. Most people who use the front door toilet (cloakroom) very rarely wash their hands in hot water, because of the long lag.

I have seen a pump on the DHW draw-off pipe activated by a flow switch. This pumps out water from a cylinder pronto reducing the lag

[Christian McArdle]

It is based on the fact that a storage based solution will be able to perform a considerable amount of secondary loop heating service before this activates the vessel thermostat, allowing the boiler to perform a slightly longer more efficient burn.

With an instantaneous combi, the combi must fire every time the secondary loop pump activates, and in the worst case, if the loop circulation is slow, it might even be below the modulation threshold, so it might even have to cycle to maintain primary temperature, although careful design can eliminate this.

Therefore, even in a best case scenario, the instantaneous combi fires every time the loop thermostat calls for heat, whilst the storage based system could service for hours without boiler assistance, with sufficient capacity and thermostat hysterysis/double thermostat.

Christian.

[Doctor Drivel]

A secondary circulation loop is well, a secondary circulation loop. Whether there is a large cylinder on the loop or an instant water heat makes no difference. The loop will cool no quicker on any of them.

You could have a few small DHW vessels on the loop, in series, like under sink units, have a pressure vessel too. So when you draw off small volumes of water at the local point, the combi does not kick in as there will be no flow through the boiler until the pressure has dropped - avoids nuisance firing. There could be a thermostat on each small vessel to fire the combi to replenish the vessel of DHW. A Dutch/Belgian company did somthing similar to this - distributed storage giving instant water at the taps. The total volume of all the vessels is enough to fill a bath.

[Aidan]

snip.

Thanks for clarifying that for us all, Dr. Drivel, I'm sure that's all crystal clear now.

Is it just me, or can anyone else detect the tiniest soupcon of utter bollocks?

[Mr Fizzion]

One would hope the boiler can handle lukewarm water at least for a couple of minutes. My loft sometimes reaches 50 celsius in the summer and there will be a few litres of water in the pipe at possibly over

20 celsius.

Having hot water in less than a second is a luxury rather than a necessity. If it were easily possible with my new combi I would do it. Since it seems not to be I shall be content with waiting 30 seconds!

Mr F.

[Doctor Drivel]

To normal people it would be....but....

It is you for sure.

Those with brains can't detect any at all, so precluding you. Sad but true.

[Doctor Drivel]

More like 30 to 35C My mains temperature is 19C right now

It does save on water too.

[Christian McArdle]

Actually, there is some environmental benefit as well as a convenience one, due to the reduced wastage of water. I would suggest that a secondary loop that has a boost button that you can press before have a bath, or before sitting on the toilet (so it is ready when you finish) means that energy is actually saved as well as water. However, a secondary loop constantly on connected to a combi is likely to be somewhat wasteful of energy.

Christian.

[Matt]

All combi firing unless it is in a high velocity trajectory towards the sun is a waste of time

You've been eating them funny mushrooms again haven't you?

Yes of course it is.

Get back to that counter Dribble. Copper sales are going through the roof but a few more sales won't go amiss.