The Atmos may do. The Keston uses they same heat exchanger on the combi as on the vented boilers, as does the Atmos. The combi version is packaged with the pressure vessel etc.
It depends on the DHW temp set at the combi, 45C condenses more than say
60C, the power of the combi and the incoming cold water temp. It may be sending back to the main heat X from the combi plate heat X water temp that is above dew-point (54C). I may be sending water back below 54C. If pre-heated water is about 50C it will condense little. But the main heat X is bigger than normal so it is more efficient than a non-condensing boiler.
Having the combi pre-heat the cold water running fast through the combi and then into the heat bank DHW plate heat X, will mean it will be in condensing most of the time as the water entering is cold and the water leaving the combi is "warm" pre-heated water.
Having the combi pre-heat cold water mean that temperature in the heat bank may be lower, say 65C, as it has to do less work. This again promotes condensing efficiency. If the heat bank is to be heated using a weather compensator then this does not apply.
The heat bank, say set at 65C and supplying rads at 65C (size rads to suit with delta T across the rads of 20C), will mean a cheaper non-weather compensator boiler may be used and condensing efficiencies are guaranteed in DHW and CH.
Interesting to consider pre-heating the DHW in the combi and then up to final temp using the Heat Bank.
I can see that would be better from a condensing point of view for the reasons you've described.
As you know I intended doing it the other way round - take the edge off with some energy from the HB and then a quick final blast in the combi.
I can see your point though. Perhaps we are into a summer/winter setting thing:
If the HB needs to be at 65C to do the CH - I was hoping it could be a bit lower but I see it depends on radiator size and outside temp as well as house heat loss - then it could do the bulk of the DHW temp rise in the winter. In the summer either the HB does nothing if the incoming water is warm enough or, if the flow is inadequate, then the occasional warming of the HB just enough to raise the mains water up say 10C should do the trick. I assume that if the HB is really well insulated and the temp differential between the water inside and the outside is small then it will stay warm enough to only require a quick burst every couple of days.
Draw out and think through both setups. List advantage and disadvantages. It gets clearer when you do that. You could stage it, 1st stage, the combi, 2nd the heat bank. The combi always cuts in. A two-port modulating valve on the cold into the HB DHW plate . The two DHW outlets of the plate and the combi combine . The DHW temp for the two-port valve is after the tee. When all cold except the HB is charged, the taps is opened the HB cut in as the valve is fully open, as does the combi. When the DHW temps say is approaching 50C the valve closes and reduces the flow from the heat bank. want high flows by opening the taps up and the valve opens up. low flows and the valve close off. Flow regulators could be on each line: combi and HB DHW plate, to ensure say 10 litres/min on each leg, giving say 20 max. It may take a bit of setting to balance between the two.
Try and size the rads 60-40, and balance them to suit. Then a 60C HB temp.
It depends on what temp you set it to. If coming in at 18C and you need 30 C to supplement the combi to run off a bath, then the HB will need heating to 35 to 40C.
If the combi pre-heats, and the DHW tap temp is set to say 47C, then the HB would need to be 52ishC in summer, maybe less. If low flows because a basin is on the combi will that and just sent DHW up to temp through the HB plate.
The DHW secondary loop with three or four 15 litre electric storage heaters on the loop may solve many things. Gives enough flow, electric backup if all on and the combi, it could give say an extra 12kW supplementing say 38kW of the combi. If large volume of water are needed of course, say like when visitors are in and are in the showers at the same time. This maintains DHW temps throughout summer and winter and gives 45 to 60 litres of DHW storage, that the combi automatically tops up.
Then the cylinder is only a simple CH buffer with electric backup. 9kW immersions are available.
That is because many don't, and also this is something not included in the sedbuk rating. The ones that do often make a point of talking about in the blurb.
Don't think you can split by make... for example there are some superficially similar WB models like the CDi and the Si where IIRC one of the differences was that one did condense on HW production and the other not.
I am closely investigating the various boiler options.
DD: you mentioned 'two port modulating valve' I assume this is just a normal two ort valve with a suitable thermostat strapped to the pipe as suggested and connected together appropriately.
Or is it a special bit of kit.
Also several times suggested 'bronze pumps' I see they cost more ++. What exactly are the benefits?
Been away. Thanks for the further info. I've searched for the Danfoss modulating two port valve but have had no luck. If someone had a URL that would handy.
I see about the bronze pump now. No rust inhibitor in fresh water so needs to be made of non-ferrous material.
Thanks again.
Soon be time to p**s or get off the pot on this project. I'll let you know what level of complexity/flexibility/expense I am going to opt for.
it seems that the nearest equivalent is a VMT (two way seated valve)
22mm plus a RAVK (self acting thermoststic element (25-65C). These together plus vat and carriage come to =A3149:00 via city plumbing. So not totally out of the question for setting the DHW temp out of the PHE. The valve goes on the inlet to the Heatbank side of the PHE and the sensor on the DHW outflow from the PHE. Okay there will be a bit of a control lag due to the water in the PHE at any given time but I suspect the volume of water held in the PHE is very small by nature of the design of the device.
Using this I could ensure that the water entering the combi to have it's heat topped up can be kept at a reasonably constant temperature anywhere in the range up to the temp of the HB. Imagine the HB will be about 65 perhaps less and the water temp into the combi about 30C all year. Limiting factor will be how fast a 30kw Glowworm can shove the water through with -say- a 20C temperature rise.
Any guesses? Quoted Lpm @ DT of 35C is 12.3 so for a DT of 20/25... I know this won't be very efficient from a condensing POV but it is the CH that will take the vast majority of the energy and that will be very efficient.
Result? Good flow from the combi whatever the season without having to have the HB temp higher than needed for the CH. In the summer the HB only needs to be topped up occasionally to a very modest temperature every couple of days (approx 30C) to ensure good DHW flow.
Seems a good compromise on complexity/flexibility/expense graph.
BTW Danfoss want to know if I want a Kvs of 5 or 2.3 on the VMT. I don't know what this is though it might be maximum flow rate as it is cubic m/H
it seems that the nearest equivalent is a VMT (two way seated valve)
22mm plus a RAVK (self acting thermoststic element (25-65C). These together plus vat and carriage come to £149:00 via city plumbing. So not totally out of the question for setting the DHW temp out of the PHE. The valve goes on the inlet to the Heatbank side of the PHE
DD thanks for that further. Yes I see that a normal DHW blending valve would work fine thanks. With the VMT/RAVK set up I was aiming at being able to adjust the flow through the PHE so that the water going back into the bottom of the HB was cooler and likely flowing slower than if the PHE was supplied from the HB by a pump either being on or off. Perhaps irrationally I was trying to avoid shoving too warm water back in the bottom of they HB unnecessarily fast and disturbing the layers. Though of course the CH return will be fairly vigorous at the times of most DHW demand so I'm probably wasting my time with pointless (expensive) frills.
On the other hand I was anticipating teeing off the HW from the top of the HB to the CH flow and PHE respectively. I hope that by regulating the flow to the PHE I would not need to stop the CH pump/shut a valve when there was a demand for DHW.
Or would I be wise to?
On a related topic... Will I need to use a surrey flange on the top of the HB cylinder? If so I'm not exactly clear what the advantages are. I think is is to do with air at the top on the cylinder but can't understand why that won't get taken round until it finds the vent pipe.
Explanation greatly appreciated. I suspect a bad place for the vent pipe would be off the top of the cylinder as under extreme circumstances there might be venturi problems. (Perhaps I've just answered my own question re: Surrey flange)
Question: Given the above where is the best place to take the vent from on the system I am proposing?
Thanks again to all who are/have assisted with this,
As it is only for pre-heat probbely not worth it. This control aspect can always be added.
The Surrey flange make one port into two. Also the centre part protrudes into the cylinders.
Usually the top draw-off pipe, if the F&E tank is high enough. If not then off the shower tapping 3/4 of the way up and have an auto air vent in the top.
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