Because I live in a cottage with the corresponding low head for the DHW, I've been pondering for some time on the idea of converting the hotwater tank to a Heat Bank. Before anyone jumps in with the pump suggestion, I am under pressure from SWMBO to supply hot water to a shower and 2 taps at a considerably better flow than at moment - eg flow at mains pressure would be liked!
I've been reading the DIY Wiki entry on this subject and realise that stratification control is an issue and would appreciate any input from anyone who has undertaken this as a project.
A couple of questions as starters :
1 I've seen heat exchangers for combis advertised on Ebay - are these the sort of heat exchanger one would use ?
2 I take the tank is left vented to the atmosphere via a header tank but that the working fluid is a closed loop viz. an unpressurised CH system.
Probably not because (a) they're a bit on the low heat output side (b) they're intended for connection via proprietary mechanisms involving O rings and clamps and are a PITA to make DIY connections to. You can buy them from DPS - as used in their Pandora heat banks, cost c.£150 IIRC or from GEA Ecoflex. I got a Copper brazed plate heat exchanger model:- BP12M-20-G1G1 Packed and delivered. £100 + VAT. It works pretty well but not perfectly: at full flow from the bath tap the water comes out cool as there's evidently not enough heat transfer at the primary flow rate I've got.
That's how mine is arranged. It means you've got very good recovery as the primary water from the boiler goes straight into the tank to be drawn on by the PHE to deliver more hot water, so in practice the supply never runs cold for long: if you do run it cold you can get hot water again in a few minutes providing the boiler is running.
When I first tested it with just the electric immersion heater it seemed to run cold quite quickly and take a long time to recover, which is what you'd expect.
I've been meaning to do more methodical tests on it and to properly calculate the spec for the PHE needed, but apart from the Tuits I lack suitable measuring kit such as flow meters and multiple temperature sensors for the practical tests, and don't have all the data I need for the calculations.
Probably the first available Tuit should go to writing it up on the Wiki so others can see and hopefully contribute.
You could, but GEA Ecobraze make some larger ones with more transfer capacity. In a combi, there may well be a space limitation but then in typical domestic appliance sizes, they don't need to be able to transfer more than
40kW and produce greater than a 35 degree temperature rise at the rated flow rate.
In a heatbank setup, you can easily go for something more substantial that will give much better flow rates because more heat can be transfered. 100kW or even more is possible and the exchanger is still not a great deal larger than a couple of bricks.
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also have a UK office in Sheffield who will supply details of suppliers.
That's one way to do it, although not all boilers are happy to work on vented primaries.
Another approach is to keep the bulk hot water in the cylinder as unvented (perhaps with a header tank built on the top - the Heatweb ones are like this) and heat the cylinder with a coil from the primary circuit which can then be pressurised.
One caveat here is to make sure that the cylinder is a fast recovery type with plenty of coil surface area. The traditional and basic ones will work, but are limited in terms of heat transfer rate. Ideally one wants to get all of the output of the boiler beginning to replensih the cylinder as heat is used.
A further point is to heavily insulate the cylinder. It will be running at 82 degrees or so rather than the typical 60 of DHW.
This is a retirement project so cost has to be minimal and one trade off is that I have an 'standard' DHW tank and a similar one unused. Neither are likely to be 'fast recovery' type and I'm not sure that that in a two person house that that is going to be that critical. I'll do the sums on filling a bath for instance but if the current tank at 60 C fills the bath perfectly adequately without compromising hot water elsewhere in the house then surely a tank at 80C is going to have more reserve. Having seen the quoted transfer ratio across heat exchangers, that is not going to be the area of any loss.
Insulation doesn't present a problem. Box the tank in and fill with something (?).
The reason for asking about header tank, etc is the problem of head room. Currently the DHW tank has 1m head (top of DHW tank to surface of CW tank) - if I place the hot tank on the attic floor that headroom is going to disappear. That would push me down the 'pressurised' primary route but that does assume that the boiler will cope with that
- and equally the CH system (!). Apart from the obvious (ie the water is under pressure!) what Is the difference between systems that will cope with pressure and those that don't ?
Very roughly, you can start on the basis of 4/3 of the volume that you would get at 60 degrees. However, it may be worse because of circulation of the water in the cylinder tending to stir it.
Yes and no but be careful. It is better to have a heat exchanger that is oversized and not let it be the limiting factor. You can control HW temperature by using a thermostatic blending valve to handle changes of CW temperature and flow rates.
Just on that point..... the standard health warning about checking for adequate mains pressure and flow before starting.
Reasonable. Rockwool or glass fibre, or if space is tight, Celotex. You could locate the heat exchanger inside the box as well.
In a lot of ways, not a great deal. Have a look at Ed Sirett's sealed system FAQ.
Are you sure that the cylinders you have are direct types? Otherwise you will have in any case the bulk water in the cylinder unvented and a choice between vented and sealed for the primary depending on what you want and what the boiler can do.
For the cylinder, if you have a coil and the water is therefore separate to the primary, the arrangement can be easy. If you look at the Heatweb units, they have a small header tank on the top a bit like a Fortic tank. You could easily create something similar by making a frame for the cylinder and extending it upwards, putting the small header tank on the top.
If you have a coil and hence a separate primary, open vented, then nothing much changes if that's what you have now. The header tank for this can even be on the floor of the attic if you want.
If you have a direct cylinder and the water is common between cylinder and heating, then you need to be more careful about pump, feed/expansion pipe and vent pipe siting in order to make sure that there is not pumping over, because now this header tank will have to be above the cylinder and the head will be small.
Stand the cylinder on some rigid insulation, say 4" of polystyrene or Celotex if you really want. Surround the cylinder with 8" of glass wool
- it's dirt cheap and will fill the gap nicely between the cylinder and the (presumably) square box. Put plenty on top of the cylinder too, and have a tight-fitting lid to stop convection.
I'm confused by this - do you mean the DHW tank or the F&E tank? My thermal store (an Albion Mainsflow Direct) sits in the loft on the "raft" that used to support the 50 gallon cold water tank. The Mainsflow has it's own float valve at the top, so it's also the F&E tank for itself and the primary circuits (the boiler and radiators are two separate circuits with their own pumps).
For your purposes, you could put the heatbank in the loft and use a pumped supply from the water tank to the heat exchanger. A small (4 gallon) F&E tank could be placed on top of your box to keep the heatbank/primary circuit topped up.
Or you could do away with the water tank and go for a mains pressure feed direct to the heat exchanger. The heatbank/primary circuit would still be under low pressure.
Or you could make the heatbank indirect and have a sealed primary circuit (the heatbank could then be filled with a hose, like the DPS Pandora).
I'd suggest you don't try to make the heatbank itself pressurised as that requires extra safety devices like an unvented cylinder has.
1 The primary circuit to/from the boiler. In my case just to complicate matters, I have a wood burner included and a Dunsley Neutraliser is used to blend the two heat sources -interestingly I saw someone once suggest that the Dunsley was in fact a miniature heatbank. Anyway that's all vented with a header tank and a pump for the CH and a second from the oil burner.
2 Then we have the heatbank/pump/heat exchanger circuit. The tank, being an ordinary one, cannot be at pressure, so I need a second header tank for that at atmostpheric pressure !
3 Not really a 'circuit' but the path from the mains water through the heat exchanger to the shower and taps.
One downer is that I suspect I will have to upgrade the feed from the riser from 15mm to 22mm. It's something like a 15m run to the attic. Could someone say ya or nay to that for me please. That could a significant problem to tackle - can that be done in plastic for instance ?
I might be (probably am) missing something but what is the function of this intermediate "circuit". Why can't the boiler/CH water be the same as that used in the heat bank? Each heat transfer stage must loose something.
If the cylinder were a direct one (meaning no coil and the bulk water in it circulates to boiler and CH) then it would be one for that and one for the mains side.
That's OK. Given that you have all of this, and if it works OK, I don't see an over-riding reason to change it to sealed as long as you are using an indirect cylinder.
The other thing that occurs to me is that with the woodburner as well, you may well not have the amount of control or output to get the temperature of cylinder up to 80 degrees.
If the setup can only manage to raise it to 60 degrees as it might now, you may even be a little worse off. You would be able to get a greater flow and pressure than at present, but for a shorter period of time. That may matter for a shower.
Exactly, and it needs to be located above the cylinder. The one for the primary (1) may need to be raised a bit as well because it if it sitting on the floor of the attic, it needs to be above the level of the coil in the cylinder. Realistically, it may make sense to build a platform to take both header tanks and done with it.
Exactly.
It should be 22mm otherwise the flow will be quite restricted unless the pressure is substantial. Plastic would be OK.
I'm sure I can find a way round that with sensors of one sort or another. There is already a logic box control the system. If I head off in my box before my wife, heaven help the CH technician if maintenance is required !
Is there a good reason why one header tank with 2 separate outlets couldn't be used ?
Then in effect, you will have created a common system. I can't see a reason why not but then if you are going to do that, you might as well go for a direct cylinder approach by putting a couple of additional connection bosses on the cylinder - e.g. Essex flanges
Although what's going to happen when water is being drawn and the boiler fires?
You wouldn't want to have one pump sucking and the other blowing at each connection because that would effectively circulate water from the boiler directly through the heat exchanger and back. In effect, the cylinder wouldn't contribute anything much and one would have a crappier than usual combi.
Operationally, I would have thought that one would want to feed hot water from the boiler to the top of the cylinder and take cool from the bottom, whereas for the heat exchanger one would want to take hot from the top and return cool to the bottom.
If the flows are arranged such that both suck and blow at the same locations, then hot is going in from the boiler and cold from the exchanger.
I have another creative idea.
- Get an Abrafile blade and suitably attach it to two cords
- Drop assembly through immersion heater boss and arrange it to loop around the pipe of the coil near the entry fitting.
- Operate saw by pulling the ends of the cord until pipe is cut.
- Relocate cord near to lower entry fitting of coil and saw again.
- Manipulate coil out through the immersion heater boss (or treat it like a channel tunnel drilling machine)
- voila
This exercise may be time consuming. Rather like solving one of those metal puzzles involving string and nails.
Depends. Assuming the DHW pump has more oomph[1] than the CH one[2] then it should draw water from the cylinder as well as what the boiler can supply.
[1] technical term :-)
[2] which might need to be arranged by, say, a gate valve in the DHW part of the primary circuit if the length of pipework, the boiler itself and the motorised valve aren't enough.
The presumed advantage of the direct system for a heatbank is that is recharges quicker. I started off this Reply thinking that I would have to stick with an indirect system, but in fact the Neutraliser set- up I have may actually make the direct system work perfectly well as it would act as a buffer between the tank and the CH system. Need to think about that. I like the concept of the abra file saws ! I take it that once the coil is in, it can't be got out again ?
On the subject of the Essex Flange, should the take off be through one of these ? I have a feeling I've seen that it should be to preserve the stratification.
AIUI the neutraliser is like a mini heat bank so if you were prepared to add tappings to your tank you could probably bung both your heat sources into it directly and do away with the neutraliser. OTOH since you've got it already them if that bit's working OK, I'd just keep it.
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I think Andy was talking about taking the coil out with the abrafile, and you certainly wouldn't have the option of getting it back in. Given the choice of trying to cut through two 28mm pipes inside the cylinder with an abrafile and bunging on a couple of flanges I'd go for the latter!
You're going to bugger up the stratification pdq by pumping water from the cylinder through a PHE, but I think the quick recovery of having the cylinder directly heated makes up for this. I used the tappings I did on my direct cylinder because they were there but I think you could equally well just use the top and bottom tappings. If I had a few Tuits (and SWMBO hadn't filled the attic with so much junk I can't get near the system) I'd change over the connections on my own system and try it that way and let you know.
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