Heat Bank, Thermal store or unvented

[Peter]

After many many years of waiting for the necessary cash it is time for me to upgrade my heating.

I have a large 45Kw modulating boiler. OK so it is a bit over the top, but it modulates down to 11kw of so and I have a large house, and ther builder paid for it. BUT, currently with knackered (inadequate) pipework and the old cylinder - it rarely gets used over about 25Kw.

I intend to loose the old hot tank, relocate to the garage and put a small rad in the airing cupboard.

One of the problems I have at present, is that my boiler controls insist on absolute hot water priority. With the current cylinder, I then feel the heating go off for the 20 minutes it takes to reheat the hot water cylinder.

Whatever solution I come up with, I want to continue to use the modulating boiler controls (together with a classy programmable thermostat) to allow the boiler to modulate all the way down to run the rads in the evening.

OK - so choice time.

Originally I thought of using a very high efficiency unvented cylinder. The Keston Spa (210l) has a very high efficiency primary coil. The thinking being that if HW is to have priority - make it last the shortest time. I still think I might go that way, but am a little put off by the thoughts of having a potential bomb in the garage, as well as the 3Bar limit on pressure (reduced I assume as well by the TMV) as well as the ongoing maintence costs.

Then there's the plethora of heat stores and thermal banks.

I'd appreciate help on the choice here, as there seems to be a lot of discussion as well as entrenched positions.

I can figure stuff out, if I have data, but that appears to be hard to get.

So - the questions are:

Should I go for a heat store where the CH is run off the boiler and the DHW off the heat store? [ Pandora arranged as S plan? ] If so - what is the reheat time? Should I install the flow switch trick on the bath and showers? [ Is this a safe thing to do anyway, as an overheat condition could occur if flow was restricted, or the switch failed. ] What size do I need.

Should I run both off one of the different kinds of integrated heat store?

Do the plates in a plate heat exchanger get dangerously hot to the touch?

Any coherent thoughts , or pointers to unbiased discussions/texts welcome.

Peter

PS I currently have 1 bathroom, 2 showers and am about to install a third barthroom. I have 6 bedrooms. I am changing to pressurised CH, in order to add rads to the attic. Similarly, I need presurrised DHW for the attic bathroom.

[David Hansen]

On 20 Mar 2007 02:54:03 -0700 someone who may be "Peter" wrote this:-

Hot water cylinders are best located near to where the water is used and where the boiler is. Putting cylinders where there are long dead legs increases water consumption as well as heat losses.

Are they and the kitchen located close together, or are they scattered all round the house? In the latter case there comes a point where local generation of hot water is better. This may be by instant heaters in some cases and smaller cylinders in other places.

It would be possible for the boiler to be connected to the shell of the unpressurised thermal store and a heating coil provided (in addition to the hot water coil) for pressurised heating. However, this sort of arrangement works better with the lower temperatures of underfloor heating.

Going for a pressurised store simply means having the same "bomb" problems as an unvented cylinder If there really is no possibility of a heating header tank in the attic above the radiators then you may well be best advised to go for a pressurised heating system with unvented hot water cylinder.

An alternative would be two boilers, one pressurised for the heating and one unpressurised for the thermal store. In effect these would be "winter" and "all year" boilers. The latter would be relatively small, as it would just need to regenerate the store more quickly than heat was drawn off. I would add solar heating of the store and probably drive some towel rails off the shell of the store so they would dry towels all year round. The attic towel rails would be electric.

[John Stumbles]

What's the heat requirement of the house? There are a couple of calculators at

(in the section 'Heat requirements')

My guess is that, even modulating down all the way to 11kW your boiler is going to cycle a lot heating your house most of the time so your modulating boiler controls aren't going to help efficiency much.

I'd be inclined to think of a f**k-off great heat bank buffering the output of the boiler to supply both CH and DHW, connected to the boiler directly in a vented system with a header tank - see

directly connected the boiler can dump its full output into the heat bank quickly and then shut off.

It does mean the boiler will need to heat it to 70 or 80C so you won't benefit from low return temperatures if it's a condensing boiler (you don't say whether it is) in which case there's a question (which I can't answer for you) of whether the gain from having it firing for very short times rather than cycling outweighs the loss of efficiency at higher return temps. If it's not condensing this doesn't apply.

With appropriate coils in the heat bank you can run all your rads as a sealed system, plus UFH if you wanted that without needing a mixing valve (just an extra pump and controls). If you're in a soft water area or have a water softener you could have a DHW coil in the tank, but if you have hard water you'd want an external PHE + pump + flow switch + TMV arrangement (as shown in

these components are built-into thermal store products such as the Pandora).

If you wanted to DIY parts of the system you could use an assembly (or more than one) such as shown above to provide DHW closer to the points of use instead of having a run all the way from the garage going cold between uses. (If the primary heating water has gone cold it'll take a few seconds for hotter water to get to your local PHE but you don't need to throw away lots of cooled DHW to get hot water.)

They'll be roughly as hot as the store and boiler flow pipes - uncomfortably rather than burningly hot. I'm curious why you ask - do you forsee a problem? You'd normally have the PHE tucked out of the way somewhere, and preferably insulated.

[robgraham]

(snip)

I've just had a look at the DIY Wiki page above and have a couple of questions :

1 Why is the motorised valve 3 position when all it is doing is to direct the flow either to the tank or the CH system ? 2 Is it acceptable to run the CH system at the 80 dec C that the store is going to want.? 3 One of the comments somewhere else in the Wiki is that feeding the PHE pump straight back into the tank can lead to de-structuring of the stratification and suggests using a thermostatic valve in that part of the circuit controlled by the DHW output to restrict the flow. Is this not a preferable option to the tmv ? 4 I also do not like the triac - a round based relay from eg Maplin is far more maintainable and easier for the average person to build and replace ? (qualified and old electronic engineer!)

Sorry that's 4 questions !

Rob

[Peter]

Amazingly, both showers, the kitchen and bathroom are all quite close.

But here is the thing ..it only needs to be 1.5bar or so for th pressurise CH, so not the same situation as a 3bar DHW cylinder.

Does a direct heat bank speced for 1.5bar exist?

Nice idea - but I cant accomadate a second one. Need to use what I've got already.

[Peter]

Actualy, the calculators both say I am about a 12kW house, suggesting boiler sizes of between 26 and 35kW. And I really really like the radiators running at a lower temp continuously when merely maintaining temp.

see

Being directly connected the boiler can dump its full output into the

Direct connection between boiler and heat store is a wonderful concept.

Is Pandora a direct or an indirect system? If indirect, what loading is the primary coil?

It will be in the garage, and in general I dont like the idea of very hot things being below 3ft off the ground - for kids safety.

[John Stumbles]

This system was a direct adaptation of an existing Y-plan system with a conventional DHW system. I might have chosen a diverter valve if I were building it from new.

Probably nearer 70C: because it's direct you don't have a temperature drop between boiler and store so there's just the drop in the PHE. The reason stored DHW is normally 60C is hygiene: you don't need or want DHW coming out of the taps at 60C so the store can run cooler than that and still give DHW that's plenty hot enough.

What configuration are you referring to here? I don't think I've come across that one.

Fair enough: I don't think Maplin had any suitable relays when I was shopping for bits so I went with the triac solution. You could also go for a flow switch which could switch the pump directly, as DPS do on the Pandora.

[John Stumbles]

Eh? If the heat loss of the house is 12k you'd add on maybe 2k for DHW, plus extra rule-of-thumb chunks for a conservatory etc, but how do you get the 26-35k figure?

I don't see a simple way of having both: if you have the boiler drive the CH directly, modulating down to low flow temperatures (if that's what it does) and you want rads in the attic then you have to have a sealed system. You could legally have a directly-coupled unvented heat bank (according to Dr Drivel in a fairly recent post) but you have all the same safety implications, AIUI it's just a loophole in the regs that it's not covered under the same regs as unvented DHW cylinders.

Indirect, fairly quick recovery I thik but nowhere near 45kW! DPS do a direct heat bank (called GL or GX or GLX or something, I think) but see above...

[The Natural Philosopher]

warm up time.

A boiler that can only just keep paces with losses takes a very long time to warm a house up.

[Ed Sirett]

see

> Being directly connected the boiler can dump its full output into the

What is the boiler? It not a Keston C40 as that's 11-40kW. It's a very large an expensive piece of kit which we obviously want to keep unless it's very wrong.

You say it uses 25kW (does the boiler tell you this - must be something really high end?) or did you deduce it from the gas meter or the rpm of the premix fan?

My guess is that a 6 bed modern house would be needing around 25kW may be a little more in the coldest weather. Anyway we will await you plugging the numbers into a heat loss program/web-site.

A house of this size would do well to have division of the heating into zones. Maybe the pipes will let you do this easily or maybe not. Once you have done that then the controls are going to be extended S plan. If you don't do heating zones (and the loft rads would surely be candidates for a zone?) then you have the choice of using a diverter or Y-plan.

A diverter valve causing the occasional outages of the heating for up to

20 minutes should not be a problem. It only turns out to be a problem for people who program the heating and HW on together in the morning at the same time. You really need to set the HW timer earlier (or even 24/7) so that the peak demands are not competing for the 6:30am show.

You could go for a secondary return loop for the DHW, this requires a bronze circulator (£150) and very well insulated DHW pipes. There should be a severe flow restriction on the loop return (but not the flow of course!) and ideally a timer on the pump. This should let you put the HW store in the attached garage. If detached think again.

200 litres sounds about right.

Before embarking on any sort of mains driven DHW system you need to verify that you have the supply. Ideally 32mm MDPE and a standing pressure of 2.5 bar min. Even then with so many baths/showers the auxiliary draw effects could be noticeable. Unvented is more tolerant of multiple outlets as the expansion vessel/volume tends to buffer the changes in draw off rates.

BTW: It's nice being able to throw my spanner in to the works on a subject like this without it causing a great inferno.

[robgraham]

The DIY Wiki on "Thermal Store and Banks" has a box on Stratification which makes reference this towards the bottom of the box. The suggestion is varying the pump speed to reduce the water flow and hence control the temperature, or use a thermostatic valve as I've mentioned.

Rob

[Peter]

Ideal Imax45

What I meant to say whas that the calculators say I need about 13kW steadstate and they recommend a boiler of 25kW.

Yup - the plumbing is all ready for 2 zones downstairs, 1st floor and attric (4 zones in total) - although I may not put air thermostats on each.

Ah - but you see the issue is that with my programmer DHW is always on. Then, when at 6.30 people have DHW drawoff, the DHW priority feature of the programmer tries to replenish the hot tank - hence the CH issue.

Dont think this is the problem.

I've got 25mm and about 5bar - as I seem to remember. Don't suppose anybody has celer ideas how to measure the pressure without expensive kit?

Even then with so many baths/showers the

Spanners are fine - they are tools. I like tools.

[Ed Sirett]

This is a modern very well insulated and/or fairly small, 6 bed house, I suppose. Who is the 'they' in 'they recommend ... 25kW'. Either the requirement is

13kW or it's nearer 25kW. Some allowance for DHW should be made. I've forgotten just why and how much but not more than 3-4 kW, and maybe some for future proofing and/or insurance.

Really if the requirement is only 13kW then the boiler should be replaced there are plenty of smaller (and very much cheaper) models to choose from.

Why have the zones then? Control each zone with a programmable thermostat. I'd also have a master CH timer over all the CH zones as well.

Set the CH to have got the radiators warm before the bathroom scramble. It must take the 45kW boiler all of about 5 minutes to get the rads to temp.

You should be able to get something to do the job for under a tenner. It only has to work a few times for you.

5 bar and 25mm should be OK, but this advice comes with no warranty. If you absolutely had a disaster on your hands then a few hundred quid on some accumulators will probably rescue you.

[John Rumm]

I would have thought that if you are using a direct cylinder[1] in this way they it would have to be rated for operation at pressure at least as great as the setting on the pressure blow off valve on the sealed primary. Otherwise you could get a cascade failure, if for example your expansion vessel failed that could cause the primary pressure to rise to

3 bar+. Ideally one would want the pressure release valve to operate before explosion of the cylinder [1] Even with an indirect cylinder you still have to allow for the possibility a leak within the primary heating coil, undesirable at the best of times, but particularly so if the cylinder is not vented.

[John Rumm]

Hmm, quiet innit! ;-)

(wonder if he is on holls, or has croaked?)

[Andy Hall]

Eyebyeza, with puffy anorak courtesy of boiler manufacturer sales promotions

[David Hansen]

On 20 Mar 2007 15:53:12 -0700 someone who may be "Peter" wrote this:-

Often vertical zones are better, as they allow for solar gains on one side of the house. North/south and a core area for example.

It isn't a problem, it is a suggested solution.

Pressure gauge that fits on washing machine connection. Screwfix sell them for less than 20 pounds.

[David Hansen]

On Tue, 20 Mar 2007 23:35:04 +0000 (UTC) someone who may be Ed Sirett wrote this:-

It does sound like it. Or perhaps the windows and ventilation losses have been ignored and the figure has been fudged upwards.

Depends how large a cylinder is to be heated and how quickly it is to be reheated after draw off. This is also related to maximum demand and is still something of a black art. 3-4kW should be fine for most houses.

Now that modulating boilers are common this is perhaps sensible. In Ye Olden Days it was best avoided as it just encouraged short cycling.

[John Stumbles]

OK I was thinking TMV not TRV(-type).

(Should have known: I wrote that bit on the wiki!)

However if you've got blindingly fast recovery (like 45kW boiler direct into heat bank) it doesn't really matter.

[Ed Sirett]

If you follow 'the book' then my 'insurance extra' is off limits even with modern boilers.