DIY Heatbank - fine tuning of system (incl CH)

[John Rumm]

let me guess, I should have fitted two?

(odd we have not had the pet solution for ages now, are you fed up with it?, it used to get an outing several times a week)

[John Rumm]

And yet still exceed 90% SEDBUK rating... how much more will your store get?

Oh free energy, interesting concept...

No, actually I did the sums. It an engineering activity you ought to try once in a while. It can save you spending 1000's trimming £10 off you gas bill.

unless it modulates down to match the load... oh hang on a minute most of them do.

Wahay! Two combis almost makes an appearance...

So let me get this straight, so far the shopping list for the system is:

two boilers, one accumulator, a thermal store, half a dozen time dual channel time switches, about 15 pumps, half a mile of copper pipe, a couple of blending valves, weather compensator, immersion heater... (did I forget anything - been losing track)

spose you might get change from £4k and a MTBF comparable to you collar size.

[Doctor Drivel]

You are getting there.

[Doctor Drivel]

In a setup where the boiler set properly. Not the case in most systems. Then the auto by-pass runs out of setting.

You really don't know.

None of them modulate low enough.

You should fit two combis.

That would be a good system indeed. Cost effective for the functionality.

[John Rumm]

Perhaps you could translate: "Then the auto by-pass runs out of setting." into English.

I know you won't see a SEDBUK rating of over 100% since they are using the gross calorific value in their calculations. So here in the real world, with normal physics and all that, 100% is the maximum you can possibly get - when all the energy from the fuel you put in, is captured and put to good use.

Think through the implications.

Say you have a boiler that modulates down to 6kW, and lets guess that the OPs three storey stone built house loses heat at a rate of 4kW on a moderate day. Now aside from the fact the boiler will probably be running at its highest efficiency at this output, you will also note that the nett contribution of power to the building is 2kW.

Now if you have ever used a nett input of 2kW from a fan heater to try and heat any space of significant size (like an entire house for example), you will realise that in real terms this is a very low rate of heat input - and you will get very little cycling of the boiler on its internal stat. What little you do get will have little effect on overall seasonal efficiency.

Well lets say it squeezed an extra 5% out of the boiler (which seems unlikely - a SEDBUK efficiency around 90% usually corresponds to a boiler efficiency of some 92% or more, so we would have to be pushing

97% recovery at the boiler to achieve that).

Your above "cost effective" system has added £2k - 3k to the install cost. Lets say the gas usage for heating is pretty high at £500/year, that extra saving could be as much as £25. So you could see a return on your investment in as little as 80 years. (assuming you don't pay that saving over for extra electricity to run all those pumps)

[frankfront]

Frank here again.

Thanks for all the further input.

I think I know now how I want to manage the zones either with an HB or a combi.

However, I suspect that the actual saving either way (HB or combi) are fairly minimal. It's more a question of initial outlay, system flexibility, ease of servicing and future proofing (ie electric immersion back up or solar energy when the Russians cut off the gas). Many of the boxes are ticked by a heat bank - perhaps a few more than the combi. There is a greater outlay of time with the HB and although the technology seems tried and tested actually building the heatbank and commissioning it is something of an act of faith as they are not an everyday domestic item.

So some more questions:

I need more info in the PHE please. Who would you say make the most reliable units and what sort of size am I looking for? How do I predict the flow rate for a given temperature of HW? What is the service life of a PHE likely to be? Great help if you could point me in the direction of more technical data of PHEs

I think it would be best in my setting to have two smaller cylinders. One for the CH and one for the HW.

I could site them optimally for each use and have the benefit of greater control of each cylinder than if I were trying to make one cylinder do both jobs. Or am I barking?

I've looked at this schematic:

hxxp://aycu27.webshots.com/image/38786/2000028197462625313_rs.jpg

and taken on board DD's tweeks to the design. It looks very buildable. My only concern would be pump noise and what *exactly* the blending valve is designed to do in this cicuit. Presumably if I used two cylinders I'd need another blending valve and they would be set at different temps because (I assume) the CH cylinder is likely to be at a lower temp setting that the HW one.

That's all for now. Thank you again for this thread.

Frank.

[John Rumm]

Yup. It is the way you need to look at it, although DD will argue to the contrary, you can't really claim that a HB solution will save much if any money over directly driven rads with a modern boiler - the numbers just don't support it. Hence it needs to come down to what else it buys you, and is that something worth paying for.

Ease of servicing is partly down to what facilities you design in, and how simple the system is in the first place.

Yup, a heatbank is handy when you want to aggregate different sources of heat - especially if some of those are "low grade" heat.

I take it you are talking about a PHE used for on the fly HW heating from the store rather than one used for isolating a vented store from a sealed primary?

Size is the easier one, you can work that one back from your HW flow rate requirements. So say you want 25 lpm of water at a final temp of 50 degrees, and this is to happen with the ground water at 5 degrees, you need to shift 4200 x 45 x 25 = 4.7MJ / min or 78kJ/sec aka kW. So 80kW or better would be required here.

rearrange the above formula...

So for a given PHE power of P, a temperature rise of Td, you get a flow rate F of:

P x 60 F = --------- 4200 x Td

will give a flow in kg/min or litres

How hard is your water?

GEA have a fair bit of info on their site:

I think it would be best in my setting to have two smaller cylinders.

You are running counter to your requirement for simplicity and serviceability. The only particular advantage it addresses is location close to point of use. Note that only really matters for HW production

- you probably are not going to care if the rads need another 20 secs to get hot.

[Doctor Drivel]

What numbers do you have? None at all. You are making this up.

A heat bank using a simple boiler, time clock in one location, Smart pumps in one location for each zone is: simple, easy to maintain and cheap to run and maintain.

They are very common indeed, maybe not around your way though. Whole estates are fitted with them.

DPS sell them for approx £70. Gledhill can supply. Go to their site and download the instructions on the Systemate and the the part No. is in their. Ring a local agent and they will give a price, usually around £80. Or you may know a local dealer who supplies them. Go for a long and thin plate of

100 kW. The Swep that Gledhill use is very good for efficiency. The Danfoss double pass plates which are great but expensive.

Large plate resist scale as the plates flex, so scaling is not an issue wuth them. Not the case on small plates used in many combis. Best use a 100kW plate as the plates are larger. 20 years minimum should be the expect life of a quality plate.

You are on the right track.

He isn't at all. He is dividing and ruling, separating the functions and optimising each. Easy to work on, maintain and greater efficiency. The ch buffer will be completely off in the summer. With all the zones he has a ch buffer is ideal, and acts as a large neutral point and header combined. A dual temperature boiler with an integrated weather compensator will keep the CH buffer at what the outside temperature dictates and rise to heat the DHW heat bank. Have DHW priority in heating the cylinders, using a 3-way "diverter" valve at the boiler. CH calling? Full boiler heat to the DHW cylinder. DHW satisfied and CH calling? The weather compensator takes control of cylinder temperature.

[Doctor Drivel]

The blending valve's function is explained in this thread.

It guarantees the highest temperature set so only very hot water enters the store at the top of the cylinder ready for use to give useful DHW, even after 2 minutes of running from cold a sink can be filled. It also guarantees the boiler is operating within its delta T (flow/return temp difference) and maximises thermal expansion. It guarantees top down heating of the store and the water is heater one pass of the boiler quickening the re-heat - this can mean a smaller cylinder in many cases as the energy of the boiler and cylinder are combined. It also increases boiler longevity as the boiler is operating in an ideal hydraulic environment.

No. Only one blending valve for DHW. The ch buffer cylinder doesn't need it.

[frankfront]

Frank here again.

Thanks to both JR and DD for their further comments. I'll get my head round the PHE info later on.

One thing DD said though had me a bit foxed. The weather compensator makes plenty of sense. CAn they be wired into any boiler - such as the Glow Worm you suggested or do I have to be careful to get one that will cope?

Also 'Dual temperature boiler'? I think I can imagine what that is but clarification would help. Sounds good - do I have to be careful what I choose or can they all be made to do it?

Thanks again,

Frank

[Doctor Drivel]

A weather compensator can be a stand alone device. Danfoss used to make a cheap one.

Many boilers now have them integrated. Keston, Atmos, even a cheapo BIASI model too.

There are a few a round. The MAN comes to mind as well. Even a Ravenheat does this too - not sure if it has a compensator though.

The Keston Qudos is dual temperature.

This is cheap for what it is. The manual can be downloaded from, DHW sensor/stat is wired into the boiler. When DHW is called it runs up to max temp (or what you select), when no DHW and CH called then it reverts to weather compensator control

The Atmos Intergas has similar specs, except I don't think it is duel temperature. This can usually be done by opening, or shorting the outside sensor making the burner run full on. Then it can be used to reheat DHW fast. Best see the makers on how get around it. Atmos are super reliable and very well made

installation manual is also on line too.

Many of these boilers have a laptop interface so you can see how the boiler is performing.

[frankfront]

From Frank

A first look at the installation guide etc. for the Keston Kudos is very promising. A whole load of functionality for around the same price mark as other less handy looking machines. I like the wiring being external and proper diagrams in the guide etc.

Are Kestons well thought of in terms of value and reliability?

Two other things at this stage I would like comments on please:

1) It looks as thought the pump sending HW to the PHE from the cylinder is either on or off. When load on the DHW is light does this not mean that quite hot water is sent back t the bottom of the cylinder and disadvantageously admixed with the colder water there? I presume that the flow switch in the mains supply is either on or off and likewise the Grunfos.

2) Using a direct cylinder for the CH heatbank may be the most efficient but risks grot from the rads getting into the boiler. Is the magnaclean generally though to be adequate? I suppose one could put a PHE between the boiler and the DHW cylinder but there comes a point...

Thought please.

Frank

[Doctor Drivel]

They are generally fine. The Celsuis was a dog, but by the time they got rid of the problems they dropped it. This appears a dropped down commercial boiler with parts from other reliable boilers. It can also have 60 metres of plastic flue pipe (normal drain pipe).

That is the case. Having spreader pipes prevents excessive disturbance of stratification.

Yes.

The Magnaclean is all you need. And just re-dose the system every 4 years with inhibitor.

[Doctor Drivel]

There was a thread on the Keston a few weeks back.

[frankfront]

Thanks for that DD.

You said 'That is the case. Having spreader pipes prevents excessive disturbance of stratification.'

As I understand it a spreader pipe in inserted through the lower flange and has holes drilled below and to the sides. Presumably if this is warmer than the water at the bottom it will pass up through it and with it. So - if I am picturing this correctly - there must be mixing and disruption of the stratification. How much of problem is this to efficiency? Are there any other ways of reducing the mixing? Or is it not a problem?

Waiting to hear back from the Gledhill rep how much for the PHE

I wonder what size the cylinders should be.

Look forward to hearing your further thoughts.

Frank

[John Rumm]

I already gave you an example.

If you look at the (externally verified and tested) specs of many modern boilers, you will note they will run at about 92%+ efficiency into a conventional radiator setup with room stat interlock and TMVs on the non room stat rads. (SEDBUK values typically adjust this down to 90% taking into account seasonal influences)

Note that this is a real efficiency figure calculated using the gross energy content of the gas - none of your "make believe 106% no use to man nor beast" style efficiencies. So any amount of extra complexity you decide to glue between your boiler and radiators, is not going to be able to raise the efficiency by more than 5% at best.

Back in the days of high water content fixed output boilers with dumb controls, the store may have made a notable difference in efficiency, and been well worth having on fuel consumption grounds alone. That is no longer the case.

So by all means use a store if it gets you other facilities that you desire, just don't be mislead into thinking it will save you any significant money

[frankfront]

Frank Here again,

Most parts seem easy enough to get hold of but there are two items I've little experience of.

Flow switch and blending valve.

Any recommendations of makes that have been found reliable and good value by others?

Many thanks,

Frank

[John Rumm]

(the reed switch versions ought to be very reliable as long as you make sure you don't overload the contacts)

Screwfix and BES both have blending valves. Can't tell you which is better though.

[frankfront]

Frank again,

Thanks for all that.

Should manage a proper schematic and costing of my proposed system in the next day or two and then bound to have a few more questions. Thanks again for all the above information.

Frank

[Doctor Drivel]

Here is how to do a DHW only heat bank and the parts...

Read

for an explanation of how heat banks work.

1. Fit a Surrey type of flange in the top tapping of cylinder. 1" if you can.
2. The plate heat exchanger connects to the Surrey flange port that is immersed in the cylinder water. Drill a few holes in the protruding tube that goes into the water to ensure heat is extracted from cylinders dome, not being pumped to the bottom of the cylinder. The protruding tube has a plate over the end to stop the water heat right to the bottom of the cylinder.
3. The boiler flow is taken from the other connection of the Surrey flange on the top of the cylinder. The return into the old cold feed tapping.
4. From the Surrey to the plate and then to the pump then to a spring loaded check valve and back into the bottom cylinder port. Make a diffuser by inserting 22mm pipe into the bottom cylinder port and drilling holes in and stopping up the end with an internal pipe stop (available from BES). File down the inside of a brass compression fitting that screws to the cylinder port, removing the pipe stop so the pipe can go straight through. This will spread the returning water mainly down, so it will not upset stratification in the cylinder.
5. Two cylinder stats can be used to give a long efficient boiler recovery burn. One stat about half way up and the other about 25% of the way up the cylinder. Set bottom say to 70-80C, set top say to 60-65C. The stats must be latched in with a relay.
6. The cold mains direct from the cold mains stoptap with no tee offs. Take into the flow switch then into the bottom connection of the plate heat exchanger and then to the DHW blending valve.
7. Have thermostatic shower mixers and take the hot supply for these directly off the plate heat exchanger "before the TMV (blending valve). No need to run DHW through two thermostatic mixers.
8. Install a phosphor de-scaler, or other de-scaler, on the incoming mains pipe.
9. Install isolation valves on heavy usage appliances such as the washing machine, and throttle back so it will not rob showers and baths of hot water.
10. Have the F&E tank top up at the bottom of the cylinder and vent at the top. You may want to vent from the boiler flow pipe.
11. Fill with inhibitor - about 1% of total system volume. An average system is approx 100 litres, so a system has one 1 litre can. If say 150 litre cylinder then two cans for the cylinder alone, which is three.
12. Fit a Magnaclean filter on the rads return pipe to the boiler.

The performance is brilliant and you will be delighted with the mains fed showers and no vibrating power shower pump noise. High pressure mixers can be on all appliances.

Gledhill will supply a Plate Heat X. The model for the 145 litre Systemate will do. If you can get another cheaper source then try them. A 100kW plate heat exchanger is needed

or Screwfix will supply most of the fittings. They don't do the plate heat exchangers.

Farnell will supply the flow switch Farnell number: 1006771 with 22mm compression joints.

Flow Switch, makers site: The FS06

flow switch is about the best - very good.

If the boiler requires to be in a sealed system then have a cylinder with a quick recovery coil, the boiler heats this ASAP. Most boilers can be fitted to an open vented system. Best have open vented.