DIY Heatbank - fine tuning of system (incl CH)

[Doctor Drivel]

The boiler flow/return blending valve? Reliance 28mm. I think onepost gives the model number.

[Doctor Drivel]

In short, none at all.

Once again no figures given. You must stop making things up.

If making a heat bank it is rather foolish not to take the CH off the store as it gives a great CH buffer and the befits have been explained in other posts. It is clear you can't see this, so please be quiet as a person here wants sound advice, not the ramblings of an amateur whose only experience is fitting a combi.

As to the efficiencies of these boilers that directly heat the rads. The system is balanced properly and by-pass valve set correctly (few are set correctly). At the weekend I was in a friends house - a big house. He said the system warms up evenly enough around the house then one side of the house is hotter than the other as the rads on one side are hotter.

I looked and found that the by-pass valve opened too early (all too common) and much of the flow went back to the boiler. The flow which never went to the rads on one side of the house. This reduced the condensing efficiency of the boiler and efficiency went right down. I adjusted the by-pass and all was well. With many boilers the by-pass has to open early as they require a minimum flow through the boiler which may be quite high.

Take SEDBUK with a pinch of salt - a guide only. Efficiencies can be bettered by engineering the system.

[Doctor Drivel]

The disturbance is minimal when using spreaders. Also, the heat is not wasted it is put back where is came from - the cylinder. Once the DHW pump is off the hotter water rises and stratification occurs naturally as the cylinder settles again.

I'll be back on that.

[John Rumm]

The sums are quite easy to do. Even for you.

Well if you can explain how you can pay for all this out of a maximum of

5% efficiency gain then be my guest.

I can't see it because it is wrong. Alas you can't see it as it is too firmly entrenched in your mind along with various other old wife's tales.

Woa there boy, you seem to have a bit of a fixation on bypass valves. Are these like plastic plumbing were you claim the whole system does not work properly based on your hacksaw escapades?

With room stat controlled zones, that have rad without a TMV there is no need for a bypass valve within any of the zones. The only time you may need a bypass outside of the zones is to allow pump overrun (which occurs when the burner is not firing and hence loses no efficiency).

If you are paranoid about your ability to set this correctly, then place your bypass on a three way diversion valve such that the flow path only exists where there is no call for heat from any zone.

They let you out at weekends do they?

Agreed, but not by enough to make all the titting about cost effective.

[Andy Hall]

"Big House" being a slang term (principally American I believe) for prison........

[Doctor Drivel]

Matt, have you been a big house?

[frankfront]

Frank here again.

Glad to sat Gledhill very co-operative. GT017 PHE for =A374 plus vat and carriage.

The flow switch looks fine too.

I was wondering about a compromise system. Please tell me what you think.

What led me to thinking about heat banks in the first place was the slow flow rate of DHW from 28-30kw combis and the desire to get a long life out of a boiler by using a HB for the CH.

Instead of HB for both DHW and CH I wonder if a reasonable alternative might be to slightly preheat the incoming mains water (7C at the moment) to say 20-25 degrees by running it through a PHE connected to a suitable part of the CH heatbank cylinder and then putting it through a combi.

What would the result be of putting this through a combi rated to give, lets say 12lts per minute at a rise of 35C if we expected it only to give a rise of 25 or 30 degrees.

Would this be a good way of getting 15ltrs/min plus of water at 50-55C out of a modest sized Combi?

Or one could take the whole 35C rise at slower rate and mix it down to a safer temp with a blender.

When the CH is off the incoming mains water is warmer. At the moment the flow rate from anything but a really big combi is going to be little more than a dribble. Anything 13ltrs/min and up of 50 - 55C water is fine for me . Mixes down to about 44 for a nice shower and hot enough to give a hot shot into a cooling bath to re-invigourate it.

We don't use much DHW but it would be good to have decent delivery when we do. I know the full HB thing will do that but this is a kind of Lite version.

Just wondering what the drawbacks of this idea are and how it might by improved or why it should be discarded.

Thanks again,

Frank

[John Rumm]

You can, with provisos. You would need to check the specs on the combi carefully to see if it has a maximum input water temperature - some don't like hot water up em.

You might be able to get enough preheating without a PHE - just by using a indirect cylinder and running the cold feed through the coil. A bog standard indirect cylinder may add 5kW that way - a fast recovery could add 20+.

Having said that, you are throwing away the best advantage of the heatbank - i.e. the ability to get decent flow rates of mains pressure water.

Well take the given spec and convert that to energy terms:

12 x 4200 x 35 = 1.764MJ / min

So, lets say you want water at 40, and the incoming main is preheated to 20:

1.764x10^3 New flow rate = ----------------- 4200 x (40 - 20)

= 21 lpm of 40 degree water.

Not especially unless you have the PHE and heatbank do most of the heavy lifting. In which case why bother with the combi.

(its a workable way of making use of low grade heat from other sources like solar thermal etc)

Or just take the preheated water into the cold side of a blending valve and the combi output into the hot.

So you need about 45.5kW of heating power available to archive this in the winter.

If you want the lite version just leave the rads off the HB and use it just for HW.

[Doctor Drivel]

Good price. The Swep is great plate Heat exchanger for DHW as it is long and thin, rather than short and fat.

What is the point of this when the Heat bank can deliver high flowrates all by itself and uses a cheaper simpler boiler that will not cycle? You will gain nothing.

Most smaller combis deliver 10 to 12 litres/min. Middle range 13 to 16, top end 15 litres plus. There are the odd tiny 9 litres/min jobs still about, however combi performance has increased dramatically in the past 5 years, to the point they must be the first consideration in the average British home. They are highly cost effective, even the top end quality models.

[frankfront]

Thank you very much for those comments.

Of course combis don't do vented as far as I know for the CH so if I used a HB off a combi for the CH presumably it would need to be heated via indirect coil or another PHE. Is that correct?

Thanks again,

Frank.

[Andy Hall]

They even defy physics these days.

[Doctor Drivel]

Matt, have you been drinking this time of the morning?

[Doctor Drivel]

Frank, I think you mean a CH buffer heated via the combi and use the combi for DHW only. Yes, you would need a fast recovery indirect cylinder or a pump and plate heat exchanger to heat the CH buffer cylinder.

[Doctor Drivel]

If using a combi and a CH buffer, then the cylinder size must be the total system volume plus 25%. This means in the morning a full cylinder of hot water is waiting to be dumped into the rads giving instant CH heat up. If the combi is used for DHW the 25% extra will buffer the heating untill the combi reverts to re-heating the cylidner.

[frankfront]

Thanks for that.

I can't find any ready reckoner for radiator volume. Without doing a drain down at this stage. If we take an average radiator of 600x1000 double convector multiply by 13 and add the volume in the combi and a bit for pipe work what sort of figure will I end up with?

Now they really tricky bit. Which boiler?

Given that I want this thing to last for as long as possible I suppose the best thing is to be prepared to pay a bit of a premium to get a make with a good track record, decent reputation for spares supply and ease of service and repair.

Boderus looks a possible or should I avoid the non SS heat exchanger?

Any thoughts?

Thanks again,

Frank Particular makes to avoid?

[Doctor Drivel]

The average modern system is 80 to 100 litres

If heating a heat bank "directly" then I would go for SS - and use a Glow Worm HXi or Vaillant equivalent open vented boiler. If indirectly, I then any open vented, SS or alum'. If having CH off the cylinder then have a Magnaclean filter on the CH return to the cylinder.

The Buderus 600 range is v good. Buderus is owned by Bosch and Worcester-Bosch service people attend, so no problems there. Buderus is a large outfit on the Continent, much larger than Worcester. Eventually the name Bosch may replace Worcester and Buderus.

[frankfront]

Thanks for the info.

I'm attracted by Buderus because they look well made and as if id would be straightforward to replace components. S you say, a big outfit so parts should be okay to fnd at reasonable prices. For instance I think I read somewhere that the HX was about =A3125 and came out on releasing some screw-clips. You mentioned the 600 range but assume your comments go for the 500 range as well.

On another tack: I've fitted half a dozen boilers and CH systems in the last decade to my own properties. I've done everything myself and got a chap to certify the gas.

Never had any problems (Vokera mostly) but have always assumed that because I'm not Corgi somehow my workmanship would magically be deemed defective and no warranty honoured.

Am I right in continuing to assume this or do you have a cunning work- round?

If you've time:

I've never had reason to use blending valves before so don't know what to expect. One possible (ironic) problem with my proposed system for pre-heating the cold (often 6-8C) mains water going to the combi - to, say, 20C - is that it might easily heat much higher. Are blending valves reliable enough to set one down stream of the PHE on the mains supply to the boiler to mix in more cold and always send the water to the combi @ 20C +/- a degree or two?

Not sure what it would do to the combi long term to send it anything much above what it might have to cope with on a hot day in Spain for instance - say, 25C or a bit more. My expectation is that the combi won't like just topping the heat up say 15 degrees (If it comes in from the PHE at 35C) without waste or perhaps damage.

If it won't mind then all well and good. Very flexible system.

Comments much appreciated as usual.

Thanks again,

Frank

[Doctor Drivel]

I'm attracted by Buderus because they look well made and as if id would be straightforward to replace components. S you say, a big outfit so parts should be okay to fnd at reasonable prices. For instance I think I read somewhere that the HX was about £125 and came out on releasing some screw-clips. You mentioned the 600 range but assume your comments go for the 500 range as well.

On another tack: I've fitted half a dozen boilers and CH systems in the last decade to my own properties. I've done everything myself and got a chap to certify the gas.

Never had any problems (Vokera mostly) but have always assumed that because I'm not Corgi somehow my workmanship would magically be deemed defective and no warranty honoured.

Am I right in continuing to assume this or do you have a cunning work- round?

I've never had reason to use blending valves before so don't know what to expect. One possible (ironic) problem with my proposed system for pre-heating the cold (often 6-8C) mains water going to the combi - to, say, 20C - is that it might easily heat much higher. Are blending valves reliable enough to set one down stream of the PHE on the mains supply to the boiler to mix in more cold and always send the water to the combi @ 20C +/- a degree or two?

[Doctor Drivel]

I've never had reason to use blending valves before so don't know what to expect. One possible (ironic) problem with my proposed system for pre-heating the cold (often 6-8C) mains water going to the combi - to, say, 20C - is that it might easily heat much higher. Are blending valves reliable enough to set one down stream of the PHE on the mains supply to the boiler to mix in more cold and always send the water to the combi @ 20C +/- a degree or two?

[frankfront]

Thanks for that.

Keston say they don't do a combi that will work unvented but no problem.

Most of the ones I've looked at seem to tolerate a high input temperature for the CW supply eg 50C.

One thing that might spoil the party a bit would be a flow restrictor in the boiler that couldn't be opened enough or was fixed. This might limit the flow through the combi's own HE even when there was still some slack in th Delta T.

I like the Keston Combi but don't need 36KW Boderus we've mentioned as possible. I has an adjustable flow regulator.

One thing I can't seem to get a straight answer from on the data sheets is the extent or otherwise to which condensing combis condense when they are in DHW mode.

Are some better that others in this respect.

Many thanks again,

Nearly there I think...