Thermal Store and COmbi Boiler for DHW

[Worcester]

Looking for a solution here, anyone any ideas?

We've got a cllent that has a combi boiler that can't take pre-heated water, no room in the property for a new unvented cylinder so can't convert the combi to a system boiler, and we've got a vented thermal store that gets intermittently heated from a solar source.

We only want to use the thermal store for DHW as there isn't enough input into it for space heating, and it won't be hot all the time and has a limited heat input.

We are happy to have blended water out at 48° as there is no store of heated water legionella and G3 are not an issue.

So - how can we taken the heat out of the thermal store to heat up the incoming water for DHW and when that runs out switch over to the combi?

Here's what one of our apprentices has come up with, can anyone else suggest a better way to manage to possible sources of DHW one which is finite, and what other controls are missing?

"The tank top stat will control S1 & S2. If the tank is above the stat value ( 48c ) then S1 opens and heats the HEX, if below...S2 open and the boilers fires. There is a flow switch where DHW is on the drawing which is a live feed to the stat...then DHW. Both valves are normally closed.

Valves will be potable solenoid valves (not 2 port C/H valves)

"Blending" valve will be a standard TMV"

Obviously needs a check valve on the incoming mains supply
And considering using a Gledhill GT152 pump controller instead of the simple on/off stat for better control - better ones?

I can see a couple of challenges with this set up - what would you guys suggest?

 

[Ermintrude]

Jesus! What's the payback 36 or 40 years?

 

[Nostrum]

Have to ask, why? Is the store pre existing? Surely this won't be RHI compliant?

 

[Worcester]

It's not RHI compliant, and not even considered to be 🙂 (if it was solar thermal actually it could be!! - it's simply a dedicated solar tank) - The heat source is Solar PV via an ImmerSUN (6kW of PV on single phase) so plenty of spare energy to go into 2 x 3kw immersion heaters in cascade - one at top of cylinder one at bottom (not shown on sketch)
Payback if had to pay for all the parts is estimated about 5-6 years over saving gas completely from beginning of May - End of September plus assistance throughout the rest of the year.

Most of this is coming out of the parts bin! - Store pre-existing so no purchase cost 🙂 PV pre-existing, we install loads and loads of ImmerSUNs so have a few development versions here from the manufacturer, so cost is a bit of pipework, a control system and some valves... so actual payback 1 -2 years and on oil possibly less 🙂

I am aware of Grant's Combisol though at a trade price of just under £200 (retail £300+) looking to see if with what we have in the parts bin we can do it without a capital outlay 🙂

It's certainly got the apprentices thinking caps on - and seems to have stumped everyone here as well 🙂 !!

 

[Ermintrude]

It's a bit more of an electrical solution. Firstly a seasonal switch is required to disable.

Which boiler is it feeding?

 

[Killy Bing]

I would say a check valve on the hot from the boiler incase s2 fails.

other than that it looks good
as ermi says you want some control over it for the time of year

a timer of which you can set the system to use the combi, disable the immersions for a period for say using the washing machines on the pv ect ?

 

[Ermintrude]

I'm thinking use solar with relays to shut out phex on boiler but yet to hear which boiler it is!

 

[Worcester]

I would say a check valve on the hot from the boiler incase s2 fails.

other than that it looks good
as ermi says you want some control over it for the time of year

a timer of which you can set the system to use the combi, disable the immersions for a period for say using the washing machines on the pv ect ?

Check valve def needed, the ImmerSUN is a proportional control device that only sends 'excess' power that would have been exported to the immersions, so the washing machine and other usage is all automatically dealt with, also on lower output days say generating 1.5kW and base load is 600W, then it just sends 900W to tyhe immersions (clever stuff 🙂 and we always install them on every PV system IF the client has a DHW cylinder and immersion.

Boiler details to follow.

The above works, thou could be supplemented by some kind of control system same as the Grant combisol does otherwise on lower generation days if the tank doesn't get up to 48° the output will be 'wasted' and if we can on those occasions somehow blend down to 24° for the combi that would be ideal - a switchover solenoid perhaps. and move the existing TMV to blend the total output. i.e just reconfigure somewhat...

 

[Worcester]

I'm thinking use solar with relays to shut out phex on boiler but yet to hear which boiler it is!

GlowWorm CXI 30

 

[Ermintrude]

Oh so they gonna need a new boiler soon anyway! I would possibly suggest an intergash as you can easily utilise a hot inlet.

Why can't you pre heat? The Thermistor is on outlet of hot and flow switch on inlet?

It requires no mods to boiler. ? She's not allowed over 65 and the Thermistor will shut out burner once at 58? So initially to combi will flick in but then it will drop out as the store pre heats? I would heat store to max of 85c run it across plate so the plates always hot and ready to go - use one of those grunny smart pumps with a remote pocket stat on hot outlet on plate? Bring cold via a double check into plate then tmv down on to DCW inlet on boiler at max of 65c.

Alternately get a DHW hydroblock from a 105e and use this to control the thermal store. Use the pcb as control system. Basically short the aps remove the gas valve and set up a trick voltage on the rectification. You will then have temp control, pump control and ability to set timer for use. I would suggest u use the instant but the pre heat is horrific. ? Just a thought? Box it all up in an ip65 box and fix hydroblock to a bar?

 

[Worcester]

Oh so they gonna need a new boiler soon anyway! I would possibly suggest an intergash as you can easily utilise a hot inlet.

Sorry mate not a gasser what's the English code translation for an intergash 🙂 (and yes he knows that he'll have to have a new one soon)

Why can't you pre heat? The Thermistor is on outlet of hot and flow switch on inlet?

Only 'coz Glow-Worm said couldn't 🙂 But I guess they don't know their boilers that well and how people actually use them 🙂 Just 'no' is the easy answer!

It requires no mods to boiler. ? She's not allowed over 65 and the Thermistor will shut out burner once at 58? So initially to combi will flick in but then it will drop out as the store pre heats? I would heat store to max of 85c run it across plate so the plates always hot and ready to go - use one of those grunny smart pumps with a remote pocket stat on hot outlet on plate? Bring cold via a double check into plate then tmv down on to DCW inlet on boiler at max of 65c.

Even I understand that 🙂 So ignore Glow-worm on the preheat words and go for it. I knew the double check valve was missing 🙂 And means we can dispense with the solenoid valves (S1, S2) 🙂 Simples.

Alternately get a DHW hydroblock from a 105e and use this to control the thermal store. Use the pcb as control system. Basically short the aps remove the gas valve and set up a trick voltage on the rectification. You will then have temp control, pump control and ability to set timer for use. I would suggest u use the instant but the pre heat is horrific. ? Just a thought? Box it all up in an ip65 box and fix hydroblock to a bar?

Come on Ermi, give me a Biomass boiler with iffy flue gasses or a Heat pump with high pressure faults and I'm Ok, now this in a new language 🙂

Presume you mean the Baxi 105e and how does the hydrobloc work on that ? - just point me to a web page or pdf and I'll be fine 🙂 and you chose that one because it's brass not plastic like on some models ? Point me to the spec of the control system / pcb as above (web page / pdf) and I'll have a go at figuring it out.

Many thanks

 

[Ermintrude]

Will a schematic , basically your chopping off the burner and adding your thermal store in. Move the overheat stat on to Primary flow from cyl. Also I would be putting a tprv on the hot outlet of the whole system along with an overheat stat to shut down each unit sequentially ?

 

[Ermintrude]

ittRun expansion 10mm back to base of header fit a new Aav blank prv blank 3/4 outlets to ch and bobs related to your mother

Pcb is solid apart from the fan relay..... But you don't need it

Your gonna need to link out pressure switch

I'm actually going to mod this up for u! Alpha 24e is the same I think.

 

[Killy Bing]

Intergas or atag can take solar pre heat 🙂

You'll want an expansion vessel between the s1 solenoid and tmv 🙂

 

[Ermintrude]

Intergas or atag can take solar pre heat 🙂

You'll want an expansion vessel between the s1 solenoid and tmv 🙂

Hense my advise for an intergash! But a duo tec 2 will also take a pre heat as will a logic plus..... How do I know this....... The manufacturer says you can use a combi save - the unvented DCW pre heat for ultimate flue salvage !

 

[Nostrum]

Really all that is needed is a TMV that can take +25 degrees cold water inlet temp, but I haven't seen any as of yet?

 

[Nostrum]

or 2 TMV's, first can blend the solar water down to 25 degrees, second can take pre heated solar water @ 25 and blend with combi hot?

If solar is less than 25 then second TMV will open up enough to raise temp to set temp? Only downside is you will always be reliant on the combi to some degree, just less so in the summer

 

[Ermintrude]

What you need is a tbv thermostatic blending valve. Myson do a lovely one.

 

[Nostrum]

Got a link?

 

[Worcester]

or 2 TMV's, first can blend the solar water down to 25 degrees, second can take pre heated solar water @ 25 and blend with combi hot?

If solar is less than 25 then second TMV will open up enough to raise temp to set temp? Only downside is you will always be reliant on the combi to some degree, just less so in the winter

I've got another config with two tmv's temperature sensor and a pair of solenoids which diverts different ways depending upon temp of water <43 blend down to 24, >43 go direct...

Need to find that tbv from myson now to have another look.

 

[Ermintrude]

BSS do one it's for ufh

 

[Worcester]

BSS do one it's for ufh

We've got those on a couple of sites - better than some of the ufh tmv's.

I gotcha, you're suggesting blend down the temp to the hex and so the temp of the dhw rather than blending down the outlet of the hex ?

or use the tbv on the "potable" dhw? It's brass and rubber ...

 

[Ermintrude]

It's not suitable for potable.