Would like a thermal store but advised otherwise???

[thompo]

Hi guys,

Im researching what route to go with for our heating system. Currently only the rads we are keeping are still in place, everything else is being replaced and we need to finalise this so it can be plumbed before getting insulation/screed done.

I like the idea and flexibility of a thermal store but im told they are only worth having if your using a stove/fire back boiler or heat pump?

We would like to utilise the back boiler on the fire but pipe work to the new tank location is difficult and unlikely to produce the required gravity circuit. Realistically this is only going to be used at weekends or a evening for a few hours so dont know if its worth considering.

Given the following setup and limitations would you say thermal store or unvented?

Tank to be located in ground floor (bungalow) utility room approx 2.3m high, have a space 900mmx1200mm for tank and pipework, valves and pumps to fit.

Fire is 9m straight line from tank, more likely 15m in pipework terms. top connection on back boiler is 700mm above the floor. This means i need a tank max height of 1500mm so it can be raised enough to get a gravity circuit (is this correct??)

Heat sources will be
Oil boiler
Solar panel
Back boiler on open fire (if we can keep it)

Heat emitters
90m² UFH - approx 650m of pipe
1 towel rail
4 rads (3 in loft conversion, 1 in conservatory)

Hot water for 2 adults (at present) main requirement mornings and evenings.
1 Bathroom with drench shower (unlikely for both bath and shower to be in use at same time), Kitchen sink, wash machine.

Given the above, ive been told to get a thermal store because they are great for UFH and i "Should" be able to get the back boiler working using a pump (i dont think this is good practice if the power goes out).

Other hand ive been told to go unvented as its more straight forward, takes up less space and has a faster recovery time, downsides are no back boiler and ongoing maintenance costs.

Cheers for any info

 

[RWhites]

It is possible to have a thermal store linked to a solid fuel appliance via a pumped circuit rather than gravity. It isn't what I've got, but I know it was an option when I specced mine 6 months ago. Obviously there are safety considerations blah blah blah, but fundamentally it is possible as long as you provide a power-free heat dump for the solid fuel stove on a gravity loop.

I went to Heatweb / Specflue and got an Excel heatbank. Overall I'm very happy with it, and I'm a big fan of the clever plumbing gizmos to provide seasonal efficiency / maximise boiler efficiency / flexibility for future alterations / hot water provision - all in a relatively neat package. Having fitted mine, I can't imagine 'building my own' from a bare tank. I'm quite competent as a plumber, but their 'off the peg' solution is really very neat - much neater than I'd have managed myself.

Having said that, if you don't end up using solid fuel / solar / heat pump, then they are a waste of money.

 

[dcawkwell]

Forget the back boiler. Waste of time and will never save the cost
of connecting it up.

Oil boiler and unvented cylinder. Sorted.

Air source heat pump also possible.

 

[thompo]

Ah i had not considered that,

We have a rad(s) directly above the fire in the loft conversion so this may be an option. Does a heat dump have to be separate from the main rad circuit?

Im guessing this works by using an electrical valve that diverts back boiler to heating coil via pump but when power is removed it springs closed diverting to the heat dump?

Would this be achieved using a 3 port valve to separate the coil and dump or just a 2port that would isolate coil?

thanks

 

[dcawkwell]

Ah i had not considered that,

We have a rad(s) directly above the fire in the loft conversion so this may be an option. Does a heat dump have to be separate from the main rad circuit?

Im guessing this works by using an electrical valve that diverts back boiler to heating coil via pump but when power is removed it springs closed diverting to the heat dump?

Would this be achieved using a 3 port valve to separate the coil and dump or just a 2port that would isolate coil?

thanks

If you have a radiator above connect that up to the back boiler as a heat leak radiator. You will need a header tank.
The heat pump will be best on a sealed system and will work well with the underfloor.
Don't like underfloor heating myself.

 

[SimonH2]

I like the idea and flexibility of a thermal store but im told they are only worth having if your using a stove/fire back boiler or heat pump?

Yes, that's the sort of thing a lot of plumbers believe.

Heat sources will be
Oil boiler
Solar panel
Back boiler on open fire (if we can keep it)

Thermal store will be the simplest way to utilise all these.

Heat emitters
90m² UFH - approx 650m of pipe
1 towel rail
4 rads (3 in loft conversion, 1 in conservatory)

And it will nicely decouple the different requirements from those of the heat sources. In particular, it will mean you can run the boiler for fewer, longer burns so it will operate efficiently and cleanly - otherwise it will be short cycling pretty well all the time you have the heating on. Install the rads with a TRV on all of them, use a modulating pump (and no bypass valve) and you'll have a nice quiet and efficient system.

Hot water for 2 adults (at present) main requirement mornings and evenings.
1 Bathroom with drench shower (unlikely for both bath and shower to be in use at same time), Kitchen sink, wash machine.

It should handle that. However, in common with an unvented cylinder, it is reliant on your water supply - so you need to assess whether that has sufficient flow & pressure (combined) to supply your prospective needs. Eg, if it's restricted then your could find the shower cuts out when the washing machine fills or kitchen tap is used.

Other hand ive been told to go unvented as its more straight forward, takes up less space and has a faster recovery time

It's only more straight forward if you ignore everything it won't do for you !
Your thermal store will combine all your heat sources, and allow any of them to contribute to any heat load.
The unvented cylinder will only allow those sources connected to it to contribute to the hot water supply. It is possible to rig up complicated systems to get round this, but then you've effectively turned the unvented cylinder into a thermal store while taking all the worst parts of both to get a complicated and inferior system.

Ideally you need to find a competent heating system design engineer (note that this is not the same as a plumber) who can advice you properly. A plumber is someone who does plumbing, a heating design engineer is someone who understands heating system design - some people combine both effectively, but sadly it is my (limited experience) that there are far more plumbers who really don't understand the design side than there are who do.

Also, be aware that you will find a lot of plumbers who will tell you that there is never any situation where a thermal store is anything but the spawn of the devil - regardless of the situation.

 

[bolshy]

Nothing wrong with thermal stores. They can be used for hot water only and be connected to "renewables" the same as an unvented cylinder can. As unvented and thermal both rely on mains flow to provide hot water, and have similar cylinder heat losses, you need to establish what your incoming mains pressure and flow rate is. Then decide which best suits your needs. And whether you intend to use your store for heating too. I wouldn't bother with the back boiler on the fire, you will over complicate the system without sufficient gains for it, unless you plan to use a large thermal store for DHW and UF/CH and can use the back boiler to help heat it.
Personally, I'd say a thermal store is more flexible if you intend to connect all of your heat sources to it.

 

[SimonH2]

On the back boiler, are there any obstacles (eg doors) between fire and store location ?
You'd need to consult someone qualified to do the design, but I can't help thinking that if you choose a tall store and get the top near the ceiling, you should be able to get a metre or more of rise in the flow pipework. The return pipe would have to be at low level. This means it would be difficult to go around any doors as the flow pipe would need to go over the door while the return would need to go below it.

I assume no possibility of lowering the back boiler ?

 

[thompo]

Thanks, thats still my thinking that the thermal store looks like a nice efficient option just a bit unwieldy to fit and since pricing unvented, how much more they cost!

Ive attached a floor plan to show where the fireplace is with the tank going in the utility room.

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• thompo
• 12 Feb 2013
• 4

. We are still only at subfloor level, want to get plumbing 100% before putting the insulation/UFH and screed in.

Unfortunately moving the back boiler isnt an option. The fireplace was one of the selling features for the other half so dont want to risk damage to it by trying to remove the fire/boiler.

I removed a panel of the fire place yesterday and discovered something interesting at the back boiler...Its already pumped and the pipe work is not what i was expecting.

Ill get a photo later today but from what i remember top pipe exits chimney breast at a high level in 22mm and runs to the pump, exits the pump in 22mm and bends to the floor.
Bottom pipe exits in 28mm with a 28mm tee'd off going toward old tank location and a 22mm going along with the top 22mm out through a wall where it was connected to the radiator loop.


Thanks

 

[thompo]

Here is a few shots of the current back boiler setup.

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• thompo
• 26 Feb 2013
• 1

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• thompo
• 26 Feb 2013
• 1

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• thompo
• 26 Feb 2013
• 2

From what i can see, the back boiler was connected to the radiator circuit, no valves etc apart from what you can see here.

It was on an open vented system, the bottom pipe in the picture is 28mm from the wall and this turns via a Tee and exited the front of the fireplace below floor level and ran towards the old tank location. The other side of the Tee was 22mm connected to the radiator circuit.

Does any of this look like it ever had a gravity circuit in operation???

 

[RWhites]

Does any of this look like it ever had a gravity circuit in operation???

I'm no expert, but that doesn't look remotely like it has anything to do with a gravity circuit (the pump is a bit of a give-away!)

Presumably there are some pipes running into the loft from the fire back boiler? - Vent & heat dump? If there is an existing fire back boiler, there should be some way for it to dump heat in the event of a power cut. And vent on a day-to-day basis.

Looking at your floor plan, and re-reading your posts.... on a practical level, how were you planning to get the pipes from fire back to thermal store? And for that matter the rest of the pipework that is not UFH (What I mean is where do you plan to run them?) Assume for a moment that you can employ a pumped ciruit for this purpose, so you can ignore trying to have a gravity circuit in operation. It worries me slightly that you plan to bury the whole lot in concrete...

Again, I'm no expert, but I suspect it will be impossible to set up a working gravity circuit to get the fire-back water from (a) to (b).

Heatweb / Specflue have an excellent 'online system designer' tool. It can be a bit clunky, but it is worth persevering, and their pre-sales technical people are really very helpful when trying to fine tune system requirements (and budget!)

 

[thompo]

Yep thats my thinking too.

We bought the property without a copper cylinder and damaged pipe work (cylinder was stolen) so no idea what worked and how so we're starting from scratch.

The pump and the way the pipe work runs would say its never been gravity. There is/was a vent in 28mm that runs off the bottom pipe towards the old tank location. As for heat dump it was plumbed directly into the radiator flow/return from the boiler but i dont think it would have dumped any heat unless the pump was going!

Im thinking for ease of plumbing and future safety the back boiler is not going to be feasible.

 

[bolshy]

They used to plumb solid fuel back boilers to the DHW cylinder, with a pipestat fitted to start the circulating pump when the primaries into the cylinder became hot. The primaries would be teed to the CH circuit with standard F and E pipework connected. The circulator then dumps the heat around the radiators too, with everything warming up together. There would usually be an override switch to start the circulator from cold. No zone valves, very simple controls. Went to look at one which needs the back boiler replacing yesterday (a small leak after 30 years use), the old boy gave me a tour of his system, including all the different coals and biofuels he had - he was proud of it.
His HW cylinder was at the same level as his back boiler. The convection in the back boiler and cylinder must be enough to generate some flow. I didn't see it working though, but he'd used it for the past 30 winters.

 

[thompo]

I think that is our system you have just described .

There is a pipe stat hanging in proximity of the old cylinder lol. The pump also appears to have an over ride, the swtich above it looks to be connected to it.

I take it this isnt that efficient as the back boiler is heated by the oil like a radiator unless its putting out more heat than the oil boiler?

Does the rad circuit in effect act as the heat dump, assuming there is some non electrical flow?

 

[bolshy]

It's probably just an old original system. It would need to be properly circulated around the rads for obvious reasons, unless you can re-pipe to give it a decent gravity flow. They take a while to warm up, but the problem is, they don't stop warming up unless you can ditch the heat produced. Your gravity circuit should be used for shutdown/warmup purposes really, pumped at all other times.