Decision on thermal store or not

[sphannaby]

Looking for advice from @Nickfromwales or some of the other experts.

I had originally gone over my plans on ebuild and took the advice to go for a thermal store with a system boiler.  Things have slightly changed now and I'm wondering if I need to adapt my plans based on this.

My setup is as follows:

2 UFH manifolds (downstairs one is about 2 metres away from boiler/TS location, upstairs manifold is about 9 metres away from boiler/TS)

House is 85m² footprint with 2 storey.  I have 2 bathrooms which comprise 1 bath and 2 showers, only other DHW demand is kitchen sink and 3 hand basins.

Original plan as agreed on ebuild was to have a 30-32kW system boiler which would feed a 300 litre Telford TS (also have a 50 litre expansion vessel).  UFH manifolds would be tee'd into TS and there would be no need for any extra pumps as manifold pumps could handle the demand to pull the HW through.

Original plan was to also have Solar PV and this would feed the immersion in the TS via iBoost (or similar).

Now the idea of Solar PV is totally gone and I can't see it happening in the near future (if at all).

I planned to still go for exactly the same setup just forsaking the free PV feed to the TS immersion.  

I was happy with this until my chosen plumber is not able to do the work so i got another guy over to go through my design and price the installation.  He is telling me I am crazy going for a TS and should just have a UVC.  I stated one of my reasons going for the TS was the fact I wouldn't need any extra pumps to feed the UFH manifolds and he reckons a UVC would aslo be able to do this and I would be saving a lot of money and making things a lot more simple.

I got the feeling he wasn't comfortable with a TS and spun me a few lines so he could fit something he was familiar with doing.  I don't think I will go with this guy anyway and I am looking for alternatives as we speak I just want some opinions on if my original setup is the best to go for still so I can be insistant on this to the other guys who I get to price it up.

 

Any advice/ideas much appreciated.

 

Cheers

[Temp]

I assume this is a fully modulating gas boiler? If it was an oil boiler I would go with a TS. If gas I don't think there is much in it between a TC and a UVC. I don't see how having a TS reduces the number of pumps.

[sphannaby]

Sorry should have stated yes it will be gas system boiler.

I thought from other discussions by using a TS it would provide enough flow/pressure to the manifold that the UFH pump that is connected to the manifold would be enough to draw through required water.

What I was wanting to know was would a UVC provide sufficient flow/pressure to do the same or would i need an extra 'supply' pump at the UVC to get it to the UFH manifolds?

[PeterW]

Slightly lost here as to what your plumber is suggesting..?

you would need two circuits - DHW and UFH - with a UVC so the boiler would directly connect to the floor when the UVC didn't need heat.

The UVC stores a bulk of hot water and does not have a secondary coil you can use to pull heat from it. 

I am not sure the plumber is au fait with underfloor heating ...

[Nickfromwales]

What peter said.

Ok......

A TS is basically a huge round radiator, full of primary central heating water, and stores zero dhw. Dhw is produced instantaneously via a coil which sits at the top of the TS which is simply heated by the surrounding water. The heated primary water can be drawn off via dedicated tappings, the same way heated primary water enters the TS from the boiler, and your Ufh would simply be drawn from those points. There is no 'pressure' other than the static primary pressure which would show on the gauge of your system boiler, which is simply showing that the system is sealed and full of water, aka pressurised. There is no force available to 'push' water to the manifolds, that circulation would be provided solely by the manifold pumps. 

For eg, if you connected the manifold flow and return to the TS, then unless it was by convection, no water would flow through the pipes as the body of water would be unified and therefore be at the same potential. Even though the boiler pump is pushing water to the TS, it's also pulling back at almost the same potential so that creates a circuit of flow to and from the TS to the boiler, only. To create external  ( beyond the TS ) circulation you need to fit a pump, hence the ones on the manifold. @Temp spoke in detail ( on EB ) about this need for a second pump still remaining with certain manifolds / manufacturers, but tbh I still am unsure as to why the manifold pump wouldn't draw through regardless. Maybe Temp can reiterate that so we can identify the type of manifold which may need the secondary circulation pump. 

An UVC is the opposite of a TS. It has a coil, fed by the heat source eg boiler, which heats the tank full of clean drinking quality 'potable' water, the water that actually comes out of the tap. UVC's don't have additional tappings for removing heat as the coil which heats it simply couldn't deal with the additional demand, eg a bath being drawn plus all the Ufh calling for hot water to heat the house from cold etc. The TS gets heated directly via the open body of water so is far far better at getting the heat from the boiler into the primary water, plus it circulates it too, spreading the heat out evenly. The flow from the boiler also hits the top of the TS, which is where the dhw coil sits, so in essence provides a kind of hot water priority. 

This is why your second plumber is one is not consider using. He should have clearly stated all this to you as a reason for his suggestion, but I doubt if he knows what a TS is, let alone how it should be utilised. My bottom dollar is on him failing to explain that he intended to run your Ufh directly from the boiler, using 2-port zone valves, and a third zone valve for the UVC, as opposed to leaving you think that the Ufh could be fed from the UVC. 

If you were having Pv I'd have probably said upsize to maybe 400ltrs, but if you feed the TS directly with that size boiler you'll get instant, constant ( as in 24/7 constant running dhw ) from the 300. As it'll never be used as a heat battery from a Pv POV, pointless in upsizing and increasing the losses, BUT, if you do upsize it, drumroll please, you'll be able to run at a lower target temp and reduce the losses a fair bit so if overheating would ever be a problem that's one way to mitigate it ( say upsizing to 500L and running at 55oC, or 65oC at 400L ), Setting up for a lower flow temperature can allow you to ensure you hit the best condensing flow return temps and maximise boiler efficiency too, so this is a multi-faceted question which needs a bit more thought / input .

With a fully modulating boiler you could probably get away with running the Ufh directly from the boiler and fitting an UVC for dhw, as long as the minimum heat load for the house ( when the house is up to temperature and the boiler is only offsetting the heat losses from the house itself ) doesn't fall below the lowest modulating point. The only problem would be sizing the UVC exactly to your anticipated consumption. This situation would see the boiler running all sorts of different flow temps though, and would reduce it's efficiency quite a bit, so my recommendation would be....

400L TS fed from a 32kw boiler. 

Ufh fed from the TS.

Have the 2 circuits of Ufh flow and return pipework split immediately after the TS so the ground floor pipework can be isolated from the upstairs via dedicated two 2-port valves. These are required more to stave off convection circulation ( a killer with TS's ) than to offer control for the heating. You will also require an additional 2-port ZV between the boiler and the TS (i) to have control of the TS temp and (ii) to stop backflow of heat from the TS to the boiler. Set the cylinder stat to control that ZV and that ZV will then fire the boiler. That can be set to get the boiler to fire for bursts at a temp suited to get the best efficiency, rather than constantly idling to maintain temp.

Written on the move so hopefully makes some sense.   

[Temp]

 

@Temp spoke in detail ( on EB ) about this need for a second pump still remaining with certain manifolds / manufacturers, but tbh I still am unsure as to why the manifold pump wouldn't draw through regardless. Maybe Temp can reiterate that so we can identify the type of manifold which may need the secondary circulation pump.

Some ready assembled manifolds have (had) a bypass built in where the flow and return connect to the manifold. I believe this is to make them easy to add-on to a system that has rads as well. I think a separate circulating pump is only needed on manifolds that have this bypass and most don't.

Our manifolds came ready assembled from Osma UFH (who I think were taken over by Wavin?) and they did have this bypasses built into the mixer. I tried persuade the installer to remove them but I was told this would affect the warranty. When I asked Osma they told me a separate circulating pump was needed because the pump on the manifold couldn't be guaranteed to be sufficient to draw water from the TS. I wasn't convinced by their answer. 

In short I'm pretty sure a separate circulating pump isn't needed on the UFH side of most systems. You need one on the boiler side to circulate water from the boiler to/from the TS but if it's a system boiler there is usually a pump in that. 

 

[sphannaby]

Got my thermal store from Telford and they have forgot to install a pocket for the thermostat to go into!

TS has to go back to be re-worked and then sent back out to me.  Caused a lot of problems as I have now had to put back other trades because I can't have the plumbing ready.  The joys of home building.....

 

@Nickfromwales with regards to to your comment on using a 2 port zone valve to stop backflow from the TS to the boiler.  Am I correct in thinking that the zone valve would be wired direct to a one channel programmer.  Then I would have a spare set of contacts within the zone valve to give a boiler enable signal to my system boiler?

Also with the 2 two port zone valve you mention to isolate the UFH.  Would these be connected onto the downstairs supply and return 22mm manifold feeds or would it be one valve for the upstairs manifold supply and one for the downstairs manifold supply?  ALso what would be the logic on controlling these?

Hope this makes sense???

[Nickfromwales]

Regarding the pocket that's a schoolboy error. Good to see they've actioned the correction.

The 2-port between the cylinder and the boiler is a part of the G3 requirement. The cylinder stat controls the ZV, and the ZV ( orange and grey wires ) control the boiler for demand. 

One ZV on each flow ( 1 up and 1 down ) will stave off convection circulation, and are of paramount importance. 

Controlling them is easy. You send the pump feed from the wiring centre to the brown and orange ( linked together ) of the ZV, and the grey connects to the manifold pump live. 

That way the pump starts only when there's an open water path. 

Ok with that? 

 

 

[Nickfromwales]

Order of controls is :-

 

fused spur

time-clock

stat

zv

boiler. 

 

[sphannaby]

got that @Nickfromwales, cheers again.

We got it up and running last week before I came away to work and it was running very smoothly.  I did\t wire full logic as I was just using stat to control everything and using the ZV manually just to get it commissioned.  I'm going to knock up a wiring diagram while I'm at work as I'm doing all my others too.  I will upload a copy to see if it meets with your approval.

 

One question I have remaining.  Can you explain what the issues are with convection circulation when related to a thermal store?  Bear in mind I'm a thick sparky and don't know anything about convection issues in any type of heating circuit.  I understand if this si not a simple thing to explain and I may need to do some more digging myself but from my google searches I don't seem to be bringing anything up of any substance.

 

[Nickfromwales]

No probs re the wiring diagram

Basically a TS is usually heated or hot 24/7. Pipe runs from the TS normally go from the upper 'flow' tappings and return again via the lower 'return' tappings. Any run of pipework from those points will inevitably rise or fall along the way, and that makes convectioncirculation possible eg there will be a 'draw' of heated water from the TS and it'll circulate through the pipework completely naturally. Same way that old heat only back boilers would heat a copper tank, eg via two large bore ( 28mm ) circs which required no pump to get the heated water up and back. Quite often these systems had basic central heating systems connected via 22mm pipework and that had a pump to get a higher velocity flow. Quite often there would be a customer complaining that radiators were heating up ( upstairs only ) when only hot water was selected ( eg summertime ) and that was caused by this convection crawling through the pump. An anti-gravity ( single check non return ) valve would stop this with ease. The additional ZV's I mention for each Ufh circuit you have provides the same anti-gravity mitigation, but selectively, ( as in you can run upstairs whilst downstairs is off and vice versa / all off / all on ) via electronic control. 

Clear as mud?

[Nickfromwales]

Better explanation here