Using a Telford DHW UV Cylinder as a CH 'preheat' Heatstore cylinder ?

[Jerry32]

I've registered onto this forum, essentially just to ask opinions on this idea.

I think I can use a Tempest 200L Indirect Heat Pump Cylinder Slimline – TSMI200HP/SL, in the opposite function to it's 'normal' setup.

i.e. The cylinder will essentially be the last 'radiator' on the return CH circuit, where instead of emitting heat, I'm going to use the immersion heater element to 'preheat' the returning CH water, before it exits out the top, on it's way to the CH inlet side of the Bosch 42CDi Combi boiler. (i.e. A CH heatstore).

The intention is to use up Solar PV power that at the moment is just being 'left on the roof', and help reduce the amount of LPG usage as well.

In summer the Combi fire up on HW demand in the kitchen, for all of 30 secs is mightily inefficient.

In winter, I'm only outputting a max of 45 degs C from the Combi to the CH radiators.

So if I can preheat the CH return by just 5 degs C, that would be 25% of the work the Combi currently does. (approx 25 deg C return temp).

If it turns out that it doesn't save that much LPG this coming winter, then I can always get it stripped back down, and setup in the 'normal' installation style and move to an ASHP instead, which is what this cylinder is intended for.
(I'm kind of loathe to jump straight over to an ASHP, even though I know from last winter that my CH radiator system, is large enough to cope with a 40-45 degs C boiler output temp).

 

The reason for choosing this particular cylinder, is that it also has the largest 'Heat Pump' coil at 3.3m2 surface area that I can find.

As I also intend to use this coil to 'preheat' ('prewarm' more likely) any DHW water, before sending it to the LPG Combi, for onward DHW heating, mainly to the kitchen HW taps.

My reason for reversing the usual setup, is that the LPG Combi is rarely used for DHW at all. It's main use is for CH during the 5 winter months.

So it seems a bit daft having a 200 litre DHW store, losing 2 kWh standing loss per day, when we could really do with more stored energy used in the CH side of things.

 

I've attached a sort of 'back of fag packet' drawing of the 'possible' cylinder setup. (well I tried to do this?)

Does this idea make any sense to people on this forum ?

The Duplex Stainless Steel spec used in the cylinder, and the 316L spec of the coil & fittings, should cope with fernox treated CH water ?

I don't think I need to use the DHW expansion tank setup, as it's only going to be at 1.2 bar CH system pressure, and the Combi already has a CH system overpressure coverage ?

The fact that it's not being used to store DHW, under pressure, I'm assuming means it does not need G3 installation either ?

Are there any other pitfalls you can think of ?

 

Reason :-

I've massively overspecced my Solaredge DC PV system, 36 x 400W panels, 2 x 6kWh Inverters, 2x 10kWh Solaredge DC batteries, I'm on Octopus Flux tariffs, and my DNO export limit is set at 4.4kWh.

So even on 'Sunny' Somerset's occasional gloomy days, there is quite a lot of power up on the roof, that the inverters do not need to convert to AC, because there is just no use for it.

This is what happens when you've got too much roof space, and don't see the need to pay income tax on your savings !

Telford HeaTelford Heat Pump UVC rough setup in reverse mode.pdft Pump UVC rough setup in reverse mode.pdfTelford Heat Pump UVC rough setup in reverse mode.pdf

 

[JohnMo]

No sure how your attachment is done, but it does not open.

1 hour ago, Jerry32 said:

've attached a sort of 'back of fag packet' drawing of the 'possible' cylinder setup. (well I tried to do this?)

 

[JohnMo]

1 hour ago, Jerry32 said:

200 litre DHW store, losing 2 kWh standing loss per day

If you store at Heat pump temps (48 deg) your standing losses are extremely low.

 

Your boiler efficiency improves with lower return temperatures, actually heating the water up 5 degs will drop boiler efficiency so be careful on that side of things.

 

1 hour ago, Jerry32 said:

The fact that it's not being used to store DHW, under pressure, I'm assuming means it does not need G3 installation either ?

Are there any other pitfalls you can think of ?

The UVC cylinder will require all the G3 bells and whistles, as the volume in the cylinder is a trapped volume, so will be pressurized to incoming water pressure. Because you are feeding to an additional heat source doesn't change anything, with respect the design premise of the cylinder.

 

I would use the cylinder as a normal UVC and let it supply your DHW only, set your boiler as priory demand hot water via the central side. To keep things simple just up the central heating flow temp a few degs at a time until you get the cylinder up to 47/48 degs. If you feel you have excess PV during winter pass the return water though a simple Willis heater.

 

Why,

1. central heating not on in the summer when you have lots of excess PV so everything excess can go in to DHW

2. Preheat to return leg of the boiler will drop boiler efficiency throughout whole heating season, when not much excess PV is available. If you do have plenty of the PV in winter excess, use a simple Willis heater.

3. Even upping the central heating flow temp to allow DHW heating with keep boiler inside condensing mode.

4. You need to do G3 regs so do it for proper DHW.

5. Preheat water requires additional diverting valves to stop boiler firing every time any hot water tap is opened.

6. UVC cylinders are way better than combi for water delivery time and flow rates

6. Its simple

[Jerry32]

Hi JohnMo,

I'd read some of your replies to other people, so was sort of hoping you might pick up on this thread, I don't know why the site wouldn't let me add a simple jpg, (I think it's worked this time.)

 

1. Yes that is my dilemma, but we use so little DHW in the kitchen / main bathroom, it's laughable really, hence my idea of flipping the usage of the main cylinder over to CH preheating use instead.
In summer I'm pretty much resigned to leaving a fair bit of power up on the roof, but in winter due to the Octopus Flux tariff, I can mix and match cheap rate electricity, with any solar generated electricity, to keep the cylinder warm enough to delay the re-firing of the Combi.

Well that was my intent anyway ?

 

2. I never thought of the condensing boiler efficiency to be honest, I'd only thought about delaying the re-firing of the Combi, so using less LPG than last year.

We actually ran the boiler practically on minimum fire position last winter, it stops heating at 43, and fires again at 28 degs C on the Combi display. (The house is roughly at 18 degs C, which is fine for us).

On sunny winter days, we don't always have the CH on constantly, just because of the passive solar heat through the windows. (Plus I've retrofitted shed loads of insulation / draught proofing etc.)

I've not actually figured out whether those figures are return temp, heat exhanger temp, or outlet temp, but most days, it was firing for about 1.5 minutes, as the temp display slowly rose to 43, then off for about 1.5 minutes, as the temp display slowly dropped down to 28 degs C.

(It didn't take us too long to realise, the now 17 year old, Bosch Combi 42Cdi when installed, was specced as per usual heating engineer calcs, and is way oversized for our usage).
My assumption was if the water returning back to the boiler took longer to drop down to 28 degs C, then the firing cycles would be less often, ergo, saving LPG gas usage ?

 

I've never heard of a Willis heater, but I'd originally thought about just using a simple 50 litre buffer tank on the CH return, with it's own Immersion heater to do much the same as above.

Then as with most things, there is always a tendancy to get carried away, in this case trying to 2nd guess when / if we fit an ASHP instead (mission creep), and it ends up being a UVC of 200 litres and a bit of DHW heating / preheating thrown in for luck ?

 

3. I think I need to research the condensing mode envelope of the Bosch Combi 42Cdi boiler, as I was likely running it too low last winter, even though the LPG consumption reduction from previous years was pretty impressive. (LPG is expensive to be honest).

 

4. G3 etc. I probably didn't explain it that well, but my intention was to use the actual cylinder on the CH circuit, not the DHW circuit, so I'd made the assumption that G3 qualifications would not be required, as it's really no different to adding a radiator to the CH circuit ?

(I'm not sure if my mate, a proper 'plumber' has G3, although he is quite used to my non standard ideas !)

 

5. I've managed to upload the 'back of fag packet' drawing this time, and I've allowed for a lot of isolating valves and NRV's, as I worked offshore in oil & gas, so do tend to like having loads of valves in a system !

 

Standing heat loss -

I actually got the 2kWh per day figure from the Telford website, so I've been 'using' it in my calcs, but I bet it's no more accurate than the Ford / Fiat mpg figures for cars ?

I have the feeling it was probably rated at 60+ degs C, and as you say, if the storage temp was lower, so will be the standing heat loss. I just haven't got to working out the reduction calcs yet.

 

6. I'm quite sure my mate would prefer me to do the simple setup, i.e. the DHW normal installation, but then he owes me a few favours, and I think he's well resigned to me overcomplicating things in the past, (as he's always said), but I do like 'proper traditional plumbing', along with loads of isolation valves, just in case for any potential 'issues' in the future.

(I can't change the habits of my offshore North Sea lifetime, along with it's 'Oil / Gas' leakage paranoia ?)

 

Would be interested if you think something like the drawing is in any way workable ?

 

I already accept that realistically I've gone well over the top on the Solar PV system, but then the roof space is there, so what else was I meant to do with it ?

Telford Heat Pump UVC rough setup in reverse mode.pdf

[JohnMo]

2 minutes ago, Jerry32 said:

solar generated electricity, to keep the cylinder warm enough to delay the re-firing of the Combi.

Well that was my intent anyway

So basically using the UVC in reverse as a buffer. I would plumb as a 2 port buffer. So basically it simplifies the install. The grey box is your boiler there is an oversized tee on the top connector, this forms a hydraulic nul point, after that you have a secondary circulation pump.

 

Water flow is to central heating first, zones closed flow is automatically diverted to buffer, no zones open around buffer only.

 

You control the secondary pump by the heating demand, you control the boiler by the cylinder thermostat. You are likely going to need a larger heating system expansion vessel to match your increased capacity.

 

How it works - So cylinder hot, house heat demand the secondary pump pulls from the buffer until the buffer calls for heat, then the boiler starts. Once house demand is finished secondary pump stops and the cylinder is heated ready to go. At the end of the heat season switch the boiler off, but it likely your solar excess will do that anyway.

 

 

The DHW would look like the top image below, with a Watts solar diverter. You need a cold water safety group and expansion vessel if not already installed.

Safety

There must be no valves (if valve the handle removed) between cylinder and heating system expansion vessel and safety group. Or you install the full safety group on the cylinder - this is a zero cost option as all the kit comes with cylinder anyway.

[Jerry32]

Thanks for the reply above, and I've now done a bit of research into Condensing Mode, 'efficiency' versus 'gas usage' reduction.

It appears that there is more in the way of gas usage cost saving, by reducing the output temperature from the boiler, so that it only 'just' maintains the house at the temperature you are comfortable with. (essentially manual weather compensation?)

Rather than there will be by maintaining a higher output temperature to get a better temperature differential, output to return.

So strangely, I might have lucked into the best cost saving mode last winter by keeping the boiler output temperature as low as practically possible.

Hence the large reduction in the number of litres of LPG that we used. It was nearly 30% less.

There's more than one article that I've read, that says condensing mode only really starts at below the ambient dewpoint temperature (which typically changes depending on the weather), but is generally below 55 degs C or less, and I'm well below that temperature, and the flue output appearance seems to indicate condensing mode was active.

The other thing seems to be that the 2nd (condensing) heat exchanger in the flue, is there to preheat the return water before it goes into the main heat exchanger.

This is exactly what I'm trying to achieve with the 'buffer' cylinder pre heating the return water.

So I'd like to think I'm on roughly the right track.

 

But, but, but, my mate 'The Plumber' has probably thrown a bit of a spanner in the works, as he's pointed out that if I use the UVC on the CH circuit, and then I eventually want to fit an ASHP after all of this, then the cylinder will really need to be put back into it's intended use, as a DHW cylinder, and 'we' (meaning him) would, as well as having to do a complete repipe, also need to flush out all the last traces of Fernox treated CH water from inside the cylinder. Before it's let loose on the DHW system. (No one wants a potentially Fernox tainted Hot Water shower).

 

So, it looks like I might be taking in your original comments after all, i.e. Stop mucking about, and just set it up to use the excess Solar PV to heat up DHW water, and live with it that way.

(I still might use it's internal high surface area heat pump coil, 3.3m2, as a bit of extra preheat on the CH return pipework, just to increase the duration between those Combi firing times).

 

This following article, on condensing mode in boilers, was probably the simplest to read through of all the ones I'd found, i.e. no complicated thermo dynamics involved !

trustedreviews.com/explainer/whats-best-temperature-condensing-boiler-set

 

Many thanks for all your comments, as it's certainly got me to where I needed to go in the end.

[JohnMo]

So a couple of wrong assumptions above.

 

Condensing mode is nothing to do flow temperature is all about return temperature and only return temp.

24 minutes ago, Jerry32 said:

really starts at below the ambient dewpoint temperature

The ambient dew point is the flue gas dew point. Which approximately 53 degs for natural gas not sure about LPG.

 

25 minutes ago, Jerry32 said:

the 2nd (condensing) heat exchanger in the flue, is there to preheat the return water before it goes into the main heat exchanger.

This is exactly what I'm trying to achieve with the 'buffer' cylinder pre heating the return water.

So I'd like to think I'm on roughly the right track.

The condensing heat exchanger is there to preheat water, but also to condense the flue gas, this is where the efficiency uplift comes from. The condensing of the flue gas also latent energy to be transferred to the water.

 

Now to burst your bubble, pre heating return water will stop or reduce the condensing effect and the efficiency gains you have made.

34 minutes ago, Jerry32 said:

flue output appearance seems to indicate condensing mode was active

The more steam visible the less likely you are condensing, the condensing is occurring as the flue gas hits the atmosphere, not where it's useful.

 

36 minutes ago, Jerry32 said:

use it's internal high surface area heat pump coil, 3.3m2, as a bit of extra preheat on the CH return pipework,

Don't - see comments above

 

Read the article it's a little simplistic so misses much of the details of what's really happening.

 

Here is a better one to read

https://www.heatgeek.com/condensing-boilers-efficiency/