Posted By: clawlorit's an opportunity to get the most efficient system possible for the flat.

Is there no way you can reduce the huge heat load - that would be by far the most efficient starting point - fabric first!

Posted By: clawlorWe cannot insulate the walls generally (Grade b listed) except in one small area.

Does grade B mean you can't do internal wall insulation (IWI)? Genuine question.

From the heights quoted it sounds like they're large rooms so the floor area loss would presumably be less of a problem than in some cases.

what sort of boiler is it? At this point from the description of the property I would assume gas!

22kW is not a big boiler, just that these days a heat load of 22kW is considered quite a lot after a referb., but given your description of the building perhaps its understandable.

A thermal store (TS)won't reduce the boiler size much it will just even out the heat load seen by the boiler, you may get a reduction in boiler size (a bit) but that would need a large TS. e.g. a TS large enough to run most of the evening heat load with the boiler running as heat input, but overall a deficit in available heat with the boiler then running through the night playing catchup ready for the morning higher load would give you the chance to have a smaller boiler. But the cost difference in boiler size won't come anywhere near helping to pay for the TS.

(For wood burning you need a bigger boiler with a TS as you batch burn to carry through until the boiler is lit again):

I'm not an expert on gas heating but I would have thought a DHW tank run from the (gas) boiler with the heating directly off the boiler might be a better option. If I have got this wrong - someone shoot me down (gently).

Again if its gas a 22kW boiler is a bit small with out a DHW tank or otherwise if you have a TS plus heat exchanger (=cost) to supply the DHW then as far as I can remember 3kW is a standard assumption for DHW (Again someone shoot me down (gently) if I'm out of touch here)

You can do away with tanks altogether with a combi boiler (again assuming gas) but here a 24kW or 28kW is usual if there is only one bathroom in use at a time.

A lot of modern boilers (gas) will modulate so this will tend to avoid short cycling.

Posted By: clawlorWe will have a large TS, we think about 280/300l

300 L is a small thermal store, even if just used for DHW!

If your heat load is 22 kW you won't get boiler cycling!

Posted By: clawlorwe are insulating to at least current building regs

That is the absolute minimum that you are legally required to do. You have no option about that. This is the green building forum where people are generally trying to do a lot better.

I would suggest taking a pause and doing a lot more reading around the subject before finalising your ideas.

clawlor, SAP does indeed allow for TS (the latest version of SAP) however it will not demonstrate any great benefits, if you are hoping for some?
As has been said above, insulate insulate insulate to reduce your CH demand where ever possible! Of course no amount of insulation will reduce your DHW demand....
Good luck..

Posted By: ringiPersonally if you don’t need to run more than one shower

One shower at a time that is
However if you do need to run 2 showers at once then you can get combies up to 40kW but you need to make sure that they modulate down low enough to avoid short cycling on the CH

Thanks JonG, that's an interesting option, using the heat pump cylinder. I will have a look at manufacturers information. I've also heard that the boiler can be set to run at lower temps for longer periods but then I don't know if this would provide enough heat to the TS. We can't have a boiler that doesn't run in condensing mode!
We initially liked the TS because we would have the option of future PV.
Thanks ringi, TPI thermostat it will be!

Hi Clawlor, with the suggestions I have put forward you would do away with the TS, it is an expense you don't need and will have standing losses to account for.

PV can feed the cylinder far more effectively by heating water over the summer when generation is plentiful, it is pointless loading a heating TS over the summer, it won't retain the heat long enough and in teh winter the contribution will be neglible.

TPI is not as effective as Opentherm, and can if set up incorrectly badly mess with a modern modulating boiler, have a look at OT first but get an installer that understands both.

<blockquote><cite>Posted By: Peter_in_Hungary</cite><blockquote><cite>Posted By: clawlor</cite>We can't have a boiler that doesn't run in condensing mode!</blockquote>
To be pedantic - you can. All boilers in the UK now have to be condensing boilers.

I know, I just meant it would be pointless/inefficient to not run in condensing mode. I've heard 20-30% less efficient.

<blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: clawlor</cite>We will have a large TS, we think about 280/300l</blockquote>
300 L is a small thermal store, even if just used for DHW!

If your heat load is 22 kW you won't get boiler cycling!

<blockquote><cite>Posted By: clawlor</cite>we are insulating to at least current building regs</blockquote>
That is the absolute minimum that you are legally required to do. You have no option about that. This is the green building forum where people are generally trying to do a lot better.

I would suggest taking a pause and doing a lot more reading around the subject before finalising your ideas.

We're aware of our obligations under the building regs and to the wider environment.
You should read the subject of the discussion and the question!!

Clawlor – we have a 24kW wood pellet boiler with a 300 litre TS (which was what was recommended by the installer of our heating system). Our floor area is 180 sq metres. We do not have gas where we live - it’s either oil or wood here. In our experience the 300 litre TS is far too small – the only benefit it brings is that when the C/H comes on we have a rapid initial “burst” of heat whilst the boiler is getting up a head of steam. The wood pellet boiler takes about 20 minutes or so to get up to the operating temperature of 70C. The heat content of the TS full of 65C water would be exhausted in about 30 minutes if the boiler was not on.

I am no expert but I would say as you have main gas, I can’t see any point in having a thermal store at all.

Posted By: JonGSome boilers now have a 10-1 turn down ratio though so with careful choice it can be eliminated or a small buffer in the return can be employed if the chosen boiler doesn't ramp far enough.

JonG, I know of the Geminox gas boiler that runs down to 1kW (can run UFH off it directly), but what others are about?

Thanks. Vokera and Ravenheat? Mmmm maybe not. Geminox were pushing their boiler as a UFH boiler. They worked very well. The boiler only up went to around 10kW - modulating from 1kW to 10kW, so no cycling. I saw one operate on UFH only, directly, no rads or water heating. It only had one pump, normal 22mm zone valves for each UFH zone (I think they were motor on motor off; mo-mo) and a tamperproof high limit pipe stat set to 55C. Boiler was set to 40-45C, so was highly efficient at all times and modulated right down to 1kW. Each zone had a wall stat. It worked well. Very cheap to install and very, very simple. The DHW and upstairs rads were supplied by a 36kW Intergas combi. No stored water and two neat white boxes on the wall. A very simple, highly effective and reliable system. Far cheaper than a large thermal store or unvented cylinder. They had only one bathroom and shower room so no large DHW needs.

I am sure the 19-1 Viessmann will not be cheap.

Is this buffer on UFH? Open Therm and weather compensation is greatly misunderstood and ignored in the UK. Domestic plumbers do not get it, or understand it. It is not their game as drains and the likes are a world away to heating with its electronic control systems. Hence we are way behind the Continent where thermal storage, weather compensation, Open Therm, buffer tanks, low-loss headers, etc, are the norm. Vokera even sell a low loss header and call it a hydraulic separator.

Intergas are great boilers and built like tanks - and well priced. I may be having one fitted in my place within the next months, the 36kW model. The DHW output on the 40kW version is the same as the 36kW. Probably no weather compensation connected as I am having a Kickspace fitted and they need higher temps to operate.

Posted By: JonGCondensing gas boilers are not necessarily a good choice with a TS. In order to use a TS for DHW and SH it needs to be maintained at a high temperature usually requiring a flow off the boiler of 80 degrees, even with a 20 degree differential this results in operating temperatures above dew point which means that it barely condenses.

Jon I totally disagree with this, this sounds like an old wives myth. ;)
A number of points:

- having a tall thin cylinder (excellent stratification),
- a sizeable DHW plate heat exchanger with a modulating DHW pump or a Danfoss modulating valve to ensure the - return water into the cylinder is of a low temperature
- CH taken from an internal coil (prevents sludge build up as well),
- two stats to also prevent boiler cycling.
- spreaders on the boiler flow and return into the cylinder preventing stratification disturbance.

These will ensure the bottom of the cylinder is quite cool. Then condensing boilers work very well. In most of the reheat of the cylinder the boiler's return pipe will be receiving low temperature water.

The great thing about a thermal store is that high pressure water can be obtained from a low pressure cylinder. They are a great, and safer, alternative to an unvented cylinder. That beside the other advantage of being a buffer if CH is taken from the store, a superb neutral point, eliminating boiler cycling, etc.

Also many thermal stores operate between 70-75C. I have seen one operate at 65C using a large DHW plate heat exchanger. A well designed thermal store will work very well with a condensing boiler.

Another good point about a thermal store is that it gives not a hoot how big or small the boiler is. A large boiler can heat the store (very quick) and still work to optimum efficiency and not cycle. I know of a 50kW boiler fitted to a store that only needed around 18kW. The boiler was a cheap deal. I have also seen an 'undersized' boiler work very well on a store. There was a large extension (no new baths, so DHW the same). I said connect the extension to the existing heating system and see how it works. If it is cannot cope then replace the boiler only. It coped - just. The buffer aspects of the store made it work, always storing heat to be drawn out when needed. The boiler worked hard though. But when the house was up to temperature the store and boiler ticked over. What also helped was that the heating was was zoned, with rarely the upstairs on at the same time as downstairs for long periods. Parts of the extension were also zoned and switched out when not needed.

Thermal stores are not just for those doing solar and solid fuel stoves.