Thermal Store/Accumulator in an s-plan system

[thekidda]

Hi All

I currently have an s-plan central heating and hot water setup but I want to replace my old oil boiler with a wood pellet boiler. I gather from everything I have read that I should install a thermal store/accumulator tank for the wood boiler to burn efficiently but I don't know how this would fit into an s-plan layout. All diagrams I have seen that include an accumulator have no hot water cylinder and just have the hot water running through the accumulator, but I'd really like to keep my cylinder for various reasons.

Can I actually have a hot water cylinder as well as an accumulator? And would I have the hot water cylinder heated by the accumulator or directly from the boiler? My gut feeling is that I would have the boiler heating only the accumulator and then both the hot water cylinder and the radiators heated from the accumulator.

I have spoken to some heating engineers in the area but very few seem to know about installing accumulators. Those who do are expensive and want to charge just to come out and look at my current system. Obviously I'm happy to pay for the work but at the moment I just want to know roughly how it will fit together so I can budget for the job.

Thanks in advance

 

[desalman]

The comments are appreciated, but......What is the logic of 2000 being too big? I had assumed that the larger the store of hot water, the less firing time needed by the boiler, assuming of course that the temperature decay in the heat store can be minimised by substantial insulation.

 

[PlumbBobBob]

no you've got that round the wrong way. the larger the amount of stored hot water the larger amount of energy needed to keep it at the required temperature notwithstanding the amount of insulation. 2000l is enough for 8 baths one after the other without waiting for the water to reheat imho.

 

[ecowarm]

2000l would need a 40Kw boiler if you work on the ratio of 1:50

 

[desalman]

Many thanks for comments, I realised I had worded my last post rather badly. I meant to say that I had assumed that a larger heat store would reduce the frequency of boiler firing rather than less firing time.

ecowarm, where does the 1:50 ratio come from? I don't doubt you are right but am curious thats all.

If I go for an 800 L store that will be a bonus on cost grounds. !!!!!!!!

 

[ecowarm]

It is usual ratio used to size an accumulator with a solid fuel/ Biomass boiler. Normally the boiler is slightly under sized for the property. With a modulating pellet boiler the ratio/ accumulator can be smaller.

 

[peteheat]

Hi desalman,

I agree with ecowarm we also work on the figure of 50 litres of storage per Kw output of the boiler, a modulating pellet boiler will usually over run the requested heat setting, for example a setting of 70c will have the boiler peaking at 77 > 80c as it modulates back because there is a lot of residual heat in the boiler itself and the probes / sensors can't react fast enough.

As well as being a heat store the buffer irons out the peaks and throughs of the boiler output to enable the temperature you ask for to be delivered into the heating circuit, of course there are valves needed to mix the flow and return from both the buffer and the boiler, our preferred system is manufactured by ESBE.

We tend to use pellet boilers that we can control the output from, for example I prefer to be able to have a little reserve that we can call on if needed so in your case I would be opting for a 20Kw output that we can programme back to 15kw as this gives us that little bit of forgiveness for a heating system that may not be performing exactly as specified.

I have found that the heat loss calculations that can be made from various sources such as guides and software are based on ideal conditions, very few (if any) can allow for the different types of radiators, the house on a hill with all four corners exposed to prevailing winds etc so that little extra output can make a big difference.

If we look on our houses are containers that leak heat like a bucket with a hole in it and take your figure of 17kw (rounded up) and convert it to the theoretical bucket with a 17mm hole that we filled from a barrel (warm day) and put a hose 17mm diameter to keep it topped up the water is constantly running just to keep the bucket topped up, for some reason the hole expands to 20mm (extreme cold weather) but our filler hose is still 17mm the bucket starts to empty (temperature drop in the house) so the reserve of 3Kw / mm can be needed.

The benfit of the modulating boiler is we can over size a little especially in old houses without it costing us in fuel being wasted from the boiler constantly cycling, a pellet boiler can use up to 5Kg of fuel every time it starts up however a boiler with a maximim output 22Kw can modulate back to as low as 7Kw, in fuel terms that translates to using 4.5Kg per hour at max output down to 1.4Kg at minimum output.

In my view a succesful installation is one where the average ouput of the boiler is 75 > 80% of the maximum it can deliver as that means we have the boiler using the features that cost the money and the customer has a warm home.

 

[desalman]

So, it looks like an ATMOS 20kw boiler with a 1000 L heat store + all the bits (Laddomat etc ). I suspect Kolty might be getting an enquiry.

 

[ecowarm]

So, it looks like an ATMOS 20kw boiler with a 1000 L heat store + all the bits (Laddomat etc ). I suspect Kolty might be getting an enquiry.

Easy people to deal with and no problem with emails in english

 

[desalman]

Thanks ecowarm, I meant Kotly not Kolty.

 

[Nordio]

So, it looks like an ATMOS 20kw boiler with a 1000 L heat store + all the bits (Laddomat etc ). I suspect Kolty might be getting an enquiry.

With biomass boilers you calculate heat loss and ventilation losses. Lets say this is = to 20kw as in your case.
You then add on the following....
10% for manufacturer efficiency
10% for being with a cold garage
10% for radiator/underfloor heat up

So thats 20kw + 30% = 26kw!

Which is in line with CIBSE, Bre, Snipef, Fidhe and HHIC guidelines.

 

[ecowarm]

His heat loss is 14Kw

 

[desalman]

With biomass boilers you calculate heat loss and ventilation losses. Lets say this is = to 20kw as in your case.
You then add on the following....
10% for manufacturer efficiency
10% for being with a cold garage
10% for radiator/underfloor heat up

So thats 20kw + 30% = 26kw!

Which is in line with CIBSE, Bre, Snipef, Fidhe and HHIC guidelines.

Heat loss including DHW is estimated at a bit less than 17 kw.

No cold garage involved, boiler will be fitted in boiler room in main house between utility room and kitchen.

Rads have been calculated as a bit oversize using appropriate coefficients.

No allowance made for point source log burner in sitting room which will 'offload' the sitting room heat requirement when its lit during winter evenings.

I suspect the idea of a 20Kw boiler is a best bet, better than my 15Kw comment. The cost difference is not that much. A 1000 l accumulator matches the 1:50 ratio quoted and is an easier option than my original 2000 l unit, and considerably cheaper.

I anticipate using a 200 l DHW unvented cylinder fed from the CH system to the heating coil. There are only two of us and our DHW requirement is not mega as we only have showers not baths.