Boiler For Heat Bank

[megawatt]

Hi all,

I'm re-posting this cos some git hi-jacked the last one and got it deleted

I'm going to install a Pandora heat bank from DPS systems following some useful advice previously (thanks all who contributed) and now need to decide on a boiler to power it.

The Pandora will be fitted with the CH sealed system components and heated via an indirect coil so I need a conventional/regular boiler in the region of 25-30kW.

My starting point is the Worcester 30cdi 30kW conventional unit as my current boiler is a Worcester Highflow 3.5 and has been a trusty steed for the past 18 years'ish but, before I go ahead, I thought I'd ask the advice of you all.

If you were going to install this configuration in your home what boiler would you use?

Thanks in advance.

MODS: If this goes tits can you please lock it rather than delete it ... Ta

MOD 2

no probs

 

[croydoncorgi]

Be careful with the specs of the boiler and the heatstore!

The Pandora product has ONE Return connection to the boiler, which means that the boiler's opportunities to condense may be limited.

Other DPS products (using 'GSV' system) have TWO Return tappings and a thermostatic mixer, which optimises the Return temperature foras long as possible.

If you're a real 'efficiency driver', you need to consider carefully whether the heatstore is going to do hot water only or both HW and CH. If the latter, doing weather compensation as well will become complicated or impossible, since you won't be able to adjust the boiler temperature setpoint at the same time as keeping the heatstore at the right temperature.

Your choice of boiler may also be limited by your requirement for an open vent system. Many are specified only for a sealed system.

 

[megawatt]

When the boiler is restoring the heatbank there is some question about condensing efficiency which has been discussed in previous posts due to the high temp of the store and the possiblity that the boiler return temp may not support condensing. WaterSystems view regarding the high efficiency of the 100kW plated heat exchanger returning cool(er) water to the bottom of the store for return to the boiler is supported by the heat store manufacturer and should allow the boiler to condense okay in most circumstances.

I agree with you on the GXV system though and I would prefer to use it due to its simplicity and the fact that the store is heated directly by the boiler with the same water feeding the CH circuit ... High condensing = high efficiency. Main problem is that GX stores are only suitable for OV CH systems which DPS have confirmed despite there being info on their site saying that they can supply pressurised GX units for sealed circuit CH systems ... Shame really

When the boiler is powering the CH, though, the Pandora isn't involved and the boiler will condense (or not) as it would when driving any sealed CH system ... Or have I missed something?

 

[croydoncorgi]

I would add that IN MY EXPERIENCE there are too many problems with Open Vent systems (in particular, leaks, baterial contamination, corrosion and sludging) to be acceptable.

If possible, I always specify sealed, pressurised systems, even for retrofit. With such a system, I will KNOW what's in it (water, inhibitor and nothing much else!) and I will KNOW if there's a leak (air in OR water out - although with pressurisation airleaks are in practice impossible).

 

[megawatt]

I agree, that's why I have specified the Pandora with indirect heating and a pressurised sealed CH system.

A pressurised GXV is better though IMO and I'm not sure why they don't supply one anymore ... I've asked DPS the question and will post their response when I get it.

Do you advocate any specific boiler?

 

[croydoncorgi]

As it happens, I'm currently looking at the spec for a small hot-water-only heatbank which needs a very low-head boiler. I'm considering several but a small Alpha CDR is current favourite. This is a situation where the extra costs of a sealed system will probably rule it out.

And (probably at risks of grandmothers and eggs!) if indirect heating of the store using an external heat-exchanger is used, instead of heating the whole volume of the store directly, you then introduce the probability of high Return temperatures at the boiler. Pump technology is pretty unsophisticated (yes - OK. Grundfos Alpha makes some attempt!) and you have poor control over the flow-rate of primary water through an external HX. If instead the entire volume of the store is circulated through the boiler, it should reach the target temperature in one 'pass'. Then, the Return will remain cool until very near the end of the recovery period.

 

[Agile]

I seem to waste a lot of time posting replies only to find the threads get deleted! This seems particularly likely when heat stores are mentioned!

The original poster has not mentioned a condensing boiler although they are required under building regs.

Many condensing boilers will not give a flow temp as high as the heat store would like, i.e. 80°C to 85°C and if they do are not going to be doing much condensing.

What is needed is a boiler which can have external flow temp settings to alter the flow temp according to the destination of the heat output.

Tony

 

[MOD 2]

the lot of you pack it in we all know who he is
and he ain't taking this thread down

rant over
please gent's i am sure the OP appreciates your input

lets just get on with it

WS whatever guise you come in under it gets filed under misc

 

[Softus]

it gets filed under misc

...for miscreant, presumably?

 

[Bigburn]

Agile wrote

, i.e. 80°C to 85°C and if they do are not going to be doing much condensing.

You dont need 80 to 85 C inside a directly heated heat bank. Thats a myth.
70 C is fine especially with a good plate heat exchanger fitted and you will still acheive a higher percentage of condensing compared to an indirectly heated cylinder heated to 60 C.
What people dont understand is you cant transfer the heat quick enough when heating indirectly via a coil.
Heating directly is faster and more efficient. A good heating engineer who understands the finer points about heat transfer in water thermal systems will know this.

 

[Doctor Drivel]

MOD 2

will you take a telling

WATERSYSTEMS UNDER ANY NAME AIN'T POSTING ON THIS THREAD

 

[Softus]

take to general

NOT ON THIS THREAD PLEASE

Since it's such a coincidence that DD has reappeared at the moment of disappearance of Tess Wets Mary, the question of the five-month gap is pertinent.

Notwithstanding that, I'm quite happy if you want to delete this post (of mine), but I still think that the credibility of DD, as with all of us, is genuinely relevant to any technical topic on which he/she posts.

 

[croydoncorgi]

In amongst the Sturm und Drang surrounding this topic recently, you may have overlooked a key difference in the features of different heatstores.

If the connections mean that it's possible to reheat the whole store via a Return pipe connected at the bottom of the store, it's reasonable to assume that stratification (and hopefully the design of the rest of the thing) will ensure that the water going to the boiler will be cool (< 56) to start with. Whereabouts in the layering of the tank the temperature goes past 56 and how many 'passes' of the contents of the store it takes before it gets to its setpoint are also design issues that will affect both the energy efficiency of the store and of the boiler.

The thermostatic mixer on the return of the DPS GXV design should provide an ideal solution but I don't know to what extent it needs it should be adjusted to match the performance of a particular boiler. Although a Return temperature of 56C degrees will ensure that SOME of the heat exchanger is below the Dew Point, the optimum temperature may be some degrees cooler than that.

 

[Bigburn]

If you are pumping direct into the top of the cylinder then you dont want to much temperature dilution. This is where a temperature control valve comes in. DPS have used a three port thermostatic mixer. Their are other types available also which have not being mentioned.
45 return is what you should be looking for and a 25 C temperature rise across the boiler heat exchanger. Their will be a point where it goes out of control as the final lower section of the store is heated up but considering condensing boilers are still quite efficient if the return temperature are above dew point then this is not a big issue.

 

[megawatt]

Thanks all for the interesting thread and thanks MOD2 for keeping it alive

Getting back to the original question ... Any boiler recommendations?