Heat Bank - space heating or not ?

[John Rumm]

Yup if you don't have the modulation then you are back to cycling to achieve less than the max effective output. So being able to consume the maximum output in big lumps (i.e. with a store is better)

Many oil fired boilers are fixed output, although you can get some modulating ones. Often the modulation range is not as wide as with gas though. Misteral seem to do quite a number.

Grab a copy of the free SEDBUK database of boilers from SDA here:

If the rads are off the boiler, I could design the system for a suitable

It is unlikely to ever be fully loaded on heating alone. HW water is the place you can sink big outputs in a short amount of time.

Remember however that efficiency is better when condensing, but it is not a magic bullet - if you stop condensing there is not a big step change. The condensing boiler will still be more efficient that its conventional sibling simply by virtue of its better heat exchanger design.

Possibly, I would focus more efforts on getting the type of system performance you want first, and then the best economy within those parameters second. Otherwise you will always be a bit dissatisfied with the system, and knowing that "at least it is as cheap to run as possible" may not offer much consolation.

[John]

"Andy Hall" wrote in message news: [e-mail]...

SNIP

Andy Whilst the principles of your argument regarding the system are correct it is not the case that the OPs intended "oil" fired condensing boiler can modulate. To maintain good (efficient) combustion with small oil burners it is neccessary to closely match the oil rate and atomisation through the nozzle with the airflow rate. The economics and the practicalities are such that turning up or down the oil rate will require adjustment of either the oil nozzle hole size or oil pressure. The oil nozzle is a precision bored unit and is effectively non adjustable. The alteration of the oil pressure leads to alteration of the quality of atomisation. Again, alteration of the airflow requires some considerable additional hardware and controls i.e. servo operated air damper or fan speed but the fan is invariably mounted on the same motor shaft as the oil pump (gear type). On some larger (intermediate) oil burners a step modulation is employed by installing twin nozzles in the burner head and a two stage (solenoid operated hydraulic ram positioner) air shutter but these are only found in burners of 100kW and above. I have seen only one progressively modulating oil burner (serving a very large office building and made IIRC by Veissman) but as it was so unusual out in the field I would anticipate its cost was prohibitive. These problems are most likely behind the reasoning to not require condensing boiler installation for oil until 2007 at the present state of affairs?

[Mike]

I think more likely was the fact that almost every oil boiler on the market is 85% efficient so the 86% requirment of part L was a little silly. Non-condensing gas boilers are more inefficient for reasons I'm sure somebody here can explain.

[Jeff]

This has been very interesting and helpful, thanks all contributors.

Trying to sort out the consistent themes and the most appropriate for me I'm coming to the following conclusions:

(From the other thread) I don't need a bypass valve of any sort as long as I have a water path at all times when pumping but I do need a zone with time and temp control per 150 m2 max. 'Bypass' rads to have 2 lockshields.

An oil condensing boiler does not have the efficiency benefit over a standard one, in comparison with gas models, and is also likely to be much more expensive (checked out the efficiency on SEDBUK site)

A heat bank will usefully buffer the boiler output including for the rad circuits particularly so for a non-condenser. A two-stat model will further reduce cycling.

So I'm thinking I'll design for the 'normal' or higher temperature range of boiler output, meaning I won't feel compelled to have slightly larger rads (cost saving), I won't have to specify a condensing boiler so since I'll probably be using an installer for the boiler and tank I should get quotations competitivie with the installer's favoured models.

Now, how to size the boiler. Calculated heat loss over 2 floors plus a bit for utility areas is around 29kW but it's unlikely we would ever want every room up to design temperature. It's an old and poorly insulated property but I'm hoping to significantly improve the main living rooms to a high standard. No modulation, should I keep the boiler rating on the low side or should I allow for the heat bank dhw load in addition?

[John]

There is an efficiency gain by using a condensing boiler but it becomes less as the load modulates down and the boiler doesn't. You imply you have looked at pricing so I guess you have made the decision?

Personally I would oversize in line with a future swap to condensing and use TRVs for room comfort. In terms of which model of boiler I have a distinct penchant for Firebird non- condensing units. They have a very good build quality and an economic price. The burners Firebird use can vary but I suggest you specify Riello RDB (which is the usual model that comes with the boiler). The boiler is available for balanced flue operation (about under-worktop height) and/or conventional flueing. Both kitchen (white cased) and boilerhouse models are available

Allow for everything being on-line unless you are absolutely, definitely, certain about not wanting all on at some stage in the future. Also make sure your oil storage facility complies with OFTEC and Environment regulations.

[Jeff]

No, I haven't looked at pricing or made a decision yet - I could still go either way. I don't have a nominated installer - my feeling was that if I ask for quotations for a specific output of *any* or a small option list of boilers I am likely to get a keener quotation; in my experience once you start asking a given supplier for something that isn't on their 'preferred' stock list it can work aginst you. Of course, I'll be examining what is offered and making any necessary mental adjustments for perceived differences in quality/list price. Or, I could source the boiler myself and see if I can get someone to fit it. Or, fit it myself, but what certification would I need and will someone actually certify my installation? I've never fitted an oil boiler or the associated tank and supply lines.

You mean oversize the rads anyway on the basis the next boiler change will be condensing? I could do this. If so, I would adjust the rad size for the reduced mean temperature if I changed to condensing per the manufacturers' tables.

So would it be say 29kW plus say 6kW (notional guess) for dhw and aim for a

35kW model?

[Andrew Gabriel]

If you are zoning with a view to not always having all the heating on (whether by simple use of radiator valves, or by use of multiple heating circuits and zoning valves), remember when doing your heat loss calculations that an adjacent room (sideways, above or below) might be cold, or you'll find you can't get a room up to temperature without all those around it also being heated, defeating the object of what you're aiming to do. (Heating just parts of a house can cause other problems too, such as condensation.) Also, you need some headroom in the boiler output to heat a place up from cold without it taking ages to do so.

[Doctor Evil]

You can use a Grundfoss Alpha modulating pumps and TRVs on all rads. take CH off heat bank.

Over 150 m2 it requires to be split into zones with time control. If you are zoning have the flow and return of each zone off the heat bank. They then will be totally independent of each other coming off a neutral point. Then no zone valves, just a couple of Alpha pumps on each zone.

Yep.

Yep.

If the boiler is connected directly to the heat bank, not via a coil, then, as long as the flow and return pipes are sized properly, minimum 28mm, you can have a very large boiler. Size doesn't matter as the boiler is disconnected from the DHW and CH functions. The larger the boiler the quicker the re-heat and still no cycling. Some larger boilers are cheaper than smaller models because of volume sales. This is certainly the case with gas boilers.

If your house is very large, then it may be worth your while having two smaller boilers heat the heat bank. The beauty of a heat bank is that boilers can independently heat with their own flow and return from the heat bank. Then you have back up if one is down. The two re-heat pretty quickly and no cycling. Price up two boilers, you may be pleasantly surprised.

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[Doctor Evil]

Yep.

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[Doctor Evil]

Read what I wrote. The bottom CH zone. This will be at a different temperature to the hotter upper DHW zone. The DHW zone is priority in that if it calls for heat, say 75-80C then the boiler only sends heat to that upper zone. then it reverts to weather compensator control of the lower CH zone. All the same body of water.

Quite clear.

Quite clear.

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[Mike]

I'd go for one with the Riello RDB 90 burner - Grant Euroflame boiler house is the one I chose - which can be easily jetted down if you find you need less poke.

As you say, with a non-condensing oil boiler you can run the radiators hotter and so have smaller sizes. This is especially useful in older houses where the easiest place to put the rad isn't always the biggest.

And as you say - insulate. Then insulate some more. From the calculations our house as we bought it was unheatable - unless we bought a commercial boiler anyway - whereas several lorryloads of Kingspan/Celotex later we used under 1000 litres this winter.

[Mike]

.....

You were doing well until this. Remember it's oil, not gas. Oil boilers get better with size. Small ones are inefficient, big ones are much better. And really big ones can put out serious amounts of poke efficiently if you have something like a heat bank to actually take the energy and store it for later use.

[Doctor Evil]

Coupling the boiler "directly" to a heat bank, the size doesn't matter. You can't oversize a boiler, as long as the flow and return pipes are sized up properly, You can oversize a boiler if it is coupled directly onto a rad circuit; it will cycle far too much, be inefficient and wear out the controls pronto.

calculations

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[Doctor Evil]

As I know more about this field than anyone here and I aways do well. I don't know the size of his house. He could get a better deal with two boilers (buy two at a discount) than buying one big one.

I know one v country large house had a large Viessmann installed. It broke down, occasionally they do. the whole place was freezing until repaired. They Viessmann because it was the best and reliable, etc. Two mid priced boilers would have been a far better way.

Look at sedbuk. You will find that is not the case in small v large = efficiency.

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[Andy Hall]

Point taken. In the first post we didn't know that it was oil or that it was (probably by implication) non modulating, though, John.

[Andy Hall]

This sounds like waffle.

Are you suggesting thermostats on different heights on the cylinder, or more than one indirect coil or what?

Come on, it's waffle and you know it.

70-59 = 11 is clear, as is 59 degrees being above the dew point. However, what was the point that you were trying to make?

[Mike]

That's a somewhat biased test I'm afraid. Bit like urban mpg figures for cars. In real life a bigger oil boiler operating for a shorter time will always be more efficient.

[Doctor Evil]

I would have agreed with you 20 years ago.

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