Heating System Design - Storage Combi?

[nero44]

Hello,

I am looking at a extension project, which includes a new kitchen with UFH (kitchen only) and a new boiler.

We are currently running on a Vaillant Ecotec plus 831 R1 and have no space for a tank. It currently feeds two showers (seldom to be used at the same time) and central heating consisting of 9 rads
2 small (single convex + towel rad)
5 medium
2 large
We don't heat massively and are generally satisfied with heat output when required.

With the new kitchen, one of the medium one will go away and we will have UFH instead. Most of the copper piping of the CH system will remain and we will definitely want to fit magnetic filter.

A friend of mine has done a similar project and he was recommended a Vailant Ecotec Plus 938 storage comb boiler, so with an inbuilt tank. Should I be considering a storage combi boiler as well or would that add no value?

I thought having a tank might help with efficiency for the new underfloor heating requirement, but now I am thinking the tank really only helps you with hot water supply, i.e. running two showers for peak times?

What sort of boiler would you fit for this setup? Combi or combi storage?

 

[muggles]

Why not just keep the one you have, unless it's broken/becoming unreliable?

A storage combi can be useful if you have a need for a lot of hot water but if you're ok with the performance you have now, keep it.

If you want a new boiler and better hot water performance then the first thing to do is measure your incoming mains flow rate. That will determine how much hot water you can get out of any boiler. No point fitting a boiler that can flow 18l/min of hot water if your mains is only delivering 12....

 

[nero44]

Why not just keep the one you have, unless it's broken/becoming unreliable?

Heh, it's +16 years old and had to be repaired already 3 times in the last two years. new PCB, new expansion vessel, new heat exchanger,sludge clearout... already dropped quite some money on it.

The reason I ask is a new storage combi will require a bit more space and I am currently designing on kitchen utility/plans. Supposed there also needs to be a little space reserved for a min manifold. But it sounds a bit like just the fact that we will be using UFH in the kitchen does not require a storage tank to be used and I could be fine just with a new efficient combi boiler.

 

[denso13]

But it sounds a bit like just the fact that we will be using UFH in the kitchen does not require a storage tank to be used and I could be fine just with a new efficient combi boiler.

Agreed, as above, no need for storage unless you use a lot of hot water. Even then you probably don't need one.

 

[Johntheo5]

I think the 938 has a 15L store so if heated to say 60C will have a effective store of 24L with mains at its present 7C and a 40C showering temp. 38kW will give a flowrate of 16.51LPM, from 7C to 40C. If the max flowrate is limited to say 22LPM (2 showers) because of mains pressure constraints, or whatever, means that 22LPM at 40C can be only be maintained for less than 5minutes (4.37) before the store runs out?, the store then has to be recharged, theoretically, should take less than 2 minutes (1.46) so maybe not a big deal but is all this hassle really worth it for a < than 5 minute double shower at 11LPM each??.

 

[Agile]

Government policies and gas supplies may be quite unstable over the next few years.

So my strong advice would be to keep the existing for a few years and avoid wasting money buying something which may not be the best solution in a few years time.

 

[Johntheo5]

This was Vaillant's response to someone's query......

"You are correct. The ecoTEC plus 938 is able to give a blended flow rate of 20 L/min for 10 minutes after that the actoSTOR will be depleted so the boiler flow rate will drop to 15 L/min, all Vaillant combi boilers come fitted with a restrictor to restrict the flow to give a hot water temperature rise of 42 degrees over the incomes mains water temperature, the actoSTOR water is held at 65 degrees when cold water is added to make it useable you will then achieve the 20 L/min for 10 minutes which is sufficient for two showers running together, to achieve this an external blending valve must be fitted.

When a small demand is needed from the boiler it would still fire up the boiler and use water from the storage tank and then recharge what has been used in the tank, if you fit a Vaillant digital programmer you will be able to time the charging of the actoSTOR this will allow the store to be changed when there is high demand and during the rest of the day the boiler will work as a standard combi."

 

[nero44]

Thanks for sharing this, very interesting. So if the price difference is not too massive, it there is actually a benefit to it.

What is an external blending valve? Do you know? Would this not be always part of the installation?

 

[denso13]

So if the price difference is not too massive, it there is actually a benefit to it.

Have you got plenty of space for it, it's the same depth as a washing machine?

 

[nero44]

Have you got plenty of space for it, it's the same depth as a washing machine?

Yes I believe so... I mean this is part of a whole extension project, so the whole lot is going the be ripped out.. new foundations, new steels, new electrics, new plumbings...

The boss and I can't quite agree on how much laundry space is needed So on the latest version we earmarked an entire standard 60m wide tall cabinet for the boiler. So surely that will hold either model + manifold and some other storage space.

 

[denso13]

So on the latest version we earmarked an entire standard 60m wide tall cabinet for the boiler. So surely that will hold either model

Not the larger one, it's too deep.

 

[Johntheo5]

Thanks for sharing this, very interesting. So if the price difference is not too massive, it there is actually a benefit to it.

What is an external blending valve? Do you know? Would this not be always part of the installation?

A external blending valve is just a thermostatic mixing valve (TMV) to reduce the maximum water temperature after leaving the boiler so presumably set to 60C, the shower's TMV will then further reduce this to whatever showering temperature required, say 36C to 42C ish.

I don't know where Vaillant pluck their calculations from because a temperature rise (dT) of 42C will only result in a continuous flowrate of 12.97LPM. If the 15L store is heated to 65C (from say 7C mains temp) then its effective vol at 49C is 20.7L which will only supply enough water for less than 3 minutes (2.95), if the store is heated to ( a unlikely) 80C, then the effective capacity is 26L, it will supply enough for less than 4 minutes (3.7),both at a shower flowrate of 20LPM.

If one uses the fairly standard norm of a 35C dT, then 38kW will give a continuous flowrate of 15.56LPM (at 42C from mains at 7C) and the 15L store will have a effective vol of 24.86L at 65C to give less than 6 minutes (5.6) and just over 7 minutes (7.04) if heated to 80C from a effective store vol of 31.29L., both, again, at a shower flowrate of 20LPM.

You will have to decide yourself if this 6 minutes or so at 20LPM is worth it.

 

[muggles]

Thanks for sharing this, very interesting. So if the price difference is not too massive, it there is actually a benefit to it.

What is an external blending valve? Do you know? Would this not be always part of the installation?

Again, get your incoming mains tested. No decisions can be made until you know your incoming pressure & flow rate.

The 940 is about £600 more than the 840 and is enormous (938 is now discontinued)

 

[Razor900]

Shower valves use roughly 66% hot and 33% cold water depending what temp you have them at.

Most of them flow between 15 and 20 litres a minute at mains pressure.

A storage boiler will only flow water at the same rate it comes into the property. So if you have 25 litres per minute coming in that is all you have to share between the hot and cold supplies giving you a still respectable shower of 12.5LPM.

The existing boiler will give you about 15 litres a minute at a decent temp rise.

A storage combi will give hot water for longer but bear in mind once they go out of warranty most storage combis become a nightmare to work on.

Assuming it's in decent condition just stick with what you have...