Heat bank & boiler connections

[muddyboots]

Hi all

I have a heatbank in our selfbuild, that is hooked up to a Vaillant boiler and has been working fine in the 3 or so years since it's been fitted.

It's all situated on the ground floor, with a feed and expansion tank up in the loft. Boiler is controlled by twin thermostats fitted low down in the heatbank. Boiler pump runs at the lowest of the 3 speeds, and the thermostatic valve in the boiler return is set as low as it can go. Return temps to the boiler start at approx 40deg when the stats fire the boiler, rising to around 50deg when they shut off again. Temp rise across the boiler is approx 25deg.

Here's a quick sketch of how it's currently setup:
(I've omitted the UFH & other connection gubbins for now)
Note that the pipe to the F&E in loft is also the vent (all 22mm pipe).

I've been intending to relocate the F&E tank, for various reasons, so it's directly above the heatbank rather than up in the loft.

I started reading the boiler manual to work out how best to connect the F&E tank, and vent.
I noticed in the boiler manual, and in most other diagrams online, that it seems more common for the pump to be situated in the flow from the boiler, downstream of the feed & vent pipes.
But on ours, the pump is in the boiler return. The thermostatic valve and pump were pre-fitted on the heatbank by DPS, our plumber just hooked it up to the boiler.
We've not had any problems running it though.

So my questions:

(1) Is it OK for the feed & vent to be taken from the top of the heatbank as per my diagram ?

(2) If not, and I were to relocate the feed & vent, where should it connect in to the boiler flow/return pipework in the above diagram ? And is it better to combine the F&V ?

(3) Could I run the vent straight out of the wall so it vents outside in case of a fault ?

(4) Is there any point in having the thermostatic valve in the boiler return ? It's set as low as it can go, I can't see any benefit compared to just running the boiler return directly from the bottom of the heatbank. Only time I can see it doing anything useful is during (very rare) startups from cold, so the boiler output is always at full temp (giving instant heat to the top of the heatbank and therefore hot tap water heat exchanger) even when the bottom of the heatbank is stone cold. In my mind the thermostatic valve is just a mass of brass radiating precious heat away!


Thanks for any thoughts or other observations !

 

[Dan Robinson]

You can't relocate the F&E below the level of the highest rads.

You can't take the vent outside.

The blending valve is used to maintain a minimum return temperature to ensure a faster recovery to the top of the store.*

The Vaillant lovers out there might be able to confirm if the boiler gets upset by too big a temperature differential.


*Might be able to confirm that better with a photo of the unit.

 

[muddyboots]

You can't relocate the F&E below the level of the highest rads.

You can't take the vent outside.

The blending valve is used to maintain a minimum return temperature to ensure a faster recovery to the top of the store.*

The Vaillant lovers out there might be able to confirm if the boiler gets upset by too big a temperature differential.


*Might be able to confirm that better with a photo of the unit.

Thanks Dan.

Rad height won't be an issue.

What's the reasoning behind not running the vent outside ? (Not disagreeing with you, I just like to understand why).

 

[Dan Robinson]

It could freeze, then you house will explode...

I take it you have a bungalow? Otherwise, what are the arrangements for the heating?

 

[JonasX]

While you are at it, thermal store makers inform me that it is best to fit a Fernox filter on the radiator (iron) return pipe to the cylinder to catch any particles that make up sludge. A good insurance.

 

[DeltaT2]

Aye, I see Dr Drivel is talking to himself again. Never mind fella, Nurse will be along with your meds soon.

 

[JonasX]

This cowboy is a mental case.

 

[Agile]

OP

The vent needs to return system water to the F&E if its pumping over.

It may just be an air pressure balancing vent under normal running but under fault conditions it can be returning water.

Tony

 

[Onetap]

Thanks for any thoughts or other observations !

Is there any point in having a heat bank connected to a condensing boiler?

You only need a thermal store if you have solar/wood burner/GSHP etc and have a need to store heat. It's been discussed here, and elsewhere, frequently. The only advocate is Drivel in his many identities, but all of them are barking. The technology could be justified pre-1987 before unvented hot water cylinders became legal. If you find a valid reason for it now, please let me know.

The 3-port mixing valve is usually for back-end protection for a non-condensing boiler. It probably pre-dates the Ecotec. It serves no useful purpose, IMHO. It reduces the boiler efficiency.

A combined F&E won't pump over.

Why don't you bin the heat bank and convert it to a sealed system? If the reason is leaks, you should fix them.

 

[DeltaT2]

Don't worry, it's Water Systems aka Dr Drivel, Big Burner etc.....etc. They've changed his medication again, so he's popping up all over the place!!

 

[Dan Robinson]

Is there any point in having a heat bank connected to a condensing boiler?

You only need a thermal store if you have solar/wood burner/GSHP etc and have a need to store heat. It's been discussed here, and elsewhere, frequently. The only advocate is Drivel in his many identities, but all of them are barking. The technology could be justified pre-1987 before unvented hot water cylinders became legal. If you find a valid reason for it now, please let me know.

The 3-port mixing valve is usually for back-end protection for a non-condensing boiler. It probably pre-dates the Ecotec. It serves no useful purpose, IMHO. It reduces the boiler efficiency.

Why don't you bin the heat bank and convert it to a sealed system? If the reason is leaks, you should fix them.

What utter twaddle.

Cylinder coils loose their efficiency as the temperature rises. A thermal store (non coil heated version) will keep a condensing boiler in condensing mode 100% of the time with no modulation and little cycling. The blending valve raises the return temperature to the boiler to enable a decent temperature rise so that the flow into the store from the boiler is close to the maximum setting (80 in my store's case) which enables full temperature hot water to be delivered quicker.

A condensing boiler will do this more efficiently than a non condensing boiler
by definition.

You can also have mains pressure hot water at Unvented cylinder flow rates without the need for unvented safety systems and discharge pipes.

You can also have an immersion heater to backup the heating as well as hot water if the boiler conks out.

You can also use ANY energy source to heat the unit.

I am I am not one of DD aliases... As you well know. I fit more unvented than I do TS, and more combi's than I do unvented. But I do speak from experience in these matters. Both personal and professional.

 

[Onetap]

What utter twaddle.

Oh, dear.

Cylinder coils loose their efficiency as the temperature rises. A thermal store (non coil heated version) will keep a condensing boiler in condensing mode 100% of the time with no modulation and little cycling. The blending valve raises the return temperature to the boiler to enable a decent temperature rise so that the flow into the store from the boiler is close to the maximum setting (80 in my store's case) which enables full temperature hot water to be delivered quicker .

Taking that first paragraph, the 'efficiency' is not relevant to the cylinder flow temperature. The temperature affects the rate of heat transfer, which is nothing to do with efficiency. I know what you mean (I think) but it would lend more credibility to your opinions on energy efficiency if you got the terminology right.

If you need the boiler flow at 80 degC to get DHW at 'full temperature' then ISTR that most boiler manufacturers recommend a temperature differential of 20 degC. I think the Vaillants have an internal by-pass to keep to NMT 20 degC across the heat exchanger (I'm sure someone will tell us; I'd look it up if I had time). If Tf is 80 degC, Tr is NLT 60 degC; you're out of condensing mode there already and the efficiency has gone to steeply downhill.

The standing losses from a thermal store at 80 degC will be greater than the standing losses from a DHW cylinder at 60 degC; more inefficiencies. The DHW cylinder will store more usable energy.

If you want to utilise WC and/or UFH (usually used in conjunction with a condensing boiler to achieve the lowest practical return temperature and so the highest possible combustion efficiency) then you need a secondary mixing system on the thermal store. The system will be heating the water to 80 degC flow and then mixing it to give the required flow temperature. That is not merely inefficient, it is daft.

It's probably not a good idea to lecture me on energy efficiency.

There's no point to a thermal store, with a condensing boiler, unless you need to store heat from another heat source.

 

[JonasX]

Is there any point in having a heat bank connected to a condensing boiler?

You only need a thermal store if you have solar/wood burner/GSHP etc and have a need to store heat. It's been discussed here, and elsewhere, frequently. The only advocate is Drivel in his many identities, but all of them are barking. The technology could be justified pre-1987 before unvented hot water cylinders became legal. If you find a valid reason for it now, please let me know.

The 3-port mixing valve is usually for back-end protection for a non-condensing boiler. It probably pre-dates the Ecotec. It serves no useful purpose, IMHO. It reduces the boiler efficiency.

Why don't you bin the heat bank and convert it to a sealed system? If the reason is leaks, you should fix them.

What utter twaddle.

Spot on!

 

[JonasX]

I think the Vaillants have an internal by-pass to keep to NMT 20 degC across the heat exchanger (I'm sure someone will tell us; I'd look it up if I had time). If Tf is 80 degC, Tr is NLT 60 degC; you're out of condensing mode

Then that is a boiler you do not use for a thermal store.

The standing losses from a thermal store at 80 degC will be greater than the standing losses from a DHW cylinder at 60 degC; more inefficiencies.

Most modern thermal stores do not run at 80C, neither does Dan's as he sets his to 65C. Which blows your ideas away.

The DHW cylinder will store more usable energy.

If you want to utilise WC and/or UFH (usually used in conjunction with a condensing boiler to achieve the lowest practical return temperature and so the highest possible combustion efficiency) then you need a secondary mixing system on the thermal store. The system will be heating the water to 80 degC flow and then mixing it to give the required flow temperature. That is not merely inefficient, it is daft.

I am going for the Geminox 1 to 10 kilowatt boiler, heating only UFH directly. What you say does not make sense as the DHW section of a therm store needs higher temperatures. UFH is taken off the cool bottom section of a cylinder. Most of the time UFH is NOT being mixed down from 80C.

It's probably not a good idea to lecture me on energy efficiency.

You do not impress me.

There's no point to a thermal store, with a condensing boiler, unless you need to store heat from another heat source.

Twaddle With the bottom of the store at 25C it makes much sense. A good condensing boilers controls will ensure the heat exchanger does not get way out of temperature range. They area perfect match.

I have the impression you are going on theory not any real experience. What you wrote counters what the makers say, who have done extensive research and testing on thermal storage.

 

[Onetap]

What utter twaddle.

Spot on!

There you have it; Drivel recommends thermal stores.

I can think of no greater recommendation for binning the thermal store as soon as possible.

78 posts in one week by this Drivel alter-ego; it would seem there is little demand for his thermal store expertise.

I rest my case, M'lud.