Sizing a combination buffer tank with solar and oil - 300L vs 1000L

Tim.Bunce

Hello.

We're building an extension to our bungalow and reworking the plumbing. To cut a long story short my (continental) plumbing team are recommending a 1000L combination buffer tank and the (possible) solar installers and recommending a 300L tank. I'm in the middle scratching my head and looking for other opinions. I've read lots of old posts here and elsewhere but haven't found sufficient information.

Here's an outline:

* Existing house:

110m2 Bungalow with B-rated double glazing plus cavity wall and 300mm loft insulation.
Typically 4 occupants, periods of 2 or 6.
Double panel radiators (1970's, no fins between panels, high water content)
Oil boiler: Firebird Popular 90, 26kWh/120BTU, 85%eff C-band.
Open fireplace in sitting room, lit most evenings in winter.

* New Extension

100m2, bungalow with 3 bedrooms, Ensuite, Shower Room, Utility.
Above standard insulation in walls, floor, ceiling. A-rated windows. South facing roof.
Radiators, with low water content, sized for low water temperature (eg 40C normally).
The main bedrooms will move into the extension.

Given that we can heat the extension with 40C water we'd like to have solar contribute to the space heating. In the longer term we plan to change the open fireplace to a stove (with external air supply) with a back-boiler which would also contribute to the space heating.

The plumbers plan is to have a stratifying combination buffer tank with DHW coil plus two solar coils (e.g. SKS-2W). Two heating zones: higher temp for the existing house and lower temp for the extension, both using high efficiency pump stations with weather-compensated TMVs. All controlled via a Resol MX.

The plumbers want a 1000L tank, the solar installers want a 300L (with 10 panels of tubes). To my (uneducated) eye, 300L seems too small and 1000L seems too big, but I've no firm basis for that view.

I'd appreciate any thoughts you may have. In particular I'd like to understand the risks/effects of sizing too small and the risks/effects of sizing too large.

Can you help?

Outkast_IRE

It sounds like one crowd are reccomending a 300 ltr hot water cylinder and the other are reccomending 1000L buffer tank . These are for 2 different things , a 300ltr hot water cylinder is common for a average sized house with solar panels , the buffer is a seperate cylinder for storing heated water for use in heating and coil in the cylinder.

I would say its a case of mixed messages, you might have your wires crossed, i have installed 1000L buffer tanks before so if its a good sized house that would sound right.

If unsure get a building services eng to check do the sums for you personally i wouldnt be looking for such important info off the net

Tim.Bunce

Outkast_IRE wrote: »

It sounds like one crowd are recommending a 300 ltr hot water cylinder and the other are recommending 1000L buffer tank.

I've just double-checked. Both are recommending a single combination buffer tank (a thermal store with two solar coils and a coil for DHW). The size is the only significant difference.

The solar crowd are concerned about a "bad summer day" scenario where the solar wouldn't lift a 1000L tank hot enough to be useful. The plumbing crowd counter that the point of big buffer is to carry over heat from good days through to bad. Truth on both sides.

If unsure get a building services eng to check do the sums for you personally i wouldn't be looking for such important info off the net

Given the experience on both sides I'm sure both systems would "work" for us. Hearing of peoples actual experience with similar systems would be very helpful. I take your point though.

Mike2006

I think you are down to how many solar panels you have and the specification on your tank.

A guide I used was 1m2 of tubes for every 50l of stored water. I put in a 1000L thermal store with 20m2 of tubes. South facing roof.

I feel that it is a good fit for my system anyway as even on very average days, the solar panels will bring my tank up to over 50DegC and I just use my oil to top it up then...
On a good day, I can hit 70DegC and on 2 good days in a row, I hit 80DegC and this would be maintained for every good day after that...

I have 2 options for a heat dump.
a. I open the upstairs and downstairs hallway UFH solenoids and 'dump' the heat to the floor
or
b. I can open a drain solenoid on my system to dump 50L of 'heated' water and replace with 'cool' water to bring down the tank temp.
Option b. only comes into play when the dump to the floors cannot shed the heat fast enough from the tank and the temp continues to increase.

Mike.

Tim.Bunce

Mike2006 wrote: »

I think you are down to how many solar panels you have and the specification on your tank.

Yes, exactly. Everything else seems to be well settled now, thankfully.

A guide I used was 1m2 of tubes for every 50l of stored water. I put in a 1000L thermal store with 20m2 of tubes. South facing roof.

I feel that it is a good fit for my system anyway as even on very average days, the solar panels will bring my tank up to over 50DegC and I just use my oil to top it up then...
On a good day, I can hit 70DegC and on 2 good days in a row, I hit 80DegC and this would be maintained for every good day after that...

It's great to see some hard data from someone with a similar system. Thanks for that Mike!

I'm curious about the heat dump... How often has a heat dump been needed during periods of hot weather? (I can't just say "the summer" anymore!) Have you ever needed option b?

One more question: what controller are you using and what cooling options do you use? (The Resol MX I'm planning to use has three: System cooling and Collector cooling both basically allow the tank to rise above the 'max store temp' up to the 'emergency shutdown temp', and the Store cooling mode circulates solar fluid through the tubes to radiate out excess heat from the store, e.g., at night. A Holiday mode does the same but aims to cool the store further.)

We'd dump heat, if needed, into the radiator circuit of the existing house. Most have TRVs but the pipe volume is high and insulation poor so I'd expect reasonable heat loss even in summer.

Mike2006

The temp in the tank has maxed out a few times a couple of months back.. After about 4 days of continuous good weather it happens on my system.
The dump to the hallway UFH does the trick. I just wanted a second option in case I was away and it got really bad. Tank is suitable for 110DegC but I never want it to see anywhere near that!

My controller is a Resol as well. Not sure what model but it doesn't have nearly as many relay outputs as I wanted so I bought a temp controller with a few extra relay outputs to do my interlocking.

Interlocks I have are as follows:
1. Oil cannot fire if solar is contributing to the tank.
2. If solar is contributing but temp continues to drop then fire oil if needed.
3. Dump to UFH hallway loops.
4. Dump tank 50 litres to drain and refill with cold.

Mike.

heinbloed

The thermal output of collectors should be available from the manufacturer, the seller.
The thermal output (kW) has to be the ruler when deciding on the storage tank.

Depending on the local situation the output of the collectors can be calculated.

See

PVGIS home

A heat dump is in closed, pressurised systems usually not necessary. The circulation pump will switch off when the desired temperature is achieved. The thermal carrier (water, brine, air, oil etc...)is designed to withstand the real temperatures, so is the entire plumbing and the collectors themself.

There are exemptions.

heinbloed

Here the direct link:

http://re.jrc.ec.europa.eu/pvgis/apps/pvreg.php?lang=en&map=europe

Outkast_IRE

Do you have the space to have a seprate buffer and hot water tank , then you can size each to suit .

Tim.Bunce

Outkast_IRE wrote: »

Do you have the space to have a seprate buffer and hot water tank , then you can size each to suit .

Using two tanks would have been an appealing compromise. We don't have space though, sadly.

heinbloed

There so called "tank-in-tank" solutions, common on the EU continent.
Try google. Some of the manufacturers trade in Ireland as well.

The smaller tank would sit in the top of the larger tank.

Tim.Bunce

heinbloed wrote: »

The smaller tank would sit in the top of the larger tank.

That wouldn't help here. The goal of two tanks would be to have one as the primary with the solar and DHW coils, say 300L, and a secondary tank acting only as a buffer tank, say 700L. They'd need to be thermally isolated. The controller would move heat into the secondary only when the primary had excess heat. So overall the system would act like a 300L tank in spring and autumn and a 1000L tank in summer and when the wood burning stove is used.

That would be great, but we don't have room (unless someone knows of a manufacturer of high-spec tall thin tanks).

Meanwhile I'm hoping that dual solar coils and good stratification will be good enough.

heinbloed

What you're looking for is not an exotic solution but common standard.
Except that " ... a 1000L tank in summer and when the wood burning stove is used.". But technically that doesn't matter.

I have a list of at least three dozens of ready available tanks in your demanded size here in front of me.
But the situation as such has to be looked at, putting a technical item into a badly designed/unsuitable overall system makes no sense.

What is the heating engineer recommending, the civil engineer?

There are EN standards to which a heating system must be designed. They will give you exact numbers on how many kW(h) are available from your thermal supply and how much is lost via transport and storage.
So you'll know when tank is 'empty'. And can accordingly match the size and insulation to get the most out of the system.
Since you don't know the energy created by your collectors or boilers we'll be hopping in circels here finding a suitable tank.
We simply haven't designed your system.

If your sales men can't calculate harvest and loss (matching the demand) and valet and available space you better look for professional advise.
These people can't be professionals but must be chancers.

As said, a heating engineer would be able to sort you out. And sign for his/her calculations. If these don't match reality then you can have your money back, the professional indemnity insurance covers these errors. Unless a wrong advise was given or wrong material installed by purpose - of course.

Since June of this year heating systems have to be calculated according to EN standard. This includes storage and distribution losses as well.
We had the discussion here before, most taking part in the discussion did claim at the time that they haven't heard about this....
But there aren't many heating engineers involved here in the forum anyhow.

So fetch the yellow pages, good advise costs good money.

heinbloed

Here the required standard for costum build ST systems:

http://shop.bsigroup.com/ProductDetail/?pid=000000000030154106

http://standardsdevelopment.bsigroup.com/Home/Project/201101440

http://www.solarthermalworld.org/node/1582


So in the OPs case the costumer has the possibility to ask for a certified performance calculation. According to EN 12977-3 .
If the builder/installer/seller does not come up with it then very likely there are cowboys at work.
The 300 l tank in the OPs case seems very dubious, even with the 1000 l tank I would have my doubts - as long as the performance can't be calculated or is actually guaranteed.

A meassuring device meassuring the thermal energy tapped from the system can be fitted at the tap/thermal consumer(shower, sink tap, washing machine tap etc.) or at the consumer's outlet of the tank. If the consumed energy per year/month/day does not not match the
predictions - money back. Incl. all cost incl. replacement off course.

Get the EN performance declaration signed and guaranteed by the seller.

The devices are called " calorie meters ". This device should be installed by the ST system installer to avoid later discussions.
The calorie meters are common standard nowadays in thermal installations. Available at the plumbing shop.

If the plumbers don't know what to do contact a heating engineer.