Thermal Store/Heat Bank - Newbie Q: Combi-Boiler

[Simes123]

House I'm buying has a year old gas Combi-boiler.  I have no immediate plans to replace it.  I'm considering ET solar (East Fife, Scotland) to augment and save energy costs. 

Reading up a little on Thermal Store/Heat Banks, I understand there are benefits in respect to storing multiple sources of heat.

Would a well-insulated large thermal store, being heated by a Combi-boiler and Solar ET be a better proposition for meeting DHW and CH needs from an efficiency perspective than a small thermal store for the ET pre-heating feed water to the Combi-boiler which then supplies HW and CH needs directly?

Intuitively, from my very rudimentary understanding, the large Thermal store would appear to offer advantages in that the boiler wouldn't need to fire so often, and that even relatively low ET outputs in the fringe seasons would always contribute to the thermal reserve; with the additional benefit that a higher output might be achievable from the thermal store to meet concurrent DHW demands, perhaps overcoming one of the Combi-boiler weaknesses?

Can a combi-boiler even be used to heat a Thermal store in the way I'm thinking?
 

[desperate]

Simes,

there is nothing to stop you zoning the heating circuit into as many zones as you need from a combi, it would work just like a system boiler. You could even blank off the hot water outlet from the boiler if you wished.
As for what would be most efficient, pre heat or thermal store, that's a bit more difficult to say, there will always be standing losses from the store that are dependant on a load of factors, temperature, insulation levels, time between use/charging etc,etc,etc. In the summer a larger store with enough tubes would do the job quite well, but as you say the fringe seasons and winter my gut feeling is that it would be a liability.

But I stress that is only a semi educated guess, we have a thermal store and ETs with a WBS all backed up by a system gasser and the thermal store is the weak point in our system, the standing loss is quite large. On the other hand it would be very tricky to do without it.

Desp

[Simes123]

Desp,

Thanks for your thoughts.  Without being able to look yet, I think it's likely that there may be 3 zones - 1 per floor.  I imagine the after engineering into more zones might in some ways be more work than engineering a thermal store.  I'm a bit of a newbie when it comes to plumbing, but I imagine that I'd need to extend a feed and return for every additional zone I created, which given the size of the house might be more trouble than it's worth.  I could (and am probably) wrong though!

Ignoring standing losses for a moment, from my basic understanding, a large thermal store would require much less cycling on the boiler, in comparison to having the boiler start up every time the hot tap is run (or the CH calls for heat).  While the ET could feed it whenever they could.

However, I hadn't considered the standing losses to be too much of a factor if the store was really well insulated, but I suppose thermal conduction through the connected pipes will always be an issue - not so much in Winter when they'll contribute to house heating, but in the summer when all you need is hot water, and are effectively wasting heat to unnecessary heating, while having to replace lost heat in the store.  Though that would be countered perhaps by the ETs.  It's probably not going to be easy to answer I suspect! Simon

[brackwell]

Simon,

You will find this most helpfull

http://www.energysavingtrust.org.uk/Publications2/Housing-professionals/Heating-systems/In-situ-monitoring-of-efficiencies-of-condensing-boilers-and-use-of-secondary-heating-trial-final-report

A combi boiler (just in time heating ) is virtually unbeatable at any time of the year except when the amount of energy from the ET is sufficient which depending on your requirements and the no. of ETs may not be as much as you would like.

Be aware that thermal stores can only use about 60% the volume of water and the water has to be held at a higher temp also. This must say double your heat losses from a standard tank.

In my opinion using the ETs to heat a seperate tank to prefeed the combi is the best.  That way the heat losses are minimal but you will also be effectively able to gain from using water which whilst warmed would be useless for much else.

Ken

[JohnS]

How big is the house?  How big is the household?

The combi was probably put in because it was the best, or cheapest, solution.  I reckon that we, as a houshold of 4, use £50 of gas for hot water plus gas hob over a 20 week summer season.  It is hard to make a case for ST on these numbers.

John

[Simes123]

Thanks guys - food for thought indeed.

to answer JohnS's question.  How big is the house - 4300Sq Ft.   Household, just the 2 of us usually, occasionally augmented by 2 Adult "kids".   Typically shower users.   It's a big place to heat - converted Church (with internal wall insulation, secondary glazing and MHRV system).   I think based on your input, I'll suck it and see for 4 seasons before doing too much in any case.  Simon

[tarrel]

I am also looking to reduce use of a combi-boiler (ours is oil-fired) and was considering a thermal store fed by solar and wood burning stove.  I hadn't thought about the idea of pre-heating the combi water.  Sounds like it could be a lot less disruptive to the existing installation.

Does anyone know;
- Would it be possible to pre-heat the combi water using a combination of back-boiler and solar heated water?
- What kind of size tank would be needed, in comparison to, say, a thermal store?

Thanks

[tarrel]

Also, would the solar thermal be eligible for RHI payments if it was being used to pre-heat water for a non-renewable heat source?

[SimonHobson]

Not all combi boilers will accept hot water in - many will trip with a fault if you try.

As for standing losses, I was able to do a side by side comparison after fitting a thermal store in the flat. I measured the standing losses (170l Gledhill Torrent with additional insulation) at 80W by running it off the immersion heater for a few days and monitoring the electricity meter - the flat was empty at the time. If heating with gas, you need to factor in boiler efficiency.

By contrast, the combi (RD532) in the house I've just bought next door used the equivalent of 160W (calculated from the gas meter readings and converted using the thermal conversion factors on the back of the gas bill) in it's default mode where it keeps itself warm to avoid that "turn on tap, wait a minute for hot water" issue familiar to so many combi users. If you turn on "eco" mode then this boiler doesn't fire up at all unless there's a demand for heating or hot water.

OK, that's only one data point, but IMO it somewhat knocks the "but thermal stores waste so much heat" argument somewhat. If you make that "lost" heat do something useful (like keep the airing cupboard warm) then it's not really wasted.

Personally, having been brought up in houses with hot water cylinders, I wouldn't give wall space to a combi by choice. To get a decent HW flow rate/temperature means having a boiler that's typically grossly oversized for the heating load - and so you get a system that's a compromise and not matched to either heating or HW requirements.


I wrote up my thermal store install here. I just stopped using the combi function on the boiler, so it now just runs as a heat-only system boiler. My current tenants are happy, the heating works much better (and quieter) with the modulating pump, and the boiler is much much much more reliable now.
Incidentally, if doing one now, I'd be tempted to look at a diverting valve (such as those sold by Esbe) in the boiler flow rather than the mixer in the boiler return as I did here (but that mixer valve was really cheap off eBay).
The main thing is that you need to ensure the return is cool enough to keep the boiler in condensing mode - which I doubt you'll manage that for a lot of the time when directly driving the heating in a conventional system.

So, whilst I clearly have a bias I'm not ashamed to acknowledge, my preference would be for a thermal store (or heat bank) and use the boiler for nothing but heating the store.

[desperate]

Simon, I sort of agree with what you are saying but three points come to mind. Thermal stores are great in providing many more options of combining heat sources, but the heat loss shouldn't be ignored. You compare a stores losses compared to a combi in preheat mode and measured half the loss of a combi, but in eco mode it would IIRC be similar.
 Secondly, you compare that combi with a store with additional lagging all boxed in with rockwool or similar, not so many installations would have room for a reasonable amount of extra insulation.
Third, as you say if the heat loss can be made to do something usefull why worry, but most airing cupboards wouldn't have the room for much else other than housing the store, ok during the heating season this would still provide usefull heat, but not in summer.

I lived with a combi in our second house quite a while back now, like you I wasn't impressed at first seeing a combi in a 3 bed house, but after 5 years of living with it I changed my mind somewhat, it was very frugal, and provided we all knew its limitations it just lived in the cupboard doing its job.

Some people love them, some hate them, but there's a place for everything IMHO.

Desp

[SimonHobson]

Each to their own. Personally it would really, really p**s me off if I had to live in a house with the combi like that RD532 (British Gas badged Worcester Bosch) in Eco mode. I timed it, and (in eco mode, from cold) literally a whole minute from turning on a hot tap to getting warm water - and that's in the kitchen so hardly any pipe to heat up It's better than the old Vokera in the flat in terms of both temperature regulation and output, but compared to a decent store or conventional open vent DHW cylinder it's still a trickle - but it doesn't show so much in the house as it only has a shower at the moment.

Yes, I agree that it will use no gas in standby in eco mode - but I couldn't live with it like that.

As to lagging, I agree but only to a point. Mine's boxed in because the property was built without anything that could be used as an airing cupboard - the other downside of combis is that developers use them as an excuse to build substandard homes*. So being in teh garage it needed boxing in to make it tidy. In an airing cupboard (as could be the case in a DHW cylinder replacement or new build) it would be fairly easy to just put an extra layer or two round it (are cylinder jackets still cheap to buy ?) and it's hard to see someone not having an inch or two of space around the cylinder.

* You may have gathered that I don't think very highly of many modern shoe boxes ... err I mean properties, and the developers that built them.

[SimonHobson]

Sorry, forgot about the other issue.

If you have a combi then it will have a huge output. It will also have a built in pump.
When you are heating the property, you then have a grossly oversized boiler, and because you have to maintain a minimum flow rate, that means a bypass to allow hot water straight back to the boiler. Typically you end up with noisy TRVs, have to compromise on where you put the room stat and a non-TRV rad, and I bet a lot of the time there are a lot of boilers not condensing because of the bypass raising the return temp - can be "fixed" by having weather compensation, but how many have that "expensive" optional extra ?

[greentangerine]

I've got a Vaillant ecoTec+ 937 combi and we're very happy with how it all works. 

We've got a large house and three showers - it'll handle two running at once.

Most of the time the DHW is bypassed because of my thermal store (heated by solar thermal and the wbs) but when the combi kicks in, you don't notice the difference.

[tz0c0s]

Simon,
   Ref your boiler. If it is a re-badged Worcester, it may be worth reading the manual. Run it in Eco mode, thus using no gas. A quick flick on then off at the tap tells the boiler to pre-heat itself. Wait less than a minute as there is no water taking the heat away & it will provide a toasty trickle of water for you 

Regards Andy

[SimonHobson]

Yeah, I know you can do this - fortunately I don't have to live with it, and the current tenant used to live with a "none too reliable" system in here previous place so has low expectations. But what a palaver to go through just to get hot water