Recommendations for a decent boiler with external temperature control?

Before the weather gets too cold I am toying with the idea of swapping the boiler. I suspect that this is one of those rare occasions where it is probably worth doing it on fuel costs alone. I would estimate the current boiler is at best 70% efficient, and the configuration is not ideal since its a S Plan system with no pump overrun. Hence with every cycle, the high water content cast iron lump will dump its stored heat out the flue. Our gas costs last year ware approx £1300[1]. So by my reckoning that's about £910 worth of useful energy, which would cost, say £1,010 to get from a modern condenser. I would expect to get a bit more improvement from splitting the heating into two zones as well. So allowing for a bit of slack in the numbers, payback in 4 years ought to be doable.

This would be a kill two birds with one stone exercise as well; the current water heating arrangement (conventional gravity indirect), is poor. There is insufficient head of water, so pressure on the hot water is poor downstairs, and daft upstairs where there is only about 4' of hight difference from bath to top of cold water tank. Also the header tank and CH feed and expansion tank fill a large cupboard which would convert into a really nice ensuite shower!

So the plan would be, rip and replace the boiler;

Convert everything to a sealed system;

and cobble together a heat bank to heat the hot water on demand.

I don't want any part of the heating system to be vented, so that suggests discrete plate HEs for charging, and extracting heat from the bank. Note the heat bank will be just for DHW, I have no intention of running the rads from it. Cold water supply is decent pressure and at fire hose flow rates.

So ideally, I want system (or possibly combi - although the DHW output would only feed the kitchen tap) boiler, with internal bypass. A good modulation range to drive the rads, and the option of remotely upping the temperature output from the boiler for the final stages of recharging the heat bank. 20kW is probably enough for the current heating load. Although I will initially install it myself, I don't want anything too esoteric that will leave an off the shelf heating "engineer" scratching his head and sucking in air through his teeth! The less Geoff sees of the PCBs the better as well ;-) Suggestions for a short list?

[1] about 70 ft^3/day in summer (HW only), and up to 5,500 ft^3/day in winter. Yes, I know insulation is the other thing that needs attention - but since that will have to be an external solid wall cover and render job, that looks like it might cost even more than the boiler!
Reply to
John Rumm
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The best value of any and very specced boilers are the Remeha Avantaplus range. Broag. They do a 39 kW combi, the 39C, if a combi is what you want. Remeha have just bought out Baxi and they are well placed in the UK now. Do a search on this group and all will come up. It is a dual-temperature boiler with Integrated outside weather compensation. Have the DHW on a DHW priority system. When DHW calls the boiler runs up to maximum, or whatever boiler stat is set to, and when on CH operates from the weather compensator. Then very fast DHW recovery and if the heat bank is connected up right, it will feed the plate heat exchanger as well. This means that the heat bank need not be as big as the boiler assists and yiou willnever run out of DHW. The boiler also has OpenTherm control.

Reply to
Doctor Drivel

There are pressurised heat banks, See Advance Appliances, they will do one. No G3 needed and all it needs is a pressure relief valve, although I would fit two. The cylinder is heated directly from the boiler so the system has a large expansion vessel and the boiler has its own PRV too. That is three on the system. The reheat is startling. Or a vented heat bank reheated via a plate heat exchanger and pump. Still fast. Use a 150kW DHW plate heat ex.

Look at the Broag 39C combi and if that flowrate is fine go for that. If you want backup fit an in-line instant electric heater, which is normally off.

Reply to
Doctor Drivel

Installed an Atmos HE32 in May of this year, it has an external temp sensor which modulates the CH water temp.N o probs so far, A bit early to report on it as it hasn't done a winter yet. Also installed a water softener which deals with our hard water (230 ppm approx). superb showers now. Cheers Don

Reply to
Donwill

Spec wise it sounds ok, but I am not sure if it passes the test of being mainstream enough. It may be that will change in time, but I would rather go with something that a reasonable collection of people have some experience with.

I potentially made the mistake with the Isar I got for the last place in buying a newly released model based just on tech spec alone. At the time no one had anything particularly negative to say on them, but I gather that their PCBs are some of Geoff's best sellers at the mo. (fortunately it always worked very well for me, and was reliable for the few years I had it).

Reply to
John Rumm

I was thinking of a DIY vented heat bank with plate HE's in and out.

Don't really have much use for combi in this case. If it were a case that the spec I wanted was only available in combi, then I could use one and just have the DHW feed say the kitchen and utility room - neither of which have high HW demands, but the short dead leg might be nice (the boiler will be in the utility room). I expect a system boiler with internal bypass would be the best fit to the circumstance.

Reply to
John Rumm

It is sold by all outlets now, and soon you may see it with a Baxi badge on. It is selling like hot cakes and pushed hard with adds on the rear of trade mags. Service is good and parts are standard. It has a Giannoni SS heat exchanger, with all you need built in.

The Avantaplus is basic industry standard parts that you will see in a Vaillant. They are "very" reliable. Made in Holland. They have bought Baxi-Pot and service can only get better.

They, and Keston, are about the only dual-temperature boilers around. The

3-way valve and cyl' stat are wired into the boiler. The 39C combi is excellent. If the flowrate is fine for you at 16 litres/min then go for that - beats the hell out the old Ideal you had. Their web site has pdf's on wiring, installation, etc. The weather compensation is "standard".

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Reply to
Doctor Drivel

Best use a very large DHW plate heat exchanger - 150 kW. These are very efficient and will work at a very low store temperature. This means if stratification is messed up somewhat or the temperature is just low they deliver the DHW.

Stratification is not messed up too much as the volume of water in the cylinder is quite large and the DHW pump pumps the right way. The boiler pump pumps the wrong way but very hot water is pumped into the top of the cylinder, so not a problem.

A spreader can be fitted on the DHW return. Drill out a compression fitting pipe stop and slide in the pipe. Have an internal pipe stop end. Drill holes in the pipe and have the holes face downwards. All pumps must pump away from the store.

The bottom of the cylinder can be very cool - 30C, while the top 75C. High condensing efficiency is guaranteed for most of the re-heat burn.

Have the CH off the heat bank as it is so much more effcient. A Smart pump on the CH flow with a check valve after. Have a Magnaclean filter on the CH loop return - essential. With a Smart pump you can have TRVs all around.

Look at: AVANTA 18v System Only Option 6

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heating boilers are vented and the system sealed.

They are dual temperature boilers. The Broag 18V is open vented with an internal weather compensator. It is very well priced and a quality Dutch product. An external 3-way valve is energised by two anti-cycle stats on the upper part (DHW) of the store. All the boilers heat at full temp and goes to reheating the DHW ASAP. When DHW satisfied the 3-way valve diverts to heating only the bottom section to the dictates of the weather compensator.

The room stat is linked out and the 3-way valve is controlled by the boiler. The cyl stat is wired into the boiler. (maybe two stats using a relay to prevnet boiler cylinder on the DHW section). Just buy an outside weather temp sensor.

Then the rad water is at the right temperature dictated by the weather compensator and a large volume of water in the store too preventing boiler cycling. No mixing valves are needed for CH as all is done by the boiler controls.

You can have two CH loops off the heat bank, each one with a Smart pump and check valve, ansd all have TRVs all around.

If cold it take a few minutes to get DHW at the taps, so no need to reheat a large cylinder to get DHW. Switch on from col. get undressed and by the time you are ready enough for a shower is there.

A vented heat bank can be heated via plate and pump.

Advance Appliances (SS cylinders) will put the probes and tappings in the place you specify. They will provide the plate heat exchanger and most of the bits and you just connect up. They don't like to connect up electrical parts as they need to test and have certificates etc.

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the dual temperature and weather compensation of the boiler is a great way to enhance a fully integrated CH/DHW heat bank. A "very" cheap was as well, no CH mixing valves or the likes - all done with the boilers internal controls.

---------------------------- Cranfield UNI....

Applied Energy Group, School of Mechanical Engineering, Cranfield Institute of Technology, Bedford MK43 0AL, UK

Cranfield Institute of Technology and British Gas plc have undertaken a collaborative programme of tests of a system which interposes a heat store between a boiler and a distributive central heating network. This has culminated in a field trial at Spa School in London. It was predicted to lead to significant energy savings, to a lower installed boiler capacity requirement, and to reductions in installation costs for new and for some retro-fitted systems. Specifically, a thermally insulated 3235 1 hot water store was installed in the circuit between the existing modular boiler set and the hydronic 'radiator' network. The pipework was arranged so that the heating system could be operated either as configured originally or in the storage mode, in which case two of the five modular boilers were valved off. The two configurations were operated alternately (for four week periods), and their performances were carefully monitored over the 1985/86 heating season.

The storage system performed successfully with a 40% lower installed boiler capacity. Boiler cycling was reduced by a factor of 15 and the mean boiler load factor rose from 37 to 70%. Analyses of results show that a 6% saving in annual fuel consumption was achieved by the storage system as compared with that of the highly efficient conventional system employing direct external compensation of the boilers. However, all this saving is attributable to the faster building preheat achieved with the store as, surprisingly, the annual efficiencies of the two systems were the same (in contradiction to the findings of other trials). The reasons for this are examined. Where direct compensation is not possible, the reduction in boiler cycling demonstrated will lead to a further substantial efficiency advantage of at least 10%.

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fuel reduction further 10% because of boiler cycling elimination Boilers 60% less in capacity.

At least 16% better in fuel efficiency. The conventional system was direct weather compensated as well.

Put in an Avantaplus condensing boiler, which can heat the CH section of the store directly to weather compensation and savings are even greater.

You install a pretty small boiler and get away with it, saving a packet in instalation and running costs.

Reply to
Doctor Drivel

Vaillant can also do dual temperature.

Reply to
David

That was ages ago!

And a commercial application.

Reply to
David

twas the plan - 100kW or more anyway...

Yup, efficiency when re-heating the store is less of a concern than total energy density, it would be nice to heat it to 70+ but without also needing the heating running that hot...

No thanks. Not wanting to go into this again now, but I really can't see enough advantage with modern condenser that can get better than 90% efficiency driving the rads all by itself.

With a S+ plan system would rather have a bypass somewhere as a fallback anyway.

That is prehistoric in boiler technology terms...

Seems unlikely you would achieve anything like that with a modern boiler (given the most you could get is about 8%)

That could be a factor on large installs - but we are only talking 20kW here so not really an issue.

Not sure I follow the logic. If the boiler's seasonally adjusted efficiency (calculated using gross calorific values) is in excess of

92%, and if AIUI, that includes allowances for DHW using "ordinary" efficiency indirect cylinder style heating. There is not much cost saving to be had no matter how sophisticated you make anything that follows.

The heat bank for DHW makes sense since it gives high flow rates at mains pressure, and will probably cost less to install than an unvented HW cylinder. For heating however I am content to let the boiler look after that - maybe with extra sensing from an external temperature sensor.

Installation cost will be much the same whatever I put it (i.e. tending to nil, so long as I can avoid too much extra external complexity)

Reply to
John Rumm

Vaillant can also do dual temperature.

Reply to
Doctor Drivel

That was ages ago!

And a commercial application.

Reply to
Doctor Drivel

The bigger the lower the stored water temperature. You can get 50C water with store temp of 55C, the plates are so efficient.

A large DHW plate can have the store at 60C.

You don't seem to understand what is going on. A modern condensing boiler will drop to around 6kW. This mat be too large an output and result in boiler cycling. Heat the bottom of the store (3-way valve to heat top DHW section and CH bottom) by the boiler using the internal weather compensation controls (no stat on the cylinder, all done in the boiler. The two CH circuits can come off the CH section and TRVs all around using a Smart pump. No wall stats are needed. The Smart pumps can inject into a CH circuit just

0.25kW if need be. A boiler directly on the CH circuit cannot do that. Anything below 6kW demand will entail the boiler cycles. The boiler will not cycle (or so infrequently) when reheating a large body of water. The store also gives "instant" heat up in the morning with the rads hot in a few minutes.

A 1" tapping can be at the cylidner. This can have a short 1" pipe and two

22mm pipe off - one to each CH circuit witha Smart pump on each.

A by-pass is best eliminated. They open up and cause a short circuit of hot water back to the boiler reducing condensing efficiency. They are invariably set incorrectly. They run out of setting over time. They are a kludge and best avoided. When the TRVs are all closed up, or mostly closed the flow through the boiler is restricted. When the boiler is connected directly to the cylinder it is in the ideal hydraulic environment with full flow. This improves efficiency and longevity.

Still the same today. Little has changed. Irrespective of boiler, whether it is super efficient or a dog, it is the difference of performance between the two "systems" that matters. The same boilers were used on both systems.

Cycling is inn efficent on any boiler.

It is the same for any Heat bank/thermal storage system. The point is that you can get away with the cheapest CH boiler and maybe save £300 or so.

You will get improvements for sure.

See above, re: by-passes, too low a kW output.

A heat bank running two CH circuits with a Smart pump on each is not complex at all. No room stat needed and TRVs all around. Simple. No kludgy by-pass valves.

Having the boiler heat the cylinder (heat bank) with a 3-way valve electrically connected to the boiler and the DHW stat connected to the boiler and using the weather compensation is simple (no connection to the lower half of the CH section. All in the boiler's controls. The heat bank has two flow connections. One at the top for DHW and one at the centre (approx) for CH. A boiler operated simple 3-way "diverter valve" sends hot water to the CH section (set by the boiler stat) to max boiler temp for rapid reheat. When satisfied the valve diverts to the lower CH section and heat to the dictates of the weather compensator.

  • Very simple to fit - the dual-temp boiler and integrated weather compensation makes it shine.
  • An effective system using a dual-temperature boiler with integrated weather compensation.
  • Everything operating in ideal hydraulic conditions.
  • Equipment longevity guaranteed.
  • Ideal comfort conditions with TRV all around and rad temp to weather conditions.

I fitted one the same config but using UFH only. The slope on the weather compensation can be adjusted to UFH. Just a high limit stat on the UFH flow as protection. The high expense and complexity of UFH was eliminated. The store always had the correct temp water diverted by the weather compensator, so no temperature manifold mixers. One UFH pump was used and a zone valve and room stat on each UFH zone. Saved a bomb in money. Works great.

Reply to
Doctor Drivel

Look at the ATAG 50plus kW combi. 25 l/min. Not cheap but quality indeed. Service and parts are good in the south.

Reply to
Doctor Drivel

Pls excuse the slight tangent but I want to replace our existing boiler before winter. Ideally I would love to install a softener too but I have always worried that the softener would restrict the flow/pressure.

We have quite low mains pressure in Cambridge, typically 2.5 bar, this correspondingly delivers only about 20L/min at ground floor on cold mains but a disappointing 10L/min in our second floor bathroom.

Clearly, Don, you are very happy with your shower now but I wonder if you think your water supply and situation means that the softener could be accommodated better in your supply than it would work for us?

What kind/make did you use - I have noticed some are limited to 1.75 bar?

Any tips/info gratefully received.

best wishes

Nick

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Reply to
Nick Holmes

Yup I realise they can work with relatively little differential temperature. However if your store is at 55 deg you are reducing the effective size of it by quite a margin. You are also never getting it hot enough to ensure to kill all the bugs...

Indeed. However cycling with a pump overrun is less of an issue since the residual heat still gets pumped into the house rather than out of the flue.

This is a victorian place with tall solid walls in an exposed location - average heat loss is about 6kW. Now even with more insulation etc, its not going to come down that much - I might be able to get down to 4kW. So with a decent modulation range there ought not be much cycling anyway. (previous house had lower heat loss, and the boiler could only come down to about 8kW IIRC - that did not cycle particularly)

Not really an issue here. I have the prog stat set at 15 overnight, and its rare for the heating to fire at all. So the reheat is not that difficult.

Hence the advantage of having an internal path that is controlled by valves...

(failing that, I could arrange a bypass rad in the airing cupboard - probably not a bad idea since the new cylinder etc will be far better lagged than the current one)

Given the variation in heatloss between different bits of the place, I doubt the flow is ever that restricted at the moment.

Other than mandatory use of condensing technology, and a >10% rise in seasonally adjusted efficency...

The impacts vary greatly with the system though. On a setup like my current arrangement it is bad news - fixed output boiler, and no pump overrun coupled with high heat content in the boiler itself. There is lots of energy to vent out the flue on each cycle. With a low water content boiler and light weight HE, with pump overrun, no more heat is wasted than when the boiler is running, The only "loss" is that you may overshoot the room stat set point - but a bit. If you are really worried about that then use an optimising stat.

At an expense of probably that or more spent on extra complexity elsewhere.

Quite possibly, I am not disagreeing - lets say I get a 20% improvement (which is probably optimistic) - that could raise the efficiency from say 92% to 94% - possibly £20/year saving in gas. I will pay that in electric for the extra pumps. We are well into diminishing returns.

In a super insulated house with diddle squat thermal mass it may make noticeable difference to comfort levels. In this place with high heat loss and high thermal mass (or medium heat loss and high thermal mass when I am done insulating it), its not an issue.

I still want room stats to allow independent control of the zones (that I will introduce).

Well you have added more pumps - so system MTBF has fallen...

Not fussed by rad temp particularly - its room temp that I care about.

For UF it makes more sense since you want significantly lower flow temps...

Quite possibly, but one size does not fit all.

Reply to
John Rumm

By second floor I take it you mean in the UK sense - i.e. two storeys above the ground? Pressure drop due to height is about 1 bar for every

30', so it sounds like you might be dropping 0.7 bar due to height (depending on how tall your rooms are etc). That does not sound like it ought to halve your flow rate all by itself. Are you sure you have not got a mass of convoluted piping, or a partly closed service or gate valve somewhere?

Don can do that bit ;-)

Reply to
John Rumm

Yes, the bathroom is on what we in the UK call the second floor, about

22ft above ground level. Pretty sure no partly closed valves but the plumbing is old and may be convoluted - the flow quoted was from bath cold tap. All CW taps are connected to mains, there is no header tank.
Reply to
Nick Holmes

I am making a point. The stored water temp doesn't matter. The temp of the DHW matters. Many combis do not raise the temp to over 50C. It is fresh water coming in and being instantly heated.

In Spring and Autum it will need less than 6kW at more times than you think.

The reheat is instant which does make a difference.

Draw out what I explained to you. What I put across was

  • no complex motorised valves
  • no inefficient by-pass valves (when these go they can ruin a boiler as no flow through the boiler, so best avoid if you can - a kludge).
  • one simple 3-way diverter valve,
  • a cyl stat for DHW
  • a boiler with outside sensor
  • Two smart pumps.

That is basically it. Simple and highly efficient.

You are guessing. Ensure max efficiency.

Take note of above passage.

Cycling is inefficent in burner startup

Again. Draw out what I have explained. It is ultra simple and cheap and highly efficient using a dual-temp boiler.

The Smart pumps wind down and use less electricity.

You missed the point(s)

You put a cheaper timer on each zone, switching the Smart pump and boiler (tell it you want CH). No need for more complexity in introducing electric room stats. The problem with room stats is that when it closes, a number of rooms may be calling for heat. TRVs give 100% room temp independence. Or one timer for the CH store section (the master, when off the CH is off and zone Smart pumps) to get instant heatup and one timer for each zone. Simple timers are cheap. They can all be in a bank together.

A Smart pump for each CH zone replacing a 2-port zone valve, and increases reliability, and the internal pump of the boiler. Simple.

That improves comfort conditions by injecting the correct amount of heat from rad. No spikes off high temp, up to temp, the off, then cool then all over again.

It make sense in rads as it is simple and cheap and highly effective.

It is not the size, it is the layout. You would be better doing what I explained. Do no go down the zone valves/room programmer-stat/bypass/rad off the boiler route. Do it cheaply, simply, properly and efficiently. You have the cylinder, just upsize it and little the rest is simple and cheaper. While doing a heat bank it is foolish not to do the CH from it as well. The cost effective dual-temp boilers have made it so much easier to install and setup.

Unlock your mind out of what you had planned. Look at things from another angle.

Reply to
Doctor Drivel

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