Solar Heat Bank Thermal Store Pre-Heat For Combi Using Plate Heat Exchanger

[ZeB]

Hi there, I've got a Baxi Duotec 40, had it for about 2 years now and seems to do exactly what i intended it to, I had it fitted just after we bought our first home 2 years ago, previous system was a back boiler with Hot Water Cylinder, I completely replumbed the house when we moved in and had the combi fitted, at the time we had a tight budget and I could not afford new Rads aswell so hav been running the system quite ineffeciently so far as I cannot run the boiler on anything other than max output on CH as the old Rads do not loose enough heat quickly enough, I am at last in the position and hav ordered 7 new rads and a weather compensation kit (Baxi Multifit), I have massively oversized the new Rads on the premise of havin an extremely low flow temp and even lower return to maintain condensing operation of the boiler, as far as i'm aware the system should run for longer periods but at the lower modulation level and condensing mode all the time, with the old Rads I'm only gettin a 6-7 degree temperature drop flow to return. As I have ordered my new rads and compensation kit I am already planning the next stage of increased efficiency.

The next stage wheneva my budget allows is to incorporate solar thermal and a Heat Bank Thermal store into the system, from what i have read and researched so far there are 2 main options:

1 - Preheat the cold water prior to the combi topping it up
2 - take a zone off the primary circuit to a twin coil cylinder

I would sway towards option 1 atm as no matter what solar heat was stored i would always hav unlimited water from the combi.

My main question being could I use a Heat Bank Thermal store to collect solar energy and then use a plate heat exchanger, CH pump and flow switch to preheat the cold goin into the combi without the need to store luke warm water for preheat. Will my combi allow me to use this configuration, Obviously i would utilise a TMV to maintain a safe temperature of hot water to distribution pipework.

Also looking at the Multifit Gasaver from baxi any1 rate it? believe it is covered by the green deal?

 

[carrpaul]

Hi there why did you fit combi ?

 

[ZeB]

It seemed to tick the boxes at the time, 19 or so litres/minute DHW and was condensing, Had good reviews, Before i had considered Heat Banks & Renewables I didn't like the idea of storing water i wasn't necessarily going to use.

 

[TBServices]

Frankly, if you wanted an efficient system, you should have had it specced, sized and installed by a professional. You're already walking down the wrong path IMO.

 

[lame plumber]

oversized rads are just a waste of money, get the right size for each room and use the combi as a system boiler and have a solar cyl and kitchen tap on the combi dhw. then you may start to save some money, get someone in who knows what they are about, you dont I'm afraid to say!

 

[doitmyself]

oversized rads are just a waste of money, get the right size for each room

But the "oversize" rads will be the right size for the room - when you take into account the OP's intention to run the system at a lower temperature from that used by the rad manufacturer when testing the rads.

 

[lame plumber]

he has massively oversized them ,not done any calculations as to requirements at a lower flow temp! so how does that make them the right size for the room, being a heating technician isnt guess work it takes a littlebit of thinking and knowhow!!!!!!!!!!!!!!!!!!

 

[ZeB]

Firstly I'd like to point out that I asked for the 40kw combi to satisfy my desired hot water requirements which as far as I am aware is how a combi is sized, not on CH demand, Secondly I was not aware that I had said anything about not doin any calculations for the rads, I sized them and added 25-35% as was recomended in a post on this forum, but also took into consideration my budegt and went slighttly larger again as a thread on this forum concluded that the bigger the better as far as rads are concerned for runnin at low temps to maintain condensing mode/possible future renewable applications like Heat Pumps etc

I was under the impression this forum was all about advice no criticism and posted hoping to have an in depth discussion with like minded people who were interested in Central Heating and Renewables.

As far as my research has lead me to believe the due point of flue gasses from the burning of natural gas is approximately 55 degrees centigrade, with my current rads I can only get a temperature driop of 6 degrees as previously mentioned, The rads do not allow me 2 run my boiler in condensing mopde at any point, If i ran my boiler with a flow temp of 60 to guarantee condensing operation My house does not heat up as the heat loss outweighs the heat output of the rads.

Thanks for the Unconstructive criticism, I'll do you all a favour and look elsewhere to seek the answers to the origonal post which No1 seems to be willing to answer.

 

[lame plumber]

part of designing and setting up a heating system is to size the rads correctly for the job being done, then to set up and balance the rads across the sytem to achieve the 10 deg drop required across each rad. then you can achieve what your looking for, but youll not know that now having stomped off despite being given some advice in the first place. I also wonder whether having a 40kw baxi and then wanting it to condense in central heating mode is a viable choice being that it uses 40kw for water heating and not condensing and then wanting to save energy when it modulates down for ch mode. bit bum about face in this energy saving world of ours

 

[Nostrum]

If you serious about this then the 40kw combi has to go, in fact you would effectively be starting from scratch . To change to a twin coil cylinder and boiler (as your boiler is totally unsuitable for anything but instant hot water at a reasonable flow rate) , plus panels is going to set you back more than you'll ever save in gas consumption. The combi was a fatal error, which I hope you can blame the engineer who installed it. If you want to try and claim any kind of incentives for the solar, it'll need to be fitted by an MCS accredited company who aren't going to want £100 to sign the install off (if I read you post right, you may be looking to do this yourself?) so you'll be paying the premium for their services unless they are desperate.

not wanting to sound negative but i can't see it being worth while financially.

Personally I hate large combis, often over sold on their flow rate capabilities even though the extra is rarely necessary. They fall between the gap of the convenience of a combi, with the performance of a cylinder and don't do a great job at either as you are probably now figuring out.

The only real way of keeping the 40kw and making it work would be to use a thermal store as a buffer, the boiler wil fire for longer and less often so should be able to modulate down, but its not the best solution and quite expensive. Probably cheaper to just replace it with a 15kw (taking a pot shot at your requirements) system boiler and twin coil cylinder.

 

[Nostrum]

Just to add, the weather comp won't really help, nor will be rads. Your problem is the minimum output of the boiler versus the heat loss of your system. Your boiler will still be pushing out 10kw regardless of the flow temp as this is all it can modulate down to, more when condensing so you may even create more problems. The boiler will reduce the flow temp, so it is more inclined to short cycling if the heat load is less than this.

whats the house its fitted in, bedrooms, age, terrace or detached etc, big small etc?

 

[Koogatubac]

Why post to DIY'ers when you give them advice (which they dont like) and say your criticising them.

Thats why i dont comment to these topics

 

[Nostrum]

Too be fair to the OP, he has done some homework, and I've seen worse ideas discussed in great length on here ( DIY underfloor heating springs to mind)

 

[doitmyself]

part of designing and setting up a heating system is to size the rads correctly for the job being done, then to set up and balance the rads across the sytem to achieve the 10 deg drop required across each rad.

If you are balancing for a 10C drop across each rad, how are you going to achieve the required 20C drop at the boiler?

 

[ZeB]

Thank you all for the follow up advice, much appreciated, On the note of boiler running at full whack for the water I did enquire in my first post regarding the Multifit Gasaver from baxi which fits between the boiler and flue and preheats the incoming cold to the boiler when in DHW operation and claims up 2 37% savings on gas consumption for DHW?

I have an end terrace house 3 beds, fairly spacious as is ex council, I believe it was built in the 60's and has had cavity wall insulation. We also had the loft insulation done when the government push for Part L kicked in, think we have 300mm loft insulation. It has double Glazing but this is fairly dated and is also on my list of things to update at some point.

 

[doitmyself]

I have an end terrace house 3 beds, fairly spacious as is ex council, I believe it was built in the 60's and has had cavity wall insulation. We also had the loft insulation done when the government push for Part L kicked in, think we have 300mm loft insulation. It has double Glazing but this is fairly dated and is also on my list of things to update at some point.

Sounds very like my son's previous house. His heating requirement was about 10kW. If you want to know what your heating requirement is, use the Whole House Boiler Size Calculator.

Many say that combi boilers are sized on HW only. But if the CH requirement is not taken into account you get the situation where the boiler cannot modulate low enough to meet the CH demand, so it has to operate in on/off mode. In worst cases it does this all the time.

Did you measure the cold water flow rate before deciding on a 40kW boiler?

 

[ZeB]

Yes, I have 4.5 bar pressure and about 26 l/m flow with the cold open full blast, I currently hav a high flow shower in place but have factored in a possible down stairs shower room in the future as I have 2 small kids and can foresee the need to strip um down and sling um in downstairs outta the way after sports etc when they get a bit older. You could argue that there is little possibility that both showers will be in use at the same time but when they are of the age for full time school etc it will make morning life much easier to run 2 similtaneously. I plan to flow reduce all the pipework and fit a PRV as 4.5 bar pressure is a tad excessive, as i mentioned before though as money is a key factor in any project it is all a work in progress and happening as an when it is feasable.

I piped the house specifically to prioratise flow to the shower/bath atm, I have 22mm cold all the way to the boiler, with the cold for the rest of the house tee'd off near the incoming service, I have 22mm hot outlet on the boiler feeding the shower/bath and 15mm hot distribution pipework to the rest of the house tee'd by the boiler.

It works absolutely fine as is, apart from the severe cold weather we had caused the hot water to not reach full temp in the early hours 5am or so i.e. I could shower with full hot selected. The plan is to flow reduce the cold water supply to the boiler near the boiler to 19l/m to allow it only ever to work at maximum rate.

Plus as i previously mentioned i'm eyeing up the Multifit Gasaver from baxi which will utilize latent heta in the flue gasses during DHW operation and preheat the cold entering the boiler.

 

[Nostrum]

Also worth bearing in mind. Using doitmyself's example, 10kw heat loss will be at approx -1 to -3 outside and so if either the outside temperature rise or trvs etc start shutting down once up to temp, the heat loss reduces and the boilers requirements do so as well. Your choice of boiler means that in all but the harshest conditions, the boiler cannot tick along and stay lit for long periods which is inefficient.

The gas saver is aimed more at specifiers and house builders trying to earn points for SAP. I'm not sure on cost, but your hot water demand is only a percentage of gas usage, so it may be 35% of 40% of your boilers consumption, maybe less. You would have to do the sums and see if it pays.

 

[Nostrum]

Based on £1000 a year gas bill ( probably a bit steep) 40% being dhw is £400. 35% of is £140 a year. Unit cost looks to be about £650 plus fitting (as it is the flue and needs to be carried out by a competent person) call it £800. So your looking at approx 6 year payback, all being well. That's just to break even. Unsurprisingly, you don't see many of them about.

 

[ZeB]

I think my boiler modulates down to 9kw? might be wrong though. I think for my situation I would probably be happy breaking even after 6 years and saving x amount ever year after on my heating, I'm not planning on moving or selling up long term so not so bothered about quick payback of investments. So the biggest flaw of my boiler is the lack of scope for lower modulation i guess then? I assume that any previously mentioned renewable ideas i have had are kinda pointless if they will cause the boiler to cycle frequently? or even with it cycling would it be worth having a heat bank as a buffer or similar setup?

 

[Inverness]

Very expensive to run and ****e flow rates! I don't see the money saving on a thermal storage with combi. Dhw running all the time costing money and the flow rates are not that high unlike With a system boiler with a mains pressures all round and a 2nd coil for your vacuecated tubes to pre warm the dhw inside cylinder.