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

[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.