heating and DHW upgrades - advice please

[ian192744]

hi all

long term lurker and reader on here, this is my first post, hopefully I follow the etiquette!

House is 1973 built 2 storey 4 bed detached, brick and block,  "too-thin" 40mm cavity wall with 20mm EPS in from build time, cannot add more due to lack of width. Loft has 400mm wool DIY'd.  Floors are uninsulated concrete. double glazed. All rooms have large radiators, 20 at the last count. flow temps of 50-55 warm the house perfectly well. EPC says 22MwH, in practice we use 15MwH per annum .Full Honeywell Evohome control of CH and DHW. Gas boiler Viessmann 28kw 100WB1B 4 years old.  DHW is in 1st floor airing cupboard - a DPS Pandora 210L open-vented store with solar thermal coil at the bottom (which is now unused ). DPS and solar thermal installed 15 years ago. thermal system has gone - it died (long story, not for here) then replaced the roof use with 12kw of PV.. Mains gas, Mains hot water (good pressure 5 bar).  2x Tesla powerwall2 batteries.

 

problems to solve:

1. need more hot water capacity in winter :

• Condensing boiler cannot achieve the designed store temp (75c). It worked fine with the old nasty non-condensing one!.
• Boiler has to be made to run extra-hot (79C flow) to reheat store, but even then - 
• 67C is hottest store temp that boiler can reliably achieve in a reasonable amount of time.
• Bottom part of store is unused capacity.

2. DHW reheat necessitates CH shutdown for 30 mins 

• always happens once , often twice an evening during peak childrens shower time in winter.
• takes boiler 5 -10 mins to ramp from 50 (CH flow) to 79 (DHW flow target) + another 20 to reheat.
• if not CH not shutdown , the CH cold return doesn't let the boiler get hot enough, so the HW reheat takes ages. Plus the rads overheat due to the too-high flow temp.

 3. Rads group 3 are poorly/not heated if groups1 and 2 are calling in large numbers.

• Pump is in PP3 mode. Group3 is a long convoluted pipe run , can't do anything about it. Pump on fixed speed3 will reach group3 but is very noisy (very loud resonances as well as flow noise)-  when load is low (1 rad open a tiny bit) so have to use a PP mode

4. shoulder season short cycling

• lowest boiler modulation is 7.5kw. Load with only 1 or 2 rads , house in daytime occupancy, is way below that.

5. increase hot water storage capacity  spring<->autumn:

• PV dump heats upper store to 90C easily  - immersion is between the upper and lower coils.
• increase storage capacity by moving heat into bottom part of store

6. Add ASHP in future, without changing everything else. ASHP would be sized down so still use the gas for the worst days.

 

existing system as per diagram1

 

constraints - no space to add any additional large tanks or stores. Loft is impractical (tiny hatch, no space for a big tank). No ground floor space that works .

Don't want to throw away perfectly good existing kit (store and boiler).

 

my thinking on solutions

diagram2: 

problem1+2 -> loop gas DHW mode additionally through solar coil . Heat in "base" mode through on lower coil only whilst keep CH active. 

diagram3:

problem5 ->add de-stratification loop to run primary water in a loop between the coils until temperature evens out 

 

diagrams2 and 3 aren't an either/or they are logically sequential steps which would in practice be implemented at the same time.

 

problem 3-> increase to a higher head pump . current is alpha2 15-50/60. If upgrade to the 80 will this help as its PP3 curve is steeper?

 

diagram 4:

problems 4, 6(+3) : rework to include a100L sealed system buffer tank (such as this) to separate CH and boiler flow sides . Mount it ABOVE the DHW cylinder - There is a 3 sided internal wall recess to fit into, so strength to build appropriate wall-mounted supporting frame for it is not an issue.  

Set a higher flow rate on boiler side vs CH side to decrease store/HW reheat times. Can ASHP and boiler share same primary tappings into buffer via Tee, or better to have additional tappings put in?

Not shown on the diagram, but also wondering to  perhaps take from the buffer via another plate HEX as a "winter preheat" for the HW, to avoid the cold water being out-of-the-ground cold when it hits the DHW store, to further improve DHW delivery capacity?

 

I'm hoping I haven't completely gone down the wrong track here - not spent any money on new stuff yet but quite some time thinking about it and analysing the behaviours with temperature monitors etc  - would appreciate any advice anyone can give.

 

thanks in advance

Ian

[Nickfromwales]

With the PV and batteries, plus the suggestion of an ASHP, you’ll need to get rid of the TS ( thermal store ) and fit a HP UVC. I’d fit a 300L  UVC. Remember the ASHP will heat the tank slowly, so factor that in. Get dual 3kW immersions fitted and then you can boost for duress.

The thermal store cannot stay. 

[ian192744]

hi nick

thanks for your advice and time. I was already seriously considering the 300L UVC option - certainly if I was doing this from scratch with no existing system fitted, thats pretty obviously the best option, have read lots of your previous commentary about that. However, ditching the TS is a big thing - both in terms of physical work , cost, and mental commitment (SWMBO view of nothing wrong with it , its paid for, why get rid?).

 

would adding the buffer be a good thing to do, regardless of any potential decision about ditching the TS to put in an UVC?

The objectives being

- separate the flow speed , flow temperature and demand curve/timings on the boiler loop from that on the CH loop

- facilitate the future ASHP (which is nothing more than an idea right now, probably 2-3 years off).

 

immersion duress boost in winter isn't that sensible - it will draw from battery (which has charge overnight on octopus go) so although only charged at 5p a unit, its a cycle through the battery that is not best use of its lifetime cycles. I was thinking instead I could run the system boiler through an external plate hex, creating a pseudo-combi for the duress boost situation. That gives an immediate lifestyle benefit of enough hot water at all times in winter!

 See new diagram below. shopping list would be 2x TMV, 1x plate heat exchanger, 1x TDV, 1x pump, 1x 3 port multiposition valve, 1x 100L buffer so nothing too major.

 

does upping to an 8m head CH pump make sense? the PP3 curve is steeper on the 15-80 , so am I correct to think  should get more flow rate at the same radiator resistance/pressure level when all are open circuit?

 

regards

Ian