LLH/Small Buffer - Pumping direction

[ian192744]

hi all

interested in the hydronics wisdom with regarding to pumping direction relative to an LLH / small buffer. Some system components like ASHP have their own pump so that is a given, it has to  at the far point away from the LLH. however for a loop that needs its own pump  (rads, and a heat only boiler), the pump needs to be sited physically close to the LLH in my case - there is no where else it can go. Should it pump away from the LLH (positive pressure into the circuit - pumps A and C ) or towards the LLH (positive pressure towards the LLH - pumps B and D). I'd have expected from the physics of it that you'd want the positive pressure going into  the circuit , but struggling to find any decent reference documentation on this?

 

reasons for system design

- existing system is not great . looked at it all sorts of ways, we have a flow rate problem. all the loops are long and bendy, big house, lots of rads, TRV's on all. boiler short cycles a lot, and in fact has been known to not fire due to not enough flow rate when TRV' drop down. separating primary from secondary will give full flow to boiler, and allow pump tuning for the needs of that loop, rather than the current which is a compromise.

- will put in an e-boiler (as backup for the gas) asap. cheap and cheerful , doddle to install, and running on 40kwh of octopus go stored in my 3x powerwalls its a wee bit cheaper than gas.

- intend to add an ASHP next year but will keep boiler. not been able to pin down a vendor or installer yet . capex for a 16kw + install struggling to make sense of right now. but 20L LLH buffer future proofs it.

looking at a buffer with internal baffles / directed take offs to maintain stratification

 

ta

Ian

[Nickfromwales]

5 hours ago, ian192744 said:

big house, lots of rads

Hi.

Alarm bells are ringing quite loudly here. a 9kW electric boiler ( won't scrape the surface in winter ) and a 16kW ASHP ( a good mono block will offer 65^oC max with quite a poor CoP unless it is a high temp split unit with internal compressor unit ) will perform much worse in winter than the typically 'advertised' figures gleefully state.

 

The LLH will be completely unsuitable for you, so forget that and look towards fitting a 100L minimum size buffer tank for all this hydraulic separation and multi-fuel integration or don't get started at all.

 

In honest, I'd ditch all of the above;

5 hours ago, ian192744 said:

running on 40kwh of octopus go stored in my 3x powerwalls

as it will be fatally flawed when these ToU tariffs end up evaporating, or ( inevitably ) the unit costs rises as we know it will, dramatically.

 

I would look at A2A space heating via 'air-con' units to best convert electricity into heat tbh. Do you have solar PV? Sucking up cheap rate electricity and then extracting it at a high rate of knots ( in expensive to buy and short-lifespan batteries ) isn't good maths imho when purchasing 'retail'. 

 

Add up the cost of all this, and then put that into a hypothetical savings account. Then work out how much gas you could have bought with all that money....... It will startle you, I'm sure.

 

Remember that a split A2A A/C system will also cool in the summer  a big bonus!

 

K.I.S.S.

[JohnMo]

I started writing, but gave up it was too late and lost the will...

[ian192744]

hi nick

The e-boiler is a backup for if the rest is broken, just heat a couple of rooms . Many ashp have a backup heater - I'd rather get one without a backup heater and have the backup contained in a separate device (so its a proper backup!) . Its also to heat  1 room regularly . the house has just 1 room occupied during the day 4 days a week - just my study - and I detest having to run the gas to heat this when I could use electricity which is either low-carbon (Tou into battery ) or from  PV, so cold sunny days in feb-march-april it will be free. none of this is predicated on cost saving with ToU tariffs (which I happen to believe will continue, but thats by the by) just a nice to have.

 

why would a 20L buffer be unsuitable for this design? not big enough? there are 16kw heat pumps (e.g daikin) that are specced for a min vol of 20L which is why I am at that figure, its not a made up number.

100L would be really difficult to find a space for. there is no intent to run heat sources simultaneously. the heat pump in the diagram doesn't exist and won't for some time.A2A non-starter - whole house is rads. This isn't a major rebuilding project.

 

any thoughts on the actual hydronics question or do I need to look elsewhere for that?

cheers

Ian

[ian192744]

+info wouldn't let me edit above post.. too slow ...

MCS heat loss for my house at -3C is 14-16kw depending on room temp assumptions. Boiler has been flow-temp limited to 55 for the last year and half. gas load monitored on 30 minute intervals, I know we're comfortably inside 16kw almost all the time.

[JohnMo]

The more you split the system into zones the bigger the required buffer.

 

If you had a big enough buffer then a couple of immersion heaters in the buffer would get rid of a need for seperate heater.  Simplifying layout and piping runs.

 

LLH have to be correctly hydraulically sized to work efficiently. So would steer away from them.  Great for commercial applications.

 

Many do not like thermal stores, but you can get them sized for heat pumps, the main difference is the DHW coil size, being huge (about 6.6m2).  That way there is about 4 to 5 degrees difference in store and DHW temp.  You keep the store at low 50s, supply heating and DHW.

 

Here is a typical drawing showing the hydraulic layout of a system with an additional heat source as well as a heat pump. Pump for the gas boiler if not already within the boiler, follow boilers manual.

 

Note: Hydronics just means a water based heating system.  The hydraulics is the use and design of the system for water in the system.

 

 

[ian192744]

thanks. sorry but your  diagram loses all detail with the screenshot resolution.  however, I have studied dozens of similar diagrams , plumbing and heating websites, watched umpteen design videos, and it does seem to be common that the pump for the secondary circuit is shown adjacent to the buffer pumping away , but nowhere can I find a reference that says that this is the right thing to do. 

 

DHW is in a 1 year old 300L UVC (which nick steered me towards after a previous discussion a year or two back) and has a HP size coil so not changing that.

 

the buffers I've been looking at are this https://lovatospa.com/gb/product/49017080/bomber-20-hybrid to fit without difficulty, or possibly, with some inconvenience and significant re-arrangement of the airing cupboard, this https://lovatospa.com/gb/product/49017084/bomber-50-c (which does have an immersion option). I REALLY want one that will flat-to-wall mount because of where it has to go. so many uk supplied buffers seem to be just cylinders and thus take up a lot of space projecting outwards. great if you have a plant room, not so great if you don't.

 

boiler is a heat only, no pump, hence the question. Currently pump is away from boiler into rad circuit (at the mid point).