Mad idea for DHW retrofit based on Mixergy?

[JamesPa]

26 minutes ago, SteamyTea said:

The environmental/embodied energy/CO2 of scrapping an old, or brand new, cylinder is minor

True, but the cost and disruption isn't!  

 

I'm also not following the below, can you clarify?

But I doubt many manufacturers want to install 2 kg of gas at 20p MPa next to a structural wall of a building.

Especially if it unnecessary with existing technology

Edited April 8, 2023 by JamesPa

[JamesPa]

1 hour ago, sharpener said:

But I was wondering what result you would get by just having the pump to circulate the water in the tank. I would guess you could easily double the heat transfer compared with simple convection off the coil, this would (assuming linearity of yr model) give DHW at 52.5 and if 3x then 55 without any HX at all.

I can't now remember where I got the heat transfer coefficient from, but the very simple model effectively assumes that the whole contents of the tank warms at a uniform rate.  That's roughly equivalent to stirring it, your suggestion.  Convection might produce stratification improving the performance, or might be insufficient to take the hot water away, degrading it.  Not sure which, I never liked fluid dynamics!

 

You research on PHEs seems to have identified some reasonable options.  Given that the mixergy 'kit' (comprising pump, phe and hp interface box) is about 400, I would expect the components are perhaps 200.  Pumps seem to be about 100 so 50-100 for the PHE seems in the right ballpark.

[sharpener]

With the standard 0.8 m^2 coil the flow inside the coil may or may not be turbulent but is fast-moving enough not to be rate-limiting. I am pretty sure that the heat transfer will be restricted in a major way by relying on convection on the tank side. If you can get the heat away faster it would be much better.

 

I think a pump on its own would do that. You can regard it as a stirrer or a de-stratification pump but mixing the contents is less important than replacing the water heated by contact with the coil with fresh colder water, though of course it is helpful for both reasons.

 

Even the smallest one in @JohnMo's link upthread would turn the tank over 10 times in an hour!

[JamesPa]

1 hour ago, sharpener said:

With the standard 0.8 m^2 coil the flow inside the coil may or may not be turbulent but is fast-moving enough not to be rate-limiting. I am pretty sure that the heat transfer will be restricted in a major way by relying on convection on the tank side. If you can get the heat away faster it would be much better.

 

I think a pump on its own would do that. You can regard it as a stirrer or a de-stratification pump but mixing the contents is less important than replacing the water heated by contact with the coil with fresh colder water, though of course it is helpful for both reasons.

 

Even the smallest one in @JohnMo's link upthread would turn the tank over 10 times in an hour!

You may very well be right, as I said I didn't enjoy fluid dynamics at uni; I think this may need an experimental determination!

 

I think I'm currently leaning, if its feasible (which it may or may not be), towards simply running a HP at higher temp for DHW only.  This is purely on grounds of simplicity.  If only I could get a handle on the modulation ratios and control strategies, which few manufacturers seem to publish, despite the importance.  If it isn't feasible then PHE and a pump looks like a good bet, or maybe a combo.

 

Whatever the outcome I'm now 100% convinced that the prevailing 'throw away your DHW system and start again' mantra, whilst undoubtedly theoretically 'the best', is hopelessly (and unnecessarily) idealistic in the real world.  It comes down to this - from a consumer point of view, fitting a HP is functionally equivalent to replacing the boiler, little or nothing more.  Most simply won't make this choice it its significantly more disruption and significantly more cost.  I regard myself as pretty 'green' and determined to go HP within 12 months, but am still reluctant to follow the received 'wisdom' of tearing everything out and starting again if there is a realistic alternative. 

 

We definitely need a broader range of options.  I suggest that anyone who doubts this spends an evening in their local pub (unless its in Brighton of course) speaking to the customers and trying to convince them both to pay £10-17K to replace their boiler (instead of 3-5K), and accept a tax increase to fund the government grant in perpetuity.  Good luck with that!

Edited April 8, 2023 by JamesPa

[sharpener]

14 minutes ago, JamesPa said:

You may very well be right, as I said I didn't enjoy fluid dynamics at uni; I think this may need an experimental determination!

 

I think I'm currently leaning, if its feasible (which it may or may not be), towards simply running a HP at higher temp for DHW only.  This is purely on grounds of simplicity.

 

Me neither but I have had to relearn some of it for a series of boiler and A2A HX concept design projects. We even engaged the author of a book on HXs as an external consultant but he was not a lot of practical help.

 

(Although I usually describe myself as an electronics engineer, in fact I have worked on everything from nuclear fuel handling to wind turbines via radio and laser systems. And if I could do it all again I might have designed bridges for a living, a wonderful discipline, WYS is very much WYG!)

 

Anyway, if anything I would go further than you and say that once you have got the 60deg + or true 70C capability then you do have a drop-in boiler replacement, and it will be interesting to see how the competition reacts to what Daikin and Samsung are offering. There have been some interesting tweaks to the refrigerant cycle and the modulation issue can only improve; no-one even notices their gas or oil boiler going on and off all the time, it's what they do even with .

 

I think however many people will baulk at having most of their rads replaced, for aesthetic reasons or needing to rearrange the furniture or re-decorate, and weather compensation (which is still far from universal) will give you all the benefits of a better CoP in less extreme weather.

 

But I wholeheartedly agree that the best is the enemy of the good and the present approach is no way to meet a target of ?600k replacement installations/year.

 

[markocosic]

5 hours ago, JamesPa said:

 

almost all new houses are being built with gas boilers

 

See also: stupid and stubborn client.  😉

[markocosic]

52 minutes ago, sharpener said:

once you have got the 60deg + or true 70C capability then you do have a drop-in boiler replacement

 

This isn't the issue.

 

(many new units can do this)

 

Handling the minimum output is the tricky part. Compressors in heat pumps can't turn down that far.

 

Samsung, to use an example that you've given, are very much amoral a-holes. They use hot gas bypass in their heat pumps to operate at heat outputs below the minimum that the compressor can turn down to.

 

How? Feed the output of the compressor back into the input once you're down at minimum output. This uses much the same power as running at minimum output; but puts out even less heat. Like driving a car with the brakes on.

 

Crappy COP? No worries! It isn't captured on the official performance tests because let's face it who would be stupid enough to run a heat pump that way. 😉

 

Heating houses with flow temperatures of 55C isn't a big deal. Replacing cylinders isn't a big deal.

 

The tricky bits are installing cylinders where there are none today; and running 28 mm primaries to the cylinder location.

 

Former is hard.

 

You can solve the latter with split units (run teeny tiny refrigerant lines to the "cylinder" instead); but this is currently hamstrung by Dupont's regulations that were written against use of usable amounts of propane (R290) indoors or separate units (dedicated unit for hot water/fancy heat pump cylinder for hot water just as they do in the USA.

 

The government grants insisting that heat pumps deliver both space heat and hot water are also an issue. We'd be better without them and a prohibition on gas in newbuild instead.

 

Samsung's "Volkswagen emissions test mode" heat pumps are not the answer.

 

Dunno about Daikin. Have they released anything new in the last two decades?

 

[markocosic]

5 hours ago, JamesPa said:

True, but the cost and disruption isn't

 

The cylinder swap is easy

 

The primary pipework swap is difficult

 

Using plate to existing cylinder; instead of replacing cylinder; doesn't fix the issue of your primaries.

 

Fix these with (a) new primaries that use refrigerant not water; (b) a solution that uses lower flow rate on primaries (easy if.ypur property only needs 5 kW space heating); (c) a solution that uses higher deltaT on primaries (easier if you have a dedicated CO2 monobloc for hot water; or (d) a separate solution for hot water (such as a standalone heat pump cylinder)

 

Otherwise all this idea for using a phe is it wonderful but you're still f**ked trying to get enough water to/from it on the primary side to work.

 

In Europe meanwhile...

 

https://www.svarienergija.lt/produktas/silumos-siurbliai/cooper-hunter-hm3/

 

Etc 

 

Split system. Outdoor block with compressor and evaporator. Indoor "fridge" with hot water tank and space heat pump. Teeny little refrigerant lines in-between. Change from £10k fitted.

 

When the new rules comes into play allowing 900 grams of propane R290 in split systems these group to propane; support high temperatures like the monoblocs; and you'll just sling the refrigerant lines up the wall, over the attic, and into a new fridge in the cylinder cupboard.

 

No bespoke nonsense. No custom made this that other. Just whack it in and move on.

 

 

 

[sharpener]

 

25 minutes ago, markocosic said:

 

Handling the minimum output is the tricky part. Compressors in heat pumps can't turn down that far.

 

Heating houses with flow temperatures of 55C isn't a big deal. Replacing cylinders isn't a big deal.

 

The tricky bits are installing cylinders where there are none today; and running 28 mm primaries to the cylinder location.

 

Former is hard.

 

You can solve the latter with split units (run teeny tiny refrigerant lines to the "cylinder" instead); but this is currently hamstrung by Dupont's regulations that were written against use of usable amounts of propane (R290) indoors or separate units (dedicated unit for hot water/fancy heat pump cylinder for hot water just as they do in the USA.

 

 

 

10 minutes ago, markocosic said:

 

The cylinder swap is easy

 

The primary pipework swap is difficult

 

Using plate to existing cylinder; instead of replacing cylinder; doesn't fix the issue of your primaries.

 

When the new rules comes into play allowing 900 grams of propane R290 in split systems these group to propane; support high temperatures like the monoblocs; and you'll just sling the refrigerant lines up the wall, over the attic, and into a new fridge in the cylinder cupboard.

 

 

Of the last 4 houses I have lived in, all Victorian, two would have been relatively easy to fit new cylinders and two extremely difficult (several days' work incl. carpentry and redecoration). One (4-storey) house was heated by a boiler two floors up which would have needed very long connections to a HP. In none would I have wanted to replace all the radiators.

 

I am not sure where all this concern about 28mm pipes comes from, I have not seen it raised as an issue before, either by installers or manufacturers. Where it is a real problem then having an HP for space heating and using E7 for DHW would seem to me to be a useful compromise and I may revert to that if all else fails.

 

I think better turn-down ratios will come in time just as they have for boilers, there is no intrinsic problem with the thermodynamics.

 

@markocosic can you provide an authoritative link to the issues surrounding R290 as a flammable gas? Surely this is a regulatory problem not an engineering problem. After all we allow unlimited quantiies of natural gas to be piped into our kitchens or bathrooms through a 22mm pipe yet in the whole of the UK gas explosions only seem to happen a few times a year (and then mostly because of tampering with the supply).

 

I agree that split systems have some advantages and that is currently what I am awaiting a quote for (I may end up with an indoor unit with integral HW tank which I will use as a thermal store). I also wonder where is the equivalent of the combi boiler to provide DHW on demand? Personally I don't like them but they do save a lot of space. Most current HP system concepts still seem to revolve around having large quantities of stored HW, is this solely a result of the "modulation problem" or is it a lack of imagination?

[SteamyTea]

5 hours ago, sharpener said:

I also wonder where is the equivalent of the combi boiler to provide DHW

I think it will come down to size of the outside unit.

You have to shift a lot of air to get 45°C water out of the tap at 20 litres a minute.

Edited April 9, 2023 by SteamyTea

[ReedRichards]

6 hours ago, sharpener said:

I also wonder where is the equivalent of the combi boiler to provide DHW on demand?

Have you looked at the power output of such boilers?  They seem to start at about 24 kW and that's not because your house needs 24 kW of heat, its because you need a lot of power to provide instant hot water.  But a "high power" gas boiler like this can still be made relatively compact and seemingly increasing the power output does not greatly increase the cost.  Heat pumps are much less compact that gas boilers (although my old external oil combi boiler was a bit of a monstrosity).  And to increase their power output they need to get a lot bigger.

 

So get the heat pump equivalent of a combi boiler you would need to find a way of making the heat pump much more compact (without blowing you off your feet as you walk past) and without increasing the cost.  You would also need a much wider modulation range, but I imagine that is relatively easy compared to the other challenges.

[JohnMo]

1 hour ago, SteamyTea said:

20 litres a minute

You would be hard pushed to find a combi that does 20l/m, summer or winter.

 

We have combi rated at 32kW for DHW, at a delta T of 28, it is rated at 18.8l/min.

 

The 23kW model is rated at 11.6l/min at the same conditions.

[JamesPa]

10 hours ago, markocosic said:

The tricky bits are installing cylinders where there are none today; and running 28 mm primaries to the cylinder location

Why 28mm primaries, one of the MCS brigade who surveyed said that too.  Is it because hps are run at lower delta t and if so is there any reason this couldn't change for the DHW circuit if we are running it at an elevated flow temp?  Or by primaries are you talking about the heating not dhw circuit (no problem in my case as the ch feed splits just after the diverter valve upstairs/downstairs each 22mm)

Edited April 9, 2023 by JamesPa

[JamesPa]

9 hours ago, markocosic said:

How? Feed the output of the compressor back into the input once you're down at minimum output. This uses much the same power as running at minimum output; but puts out even less heat. Like driving a car with the brakes on.

Not sure I understand this, can you explain how it works and/or provide a reference.

 

Also what is the modulation ratio of  Samsung hps , I can't find any specs (not that Samsung are alone in this respect, only Mitsubishi seem to provide really comprehensive specs)

[JamesPa]

8 hours ago, sharpener said:

. In none would I have wanted to replace all the radiators.

I grant it's horses for courses.  In my case the room insulation had been increased since the rads were installed so most of the upgrades were 11 to 21 or 21 to 22 with little change in dimension.  I found I could easily do 2 per day in a fairly relaxed 5 hour day, a plumber would presumably be twice as quick.  I have a couple of difficult ones yet to be tackled where I need 33 or possibly fans.  But overall much less painful than anticipated.  I think what this whole thread shows is that a) we all (pretty much) agree that there is a problem and b) we need a range of options to fit various retrofit scenarios and customer preferences, not a one size fits all approach.  

 

Edited April 9, 2023 by JamesPa

[JamesPa]

16 hours ago, SteamyTea said:

Basically it can be done (high temperature and high how) and be modulated to lower levels quite easily, from an engineering perspective.

But I doubt many manufacturers want to install 2 kg of gas at 20p MPa next to a structural wall of a building.

Especially if it unnecessary with existing technology.

 

The environmental/embodied energy/CO2 of scrapping an old, or brand new, cylinder is minor.

Can you explain this please.  I cant work out what the challenge is (and I feel I should be able to!)

[JamesPa]

15 hours ago, sharpener said:

 

I've had a bit of a play on the web site he linked to. So far this is the best solution I have found, Eur125 list, or  cheaper equivalent Eur51. (N.b. you have to use the tabs to get the price for the correct number of plates for these particular configurations.)

 

Performance as under, will transfer 4 kW with a 5 deg drop from side 1 to side 2, with ~ 12 l/min flow rate and ~0.5m head loss.

 

But I was wondering what result you would get by just having the pump to circulate the water in the tank. I would guess you could easily double the heat transfer compared with simple convection off the coil, this would (assuming linearity of yr model) give DHW at 52.5 and if 3x then 55 without any HX at all.

 

Solarcoil looks interesting and not expensive but I wonder what its surface area is?

 

 

 

 

 

Looks good if you have a 4kW pump.  I think the ideal design objective is to transfer the full output of the heat pump at the desired final DHW temp with the delta T on the hot side equal to 5 or 7 or whatever you are running the HP at.  A lesser (the minimum?) design objective it to transfer at least the minimum output of the heat pump.  The advantage of the former is (obviously) faster reheat time at the expense of a larger PHE and higher pressures in the PHE circuit.

 

Solarcoil doesn't look to have a particularly large area to me so I cant see it does the job, and anyway, for me, I want to retain my immersion heater for backup and for solar PV.

[JamesPa]

3 hours ago, SteamyTea said:

I also wonder where is the equivalent of the combi boiler to provide DHW on demand?

 

I think it will come down to size of the outside unit.

You have to shift a lot of air to get 45°C water out of the tap at 20 litres a minute.

I cant see this is do-able.  20l/min at a deltaT of 28C (which is optimistic in winter assuming you want tap water at 48C) is 39kW.  Even 10l/min is 19kW.

 

But there might be some middle-way combination.  Point of use hot water heaters often store 15l of hot water.  That's enough for most requirements and, combined with a 3kW heater enough to satisfy everything domestic other than a bath or a shower.  The recommended shower temperature is actually only 38C, perhaps there is some more 'finely tuned' engineering to be done here.

[sharpener]

18 minutes ago, JamesPa said:

Looks good if you have a 4kW pump.  I think the ideal design objective is to transfer the full output of the heat pump at the desired final DHW temp with the delta T on the hot side equal to 5 or 7 or whatever you are running the HP at.  A lesser (the minimum?) design objective it to transfer at least the minimum output of the heat pump.  The advantage of the former is (obviously) faster reheat time at the expense of a larger PHE and higher pressures in the PHE circuit.

 

Solarcoil doesn't look to have a particularly large area to me so I cant see it does the job, and anyway, for me, I want to retain my immersion heater for backup and for solar PV.

 

Happy Easter one and all!

 

There is an option to have an electric element up the middle, IIRC £119-95. But they don't tell you the rating of that either, I suspect it might not be 3kW.

 

11 minutes ago, JamesPa said:

I cant see this is do-able.  20l/min at a deltaT of 28C (which is optimistic in winter assuming you want tap water at 48C) is 39kW.  Even 10l/min is 19kW.

 

But there might be some middle-way combination.  Point of use hot water heaters often store 15l of hot water.  That's enough for most requirements and, combined with a 3kW heater enough to satisfy everything domestic other than a bath or a shower.  The recommended shower temperature is actually only 38C, perhaps there is some more 'finely tuned' engineering to be done here.

 

We seem to be getting there. You won't achieve a "power shower" - but they are the work of the devil from the pov of water consumption also. I get a more than adequate shower with 10 l/min, and when the rainwater filters are newly changed it even needs a restrictor in the flow (yes there is a UV steriliser as well).

 

But many ppl have instantaneous electric showers, which have crept up in rating to 9.5 kW now (and 7kW was definitely on the meagre side), so a 10kW HP would give you the same performance if you need that much for space heating anyway. Some combi boilers have small HW stores inside to give a quicker response (though keeping them hot does waste energy) and you might want to do that too.

 

The stuff about R290 seems to be something to do with flammable refrigerants only being allowed inside domestic premises in limited quantities if at all, hence my comments upthread about natural gas. But I don't fully understand it and last night I was unable to find anything on the web so I hope @markocosic or someone will be able to give us chapter and verse on the current and future positions in the UK. Meanwhile R32 seems to give adequate HT performance for the moment - but have you read about Vattenfall and supercritical CO2? The operating conditions for the compressor are pretty extreme!

[JohnMo]

We lived in Singapore for a year or so.  The showers there are somewhat different to ours.  We had no boiler or stored hot water tank.  The showers were not electric showers as such, not the same as we have here. But seemed to have a store of hot water in the wall, at point of use, assume heated by an immersion, shower performance was good as well. Electric bills were not excessive from I recall.

[JamesPa]

1 hour ago, sharpener said:

But many ppl have instantaneous electric showers, which have crept up in rating to 9.5 kW now (and 7kW was definitely on the meagre side), so a 10kW HP would give you the same performance if you need that much for space heating anyway. Some combi boilers have small HW stores inside to give a quicker response (though keeping them hot does waste energy) and you might want to do that too.

Building on this, 11kW HP (larger houses) will do instant hot water at the same speed as 9.5kW electric shower.  8kW HP (medium houses) plus a 3kW willis heater ditto.  Is this plus saw 15-30l stored at say 50C (ideally 15 because then you don't need the venting arrangements) sufficient?  Biggest issue is probably all the water in the distribution pipes!

[sharpener]

1 hour ago, JamesPa said:

Is this plus saw 15-30l stored at say 50C (ideally 15 because then you don't need the venting arrangements) sufficient?  Biggest issue is probably all the water in the distribution pipes!

 

Something like this? Seems expensive for a 15litre/4.5kW unit but the 3kW is even dearer and you are now down to a 6kW HP. Still needs a drain for the pressure relief valve but if fitted in place of the combi it will be there already along with all the other connections needed.

 

Add a flow switch and contactor to call for heat, and a timer to recharge from the HP every 30 mins during the day and conceptually we are there, obvs a purpose-designed unit would come into being with all this inside.

 

Edited April 9, 2023 by sharpener

[SteamyTea]

5 hours ago, JamesPa said:

Can you explain this please.  I cant work out what the challenge is (and I feel I should be able to!)

May have been my autocorrect changing things.

Basically, having a store of compressed gas is quite dangerous. To get large modulation the pressures will have to be quite high i.e. 200 MPA.

 

Also how would a heat pump work as an instantaneous heater for only a few seconds, current combis don't cope brilliantly.

[JamesPa]

1 hour ago, SteamyTea said:

To get large modulation the pressures will have to be quite high i.e. 200 MPA.

It sounds plausible, but why is this (my knowledge of compressors is small)

 

1 hour ago, SteamyTea said:

Also how would a heat pump work as an instantaneous heater for only a few seconds, current combis don't cope brilliantly.

It wouldn't. Hence the talk about point of use heaters having a 15l local store.

 

2 hours ago, sharpener said:

Something like this? Seems expensive for a 15litre/4.5kW unit but the 3kW is even dearer and you are now down to a 6kW HP. 

Yes.  I have used these in community centres where we have replaced gas with ashp.  They work a treat, but of course people aren't taking baths or showers.

[sharpener]

Don't understand positive correlation between modulation ratio and gas pressure. Anyway for a monobloc it is all outside the building (like the 2 x 50kg propane bottles I saw this morning).

 

Proposal is small (15 litre) intermediate store combined with inline boost heater as per discussion above.

 

Edit: cross posted. Was distracted by finding this Samsung data sheet but it does not shed any light on modulation ratio. Seems to need boost heater to achieve 70C but I am not sure I am reading it correctly. Diagram shows hot gas bypass arrangement as discussed upthread, there is also an intercooler which will improve the efficiency. Is a monobloc so perhaps more versatile than the Daikin Altherma 3 HT.

 

Edited April 9, 2023 by sharpener