Basic System Design Idea

[Barney12]

OK, so I'm getting close to really needing to decide on my proposed system for heating and DHW.

ignoring all the calcs for a second but broad brush; passive house standard, 2,5000sq feet, 3 bed, 2 bath, 2 adults no children.

So, I think where I am is something fairly simple:

Underfloor heating 

ASHP

250/300ltr hot water cylinder (unvented)

Electric Immmersion

Sunamp

The idea being:

The ASHP will heat the floor as required. 

The ASHP will pre-heat the water tank to a level which is cop efficient.

The sunamp will 'top up' on demand DHW to the required temp.

The electric immersion will provide the weekly legionella heat up cycle.

The sunamp will be powered (where possible) using diverted power generated from our off grid PV. The rest of the time by off peak electric.

Does this sound sensible?

 

 

 

 

[PeterW]

Replace the UVC with a TS - use it as the buffer for the UFH and also have a secondary dump load immersion in it for the times the sunamp is full. 

No legionella issue, UFH as a buffer and the ASHP doesn't cycle as much....

 

Edited June 2, 2016 by PeterW
Typo

[Nickfromwales]

+1, bit increase the TS to 400 ltrs. Lower temp so better off with more volume. It's about £100 more to go from 300 to 400 . 

[Barney12]

Yep, ok got it. Newbie questions comming..............

so will a TS be just as efficient at transferring its heat into the flowing water at low temperatures. I don't know why but my brain thought that a TS would need to be at a reasonably high temp to allow efficient transfer? 

I assume dual coil rather than some form of s plan configuration between UFH and DHW demand?

[PeterW]

If you are using the TS as a pre-heat to the sunamp then it won't really matter - there is some whizzy thermodynamic calcs you can do that show how a TS works against a UVC in terms of heat delivery, but increasing its capacity from 300 litres to 400 litres offsets this.

Not sure what you mean by dual coil..? Essentially a TS is the full body of water as the store, and there is a finned coil in the tank that allows mains pressure cold to be heated as it passes through. It is open to all sources and the UFH so has no coils needed in reality.

The benefit of the TS is that you can draw off the UFH low down (directly) and not affect the stratification - you have to watch the positioning of the immersions though as you can cause problems when you heat right at the bottom - tends to upset the tank so I would go for 2/3rds down and "live" with the bottom of the tank being the last to be heated by convection but by the time you're getting to 60+ at the bottom of the tank, you will have 70+ at the top and no issues with hot water !

 

[Jeremy Harris]

Using a low temp thermal store is sort of OK if you have the room for a large one, but because of the drop in delivery temperature from the instant you start drawing water through a thermal store, when compared with a UVC that will deliver a near-constant delivery temperature until nearly exhausted, I'd opt for a UVC with an immersion and use that immersion as a second solar dump and with a timer set to run once every couple of weeks as an anti-legionella system.  You would only need around half the capacity if using a UVC, so lower losses, less capital cost and less space taken up.

The problem is that all the water in a thermal store starts to reduce in temperature from the moment you start to draw water through it.  A UVC doesn't behave like this, it allows you to draw off maybe two thirds to three quarters of the hot/warm water off at a near constant temperature before the temperature starts to reduce.  This really makes a thermal store pretty useless if it is running at a temperature that is below the desired DHW temperature, as a pre-heat device.  If I was designing our system from scratch (ours very much evolved during the build) then I'd have had a 200 litre indirect UVC pre-heated to 35 to 40 deg C by the ASHP and have that feed warm water to the Sunamp PV, or if that wasn't charged enough, to the 9.6kW inline water heater.

[DeeJunFan]

5 hours ago, JSHarris said:

If I was designing our system from scratch (ours very much evolved during the build) then I'd have had a 200 litre indirect UVC pre-heated to 35 to 40 deg C by the ASHP and have that feed warm water to the Sunamp PV, or if that wasn't charged enough, to the 9.6kW inline water heater.

Thats an interesting design, i was under the impression that the Sunamp wasn't suited to accepting pre-heated water (not sure why I thought that).

My main concern is the Capital cost of the split DHW/UFH system.  You require 2/3 heat sources (ASHP, PV & Immersion)  Then you also require 2 Storage/Distribution devices either Buffer/UVC or TS/Sunamp

I was hoping to bring the capital costs down by going Immersion over ASHP but there doesn't seem to be much confidence that the system would perform well

head melted

[Barney12]

4 minutes ago, DeeJunFan said:

head melted

I share your pain!

[Barney12]

5 hours ago, JSHarris said:

Using a low temp thermal store is sort of OK if you have the room for a large one, but because of the drop in delivery temperature from the instant you start drawing water through a thermal store, when compared with a UVC that will deliver a near-constant delivery temperature until nearly exhausted, I'd opt for a UVC with an immersion and use that immersion as a second solar dump and with a timer set to run once every couple of weeks as an anti-legionella system.  You would only need around half the capacity if using a UVC, so lower losses, less capital cost and less space taken up.

The problem is that all the water in a thermal store starts to reduce in temperature from the moment you start to draw water through it.  A UVC doesn't behave like this, it allows you to draw off maybe two thirds to three quarters of the hot/warm water off at a near constant temperature before the temperature starts to reduce.  This really makes a thermal store pretty useless if it is running at a temperature that is below the desired DHW temperature, as a pre-heat device.  If I was designing our system from scratch (ours very much evolved during the build) then I'd have had a 200 litre indirect UVC pre-heated to 35 to 40 deg C by the ASHP and have that feed warm water to the Sunamp PV, or if that wasn't charged enough, to the 9.6kW inline water heater.

Thanks Jeremy. Your post shows exactly why to the average layman this whole area is a minefield and why us laymen are open to so much manipulation by snake oil salesmen!

You logic and reading similar posts is where I got my original logic from.

[Jeremy Harris]

To answer some basics, the Sunamp is fine with pre-heated water.  It delivers water at around 75 deg C when charged, and the amount of hot water you get out depends on the temperature that the Sunamp has to raise the water by. 

If you feed it with water at 30 deg C, say, and you set the Sunamp TMV to 45 deg C (the delivered DHW temperature) then it only has to raise the water from 30 deg C to 45 deg C, in effect, so will deliver twice as much hot water on a charge than if it was fed with water at 15 deg C.

Our cold water comes in at around 8 deg C, all year around, but mains can vary from around 4 deg C to 8 deg C with the season, I believe,

The thermal store thing is only a problem in this application because thermal stores are intended to operate at high temperatures, much higher than an ASHP could deliver.  The design temperature for most is around 75 deg C, so with hot water coming out of the thermal store TMV at 45 deg C, and a 5 deg C temperature loss across the coil in the thermal store (perhaps a bit on the high side) then the thermal store can deliver hot water at a constant temperature to the DHW system until it cools down to below about 50 deg C, and then the temperature will start to drop.

For pre-heat, then the thermal store temperature will start to drop immediately, as it's already below the DHW temperature, which is why a UVC would be a better bet.  If I get time later I'll do some example calculations and perhaps a graph that illustrates how a low temperature UVC compares to a low temperature thermal store when used as a pre-heat system.  Right now I keep popping in here as a break from sorting out hundreds of receipts and filling in our VAT reclaim...............................

[SteamyTea]

Maybe a quick diagram would help.

[Guest Alphonsox]

On 03/06/2016 at 12:16, JSHarris said:

For pre-heat, then the thermal store temperature will start to drop immediately, as it's already below the DHW temperature, which is why a UVC would be a better bet.

The problem with using a UVC to feed preheated water to the Sunamp is that you end up with hot water at cylinder pressure not mains pressure. This would be a non-starter for us. I think your solution of using a heat exchanger to warm-up incoming mains pressure water before it gets to the Sunamp is conceptually better in this regard, although a little  more complex.

IGNORE THIS RUBBISH !

Edited June 6, 2016 by Alphonsox
Brain stall

[Barney12]

8 minutes ago, Alphonsox said:

The problem with using a UVC to feed preheated water to the Sunamp is that you end up with hot water at cylinder pressure not mains pressure. This would be a non-starter for us. I think your solution of using a heat exchanger to warm-up incoming mains pressure water before it gets to the Sunamp is conceptually better in this regard, although a little  more complex.

This just gets more confusing!

I thought the whole concept of a UVC was that the pressure remained constant with mains pressure. Its certainly the case with the small (100ltr) Aquapoint un-vented cylinder we have in the flat, there is absolutely no noticeable drop in pressure at the hot tap.

[Jeremy Harris]

24 minutes ago, Alphonsox said:

The problem with using a UVC to feed preheated water to the Sunamp is that you end up with hot water at cylinder pressure not mains pressure. This would be a non-starter for us. I think your solution of using a heat exchanger to warm-up incoming mains pressure water before it gets to the Sunamp is conceptually better in this regard, although a little  more complex.

As above, the whole point of an un-vented cylinder is that it operates at high pressure, so is the same as a mains feed.

A vented cylinder does reduce the pressure to the head from the cold water tank, but with a UVC there is no cold water tank, the cylinder is fed with mains pressure and sealed (with the rider that the incoming mains pressure must have a pressure regulating valve fitted for safety, to ensure that cylinder doesn't exceed it's rated working pressure if you live in an area with exceptionally high mains pressure).

[Guest Alphonsox]

Sorry - Insufficient coffee - I read that as a vented cylinder, not unvented !

So using a UVC would require Part-G installation, sign-off and annual servicing - Probably more cost efficient to install a second Sunamp.

[Jeremy Harris]

Officially, yes, but I've heard of a lot of installations where the building inspector hasn't asked to see any paperwork and has accepted whatever has been scribbled on the commissioning sticker on the unit.

At the time I though G3 was a show stopper for self-installation, but now I'm far from convinced that it is.

As for annual inspections, these aren't enforced or checked by anyone and it's only a visual check anyway, as there is nothing really to test.  It's just a matter of making sure the pressure is OK, that the expansion vessel still has the right pre-charge and that there are no leaks or indication that the pressure release valve has operated.  I can't see any reason why, in practice, a home owner couldn't do these checks.  The only tools needed are a car tyre pressure gauge and a tyre pump to recharge the expansion vessel if it needs it.

[Stones]

I suppose a lot depends on how you assess the 'risks' attached to not having an annual inspection carried out.  How many houses in this country will be sold in circumstances where boilers / heating systems have not been checked / serviced for years?  What is the worse that can happen, a buyer insists the cylinder is inspected before completion? I can however see the manufacturer using it as an excuse not to honour a warranty.  It's the insurance question that I wonder about - would  /could an insurance company use the lack of an annual inspection to refuse paying a claim linked to failure of a cylinder / plumbing system?  If you can do it yourself (as Jeremy describes) keeping a record seems to be the obvious thing to do to mitigate that risk.

[JanetE]

On 03/06/2016 at 12:16, JSHarris said:

right now I keep popping in here as a break from sorting out hundreds of receipts and filling in our VAT reclaim...............................

Good luck with that one Jeremy 

[Jeremy Harris]

19 minutes ago, JanetE said:

Good luck with that one Jeremy 

Thanks, I'm just about finished, just waiting for a few suppliers to get back to me with their VAT numbers.......................

[willbish]

I've found this thread to very useful. Just wondering @Barney12 did you go with your setup as specified at the start of this thread? 

And is the Sunamp acting as a buffer for your UFH too?