Design Help - Plumber not helping

[DeeJunFan]

Digging this bad boy up again.

 

During a recent visit to the self build show.  The stanley stove rep told my wife in no uncertain terms that we shouldn't get a stove with a back boiler if we will have a well insulated house with good air-tightness.

 

I can understand the theory in that the stove will release too much heat to the room etc.  But what are your thoughts on this?  

 

Should we go with boiler or no boiler?

[Nickfromwales]

It's pretty well documented by many, with a few on each side of the fence. 

1) Do you have a wbs ?

2) Do you have a wbs + b/b ?

3) No wbs at all. 

This is mostly down to heart over head imo. If you REALLY want to light a fire, I know I do ( outdoors chimnea for BBQ ) then you'll go for one regardless. 

The relative question here is about yield. Do you really see yourself lighting it that often, and keeping it alight for long enough to store the heat from it ? During the summer it would probably see you vacating the room at the least, vacating the ground floor at worst, but it's also well documented that it's pretty much summer all year around inside if your up to PH or very near to. .  

@DeeJunFan  Whats your fuel source for heating and water? And don't say wood 

[DeeJunFan]

1 hour ago, Nickfromwales said:

It's pretty well documented by many, with a few on each side of the fence. 

1) Do you have a wbs ?

2) Do you have a wbs + b/b ?

3) No wbs at all. 

This is mostly down to heart over head imo. If you REALLY want to light a fire, I know I do ( outdoors chimnea for BBQ ) then you'll go for one regardless. 

The relative question here is about yield. Do you really see yourself lighting it that often, and keeping it alight for long enough to store the heat from it ? During the summer it would probably see you vacating the room at the least, vacating the ground floor at worst, but it's also well documented that it's pretty much summer all year around inside if your up to PH or very near to. .  

@DeeJunFan  Whats your fuel source for heating and water? And don't say wood 

 

Current plan was as your mammoth plan above.

 

9kw ASHP for heating UFH - then also used from cold water uplift.

PV with immersion divert & Stove (In winter) & Immersion Top up for hot water (summer)

 

Our Heat recovery unit will also have pre & post heaters.

 

D

[Temp]

2 hours ago, DeeJunFan said:

During a recent visit to the self build show.  The stanley stove rep told my wife in no uncertain terms that we shouldn't get a stove with a back boiler if we will have a well insulated house with good air-tightness.

 

We have a wood burning fire in our living room. Our house was built 10 years ago and isn't super insulated yet we have trouble keeping the room temperature down when it's lit. Even with the smallest of fires it can quickly reach 24C.

 

Work out how much power will be needed to heat your living room. Then if you look here..

 

http://www.stovesonline.co.uk/stoves_with_backboilers.html

 

..you see that they typically have a room to boiler ratio of about 3 to 5 times so you can work out how much heat you can get from the boiler without the room overheating.  

 

A real fire is great for winter weekends but I wouldn't want to rely on one for DHW. Way too much work feeding and cleaning. Back in the 1960s my parents were over the moon when gas came to the village and they could ditch their coal boiler. 

 

 

 

  

[Declan52]

I agree that the room can very quickly overheat. If I left my double doors closed it would very quickly get over 30oC. Once lit we open all the doors leading to my hall so the heat will reach everywhere including upstairs within 2 hrs. The heat will then stay in the house for a few days. It was lit on Sunday and no stats have called for heat so far this week.

Could you put all the pipe work in to suit a back boiler but just put an ordinary stove in and see how that works out. If you need the back boiler version then put the first stove on gumtree and buy another one.

That architect guy whose house we visited near Michaels had only a stove with a back boiler to heat his water but his tank was huge. Could have had dolphins in it!! Must have been close to a 2500l tank.

[Jeremy Harris]

I think I've related this before, but a couple who came to the open weekend event we held, under the Green Doors scheme, had just completed a house that was very like ours in terms of spec, but they'd fitted the smallest room-sealed woodburner they could find, as they wanted a real fire in the living room.  The unit they fitted looks like one intended for a canal barge, it's that small, and has an output of around 3 or 4 kW, IIRC.  The Christmas before last they lit it for the first time.  Within half an hour they all had to abandon the living room, with all the windows left open, as the room temperature got well over 30 deg C.  It pretty much spoilt their Christmas, as the room stayed too hot to enter until Boxing Day.  They've never used it since, and when I last spoke to them they were looking at fitting an LCD screen inside the fire so they could make it look as if it was lit.  Like us, their house barely needs any heating in winter, and chucking around ten times the whole house heating requirement into one room was the cause of their problem.

 

When it comes to MVHR with heating/cooling, then I can say from experience that delivering heat this way is unpleasant and not very effective.  We have an MVHR with a built in reversible air to air heat pump, and it's pretty good for summer cooling, but it makes the air very dry if used for winter heating.  I have the heating part disabled, because of this, and just use the cooling function on very hot days.

Edited March 1, 2017 by JSHarris

[joe90]

Well we have bought a wood burner for our new build but not with a back boiler as I can't see it being lit long enough to heat the water. I am sure we are going to overheat when it's lit but we have large French doors opening to a large conservatory so I hope this will " suck" some of the heat out of the lounge and heat the extra space which is very poorly insulated. ( I will let you know in about 9 months.

 

p.s. Just had a thought, I know MVHR does not shift heat very well but like Declan above I can see us opening all the doors to distribute the heat around the house. Our lounge has an " input" from the MVHR , if this was put on boost would this over pressurise the lounge ( slightly) and aid distribution of heat?

 

I did think of putting ducts from above the stove to upstairs to spread heat ( with intumescent sealers)  but decided not to on account of air being a poor transporter of heat. ( er indoors loves the smell of a wood stove and the thought of this smell and heat in the bedroom appealed for a short time.)

Edited March 2, 2017 by joe90
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[ProDave]

Now shoot me down on this one, but I have a "cunning plan".  On the assumption that a WBS in one room may overheat that room, and mvhr won't re distribute the heat very well or very quickly. What about an extract vent in the ceiling above the stove, ducted to another room in the opposite corner of the house, with an in line fan that you can turn on, to move head from the room with the stove to other room(s)  A sort of blown warm air heat distribution system?

 

[joe90]

Dave, great minds think alike, this is what I was thinking. In fact I am having a small inglenook built so hot air would collect above the stove and this would be where it would be ducted from. Perhaps I should resurrect the plan?

[ProDave]

My thought is to build in such a duct and a fan costs very little at build stage. So I will almost certainly fit it. If it ends up not working or not getting used, you have not wasted much. By comparison, if you decided to fit it later, it would be a lot more work and a lot more disruption to the building, that's if it's possible at all.
 

[PeterW]

2 hours ago, ProDave said:

Now shoot me down on this one, but I have a "cunning plan".  On the assumption that a WBS in one room may overheat that room, and mvhr won't re distribute the heat very well or very quickly. What about an extract vent in the ceiling above the stove, ducted to another room in the opposite corner of the house, with an in line fan that you can turn on, to move head from the room with the stove to other room(s)  A sort of blown warm air heat distribution system?

 

 

Stove is in the kitchen/diner for us - it has double height ceiling to the dining and single height to the kitchen. Dining has a single supply point (med/high flow) and kitchen has 2 extract points, one near the door, and thus I "hope" this will cause enough of a convection current to move some of the heat about but also circulate it in to the MVHR.

 

Reason for going for that room is its 7.2 x 4.5m and has a high ceiling, where the lounge is 3.5 x 4.5m and the stove kicks out 4.5kw

 

 

[Nickfromwales]

On 1 March 2017 at 18:23, Declan52 said:

That architect guy whose house we visited near Michaels had only a stove with a back boiler to heat his water but his tank was huge. Could have had dolphins in it!! Must have been close to a 2500l tank.

Exactly what I would do. Light it once, heat a giant tank, and enjoy. Same principal as charging a battery. If you've got a big charger, charge a much bigger battery. . 

[ProDave]

11 minutes ago, Nickfromwales said:

Exactly what I would do. Light it once, heat a giant tank, and enjoy. Same principal as charging a battery. If you've got a big charger, charge a much bigger battery. .

I wired a straw bale house that had a similar setup. Massive thermal store. WBS that put 12KW to water and only 2KW to the room. The theory wa you light the stove about once a week.
 

[Declan52]

He had to get the tank in place and then build the walls afterwards. 

[ProDave]

I remember a strange thing about the big thermal store in the straw bale house. It had no water feed to top it up. It was just a VERY big bucket and you hat to periodically check the water and manually top it up to allow for any evaporation.
 

[Nickfromwales]

40 minutes ago, ProDave said:

I remember a strange thing about the big thermal store in the straw bale house. It had no water feed to top it up. It was just a VERY big bucket and you hat to periodically check the water and manually top it up to allow for any evaporation.
 

Got to love the simplicity, but I think I'd have at least put a tap next to it with a hose. . 

One of my 'I'll do it one day' ideas is to put a huge TS ( 2000 litres or more ) in my shed, entombed in poly beads or similar, and use my chimnea to heat it. 

When that thing is going flat out I can barely stand 8' away from it. Can't help wondering how much DHW ( cold mains uplift via a instantaneous coil in the TS ) I could generate by my evening BBQ sessions. ?  

Was thinking of a home made ST rig ( old rads painted black ) on the flat roof of the shed as a means to keep it from going 'cold' for background input. Theoretically, after two 'burns' per week, it should always be above the flow temp of ST so weighing up the options. ??

DIY PV becomes more sensible too then as I'll always have somewhere to dump excess. 

Hopefully I'll live long enough to find out, maybe another 200 years would suffice ?

 

[Jeremy Harris]

On 02/03/2017 at 11:00, ProDave said:

Now shoot me down on this one, but I have a "cunning plan".  On the assumption that a WBS in one room may overheat that room, and mvhr won't re distribute the heat very well or very quickly. What about an extract vent in the ceiling above the stove, ducted to another room in the opposite corner of the house, with an in line fan that you can turn on, to move head from the room with the stove to other room(s)  A sort of blown warm air heat distribution system?

 

 

You need a very high air flow rate, so make the duct very large - think US-style warm air heating duct size.  Air has such a low heat capacity that you need to move a lot of it to shift a reasonable amount of heat, especially if the temperature difference between the "hot" area and the "cool" area is small, say a couple of deg C.

 

The sums are reasonably easy.  Air has a heat capacity of around 1.297 J.l.K, so 1 litre of air that's 2 deg C warmer than the surrounding air will hold around 2.594 J of heat energy, or about 0.0000007205555556 kWh of heat energy.  If you had an air flow rate of 10 times more than the MVHR kitchen extract rate in the regs, of 130 l/s, then, with a 2 deg C temperature differential the heat transfer rate would be around 337 W. 

 

If you want to do the sums for any flow rate and temperature differential, then 1 litre per second of air flow rate, with 1 deg C temperature differential between the "hot" and "cold" ends, gives a heat transfer rate of 1.297 W.   You can determine the heat transfer rate for any air flow rate and temperature differential you like from these numbers.  For example, for a 10 deg C temperature differential and 10 litres/second air flow rate the heat transfer rate (in watts) = 1.297 x 10 x 10 = 129.7 W.  

 

As another example, let's say you wanted to shift 1 kW of heat energy from one room to another, with the "hot" room being 22 deg C and the "cold" room being 20 deg C.  A 2 deg C differential temperature means a 1 litre per second air flow rate can transfer 2.594 W of heat.  To transfer 1 kW would mean having an air flow rate of 1000 / 2.594 = 385 litres per second.

 

These poor transfer rates illustrate one of the reasons why MVHR cannot move a significant amount of heat around; the flow rates and temperature differential is just too small.  It's also why warm air heating systems need very large ducts (US ones I've seen are around 18" wide and maybe 12" high) to allow high air flow rates with a relatively low flow velocity.

[DeeJunFan]

so i'm back to being totally confused.

 

Our plan as above was to have 2 seperate systems.

 

UFH

ASHP heating a 200/300L tank to 40° this would then be used for UFH and cold water uplift into the second DHW system

 

DHW

500L TS/UVC that would have its cold input uplifted by ASHP.  Heated by Immersion diverter during summer/shoulder months. Topped up by Direct-electric immersion.  In winter months to be topped up by Stove with Back boiler.  If back boiler was over heating the tank then the 500L tank could start to heat the UFH tank to make the best use out of the stove.  Also be fitting a instant water heater on the hot water out to uplift temp if it drops below set point.

 

I was pretty sure we could have worked out ok with that system.

 

But if we do away with the back boiler then the plumbing simplifes somewhat so maybe that would be the best idea??

 

UFH

ASHP heating a 200/300L tank to 40° this would then be used for UFH and cold water uplift into the second DHW system

 

DHW

500L TS/UVC that would have its cold input uplifted by ASHP.  Heated by Immersion diverter during summer/shoulder months. Topped up by Direct-electric immersion.  In winter we would be relying on the direct electric immersion for DHW uplift from the ASHP tem. Also be fitting a instant water heater on the hot water out to uplift temp if it drops below set point.

 

If we are going without the back boiler should we downsize the DHW tank to say 300L?  so say run with a 200L tank for UFH and a 300L tank for DHW??

 

we are very close to starting on the first fix plumbing.  1st fix electrics starting this week so really need to make this decision! @Nickfromwales @JSHarris any sparks of genius for me??

 

Thanks guys!

Damian

[Nickfromwales]

Forgive me if we repeat things here but I'm pillar to post at the mo

Why aren't you connecting the ashp to the DHW cylinder? That should be your back up, not the instant water heater IMO. Reasons are many. 

• You've got a system there already ( ashp run at 55oC ) to provide DHW direct to the UVC so why pay for another bit of kit ( instant ) ?
• The instant would give a choke point from the 22mm outlet of the UVC, thus restricting your DHW flow rate year round when you may never even use it. They're almost all 15mm, unless it's a commercial unit, so if there was an instant going in I'd only ever put it inline to one bathroom as failsafe. Complexity / cost etc. 

 

Id probably go 300l for both cylinders. 

 

• 300l is a good size to store lots of low grade preheat temp water for uplift. That's preferential as it'll do around 35oC of the ~42oC you'll need for average bathing. The UVC will fill the gap there according to demand. A thermostatic bath filler is your friend here so no wasted hot water goes into the bath to then be cooled down with cold. 
• If your expected DHW demand is to rise ( little ones getting bigger etc ) then I'd upsize the preheat tank to 500l now as its not much more money and it's the cylinder which will have the lowest latent loss, due to the low set temp. 
• 300l is also a good size for the Pv driven UVC, as you want to harvest and store as much of that input as possible, so I'd go no less than 250l but 300l better IMO. 

 

 

Id very much reconsider the inline instant water heater as I think it'll end up redundant. You have Pv and grid driven DHW, plus you could put additional immersions ( x2 ) in the preheat / Ufh TS so you could run without the ashp and still have grid electric supplying both UFH and DHW ( admittedly with far less response but still you'd not be left with nowt  ). 

Remeber that at these sizes the cylinders will both come with dual 3 kW immersions, so you can pump 6kw directly into each if the ? hits the fan. 

 

Edited to add : cylinder sizing is according to DHW usage so they could be downsized after a discussion on those specifics. . 

[Nickfromwales]

Make sure that your sparky provisions for the immersions at each cylinder if this sounds like a route you'll be going down

 

[le-cerveau]

What you are looking at is very similar to my original plan, I then went to a stratified TS (discussion on EB about a year ago) and I am now going with a custom Sunamp stack with two different types of cell, the standard PCM58 cells which will be heated by the boiler and some PCM34 cells which will be heated by the ASHP.  We are still working out the specifics as this is a custom build but as I have a good few months yet we will get there.  It is probably a bit close to the wire for you but you may consider it, they are amenable to custom work, all be it at a cost!

[DeeJunFan]

1 hour ago, Nickfromwales said:

 

Why aren't you connecting the ashp to the DHW cylinder? That should be your back up, not the instant water heater IMO. Reasons are many. 

 

Id probably go 300l for both cylinders. 

 

Id very much reconsider the inline instant water heater as I think it'll end up redundant. You have Pv and grid driven DHW, plus you could put additional immersions ( x2 ) in the preheat / Ufh TS so you could run without the ashp and still have grid electric supplying both UFH and DHW ( admittedly with far less response but still you'd not be left with nowt  ). 

Remeber that at these sizes the cylinders will both come with dual 3 kW immersions, so you can pump 6kw directly into each if the ? hits the fan. 

 

 

ok, more confused again.

 

When you say connecting the ashp to the DHW cylinder.  I was thinking it would be connected.  As in ASHP will heat the pre-heat (UFH) tank and cold water for the DHW tank will go throught the DHW tank to get pre-heated.  Or would you suggest a direct feed from the ASHP to the DHW tank and run the ASHP in DHW mode?  I was planning on keeping the ASHP at 40° output to keep COP high.

 

can do 300L for both cylinders.  (assuming UFH should be a TS and DHW shoule be a UVC) 

 

Hadn't figured on the pipe limitation from the Instant.  Was planning on the unit similar to @JSHarris really just for ass coverage.

 

Could always have the immersions on some sort of auto switch depending on tank temp.

 

 

[DeeJunFan]

@Nickfromwales

 

So you would suggest get rid of the back boiler.  Get rid of the In-line instant at the tank and go 300L TS fed from ASHP and 300L UVC pre-heated from the ASHP TS Coil and topped up by Solar PV/Immersion.

 

Backup plan would be Direct Electric Immersion to top up temps.

[joe90]

I didn't realise the instant water heater would cause a restriction!!! So this is making me re think using one. Jeremy, does your inline heater cause much restriction?, perhaps it would be possible to just put it in line with one shower and/or kitchen tap so that If you run out of hot water you don't have to wait for the immersion. Also the modulating water heaters top up warm water to hot!!!

[Jeremy Harris]

55 minutes ago, joe90 said:

I didn't realise the instant water heater would cause a restriction!!! So this is making me re think using one. Jeremy, does your inline heater cause much restriction?, perhaps it would be possible to just put it in line with one shower and/or kitchen tap so that If you run out of hot water you don't have to wait for the immersion. Also the modulating water heaters top up warm water to hot!!!

 

Given that we have 3 bar of incoming pressure, the 15mm on the instant heater has no noticeable effect on flow rate at all.  I did flow tests with it in place and could get over 20 litres per minute (I think the bath tap was delivering around 24 litres/min before being temporarily restricted to meet building regs).  In reality we're never going to pull more than about 10 litres per minute from the DHW system, most probably less than that when cold mixing has been taken into account at the shower mixer, so it's not a problem at all. 

 

Our old house has a 15mm cold rising main, that runs both the hot and cold systems, with all the pipework in 15mm, and that's perfectly OK in practice, in terms of flow rates at all the taps, shower etc.  The only thing with using all 15mm like this is that there is a little bit of flow noise when the bath taps are fully on, but other than that there's no noise, and we rarely use the bath, anyway.

Edited March 7, 2017 by JSHarris