Heating system for an ICF house with UFH

[Nickfromwales]

49 minutes ago, Nelliekins said:

So will the cylinder deliver that much heat

Erm…...the cylinder doesn't deliver the heat, that big white box sucking in gas does Your cylinder could be a 15L low loss header and still provide all the heat your boiler does, because its coming from the boiler VIA the cylinder. As I said, the 140 will do it, but only if the boiler is allowed to replenish heat to it via the cylinder stat.

As far as DHW is concerned, if a big boiler is connected to it then the 140L TS will be able to give you constant DHW to 2 showers with ease. 

[Nelliekins]

1 hour ago, JSHarris said:

 

Why would you want a heat source for the MVHR?  The fresh air that the MVHR feeds into the house will always be colder than the air in the house, no matter what you do in terms of adding a bit of local heat by one extract terminal. 

 

For example, right now my MVHR control panel is telling me that the extract air is at 23.1 deg C and the fresh supply air to the rooms is 19.2 deg C.  The room temperature is currently 21.9 deg C and the outside air temperature is 8.6 deg C.

 

Sorry, I didn't explain myself very well... 

 

My thinking was that at least some of the extra heat energy output from the cylinder would be taken via an extract vent through the MVHR, which would then transfer the heat back to the fresh incoming air. So in winter some of the excess therms from the TS go back into the fresh air which is circulated around the whole house. In summer, its wasted energy because it would be lost via the summer bypass. 

Edited December 28, 2018 by Nelliekins
Auto correct corrections

[Jeremy Harris]

I doubt there would be any significant beneficial impact at all, in reality.  The heat would be largely absorbed by the surfaces in the room, then a bit of it would end up in one extract duct, which would then be diluted with cooler air from every other extract duct in the house.  The net result is there might be a very tiny increase in the temperature of the air entering the heat exchanger, and around 80% or so of that tiny increase in sensible heat would be recovered and passed to the incoming fresh air.  As MVHR only ventilates at a relatively slow rate (typically around 1 air change every two hours or so) there is loads of time for surfaces to absorb heat before the air in any room is changed.

 

Normal passive MVHR always cools the house in cold weather, just not as much as conventional ventilation.

[Nelliekins]

1 hour ago, Nickfromwales said:

Erm…...the cylinder doesn't deliver the heat, that big white box sucking in gas does Your cylinder could be a 15L low loss header and still provide all the heat your boiler does, because its coming from the boiler VIA the cylinder. As I said, the 140 will do it, but only if the boiler is allowed to replenish heat to it via the cylinder stat.

As far as DHW is concerned, if a big boiler is connected to it then the 140L TS will be able to give you constant DHW to 2 showers with ease. 

 

So from what @JSHarris has said re running the UFH from the cylinder, and what you have said re the DHW, it'd be better to use the 140L cylinder for DHW and a larger cylinder for UFH? Or is a 2h24 cycle OK for a boiler providing heat for UFH as the worst case scenario? Assume that we get 24kW from the boiler, it'll take maybe 15 minutes to recharge the cylinder (assuming 6kW capacity). 

 

We could add another cylinder (170/210L?) for either task, and heat from the boiler could go to both cylinders as required through a 3 position diverter valve.

 

Or I could bite the bullet and just get a small Sunamp for the DHW... Depends on the cost, of course - I would only pay 300 quid for another cylinder and pump, whereas a Sunamp is probably 2k?

 

I guess the key is to avoid short-cycling the boiler, and let it have a single continuous burn, correct? 

Edited December 28, 2018 by Nelliekins
Added para re short-cycling

[PeterW]

18 minutes ago, Nelliekins said:

Assume that we get 24kW from the boiler, it'll take maybe 15 minutes to recharge the cylinder (assuming 6kW capacity). 

 

That assumes a perfect cylinder with a coil large enough to push the heat into the cylinder. That is not the case and it will probably take nearly twice as long to heat the cylinder. 

 

 

[Nickfromwales]

16 minutes ago, PeterW said:

That assumes a perfect cylinder with a coil large enough to push the heat into the cylinder. That is not the case and it will probably take nearly twice as long to heat the cylinder. 

My statement is for an open pipe arrangement where the boiler doesn't heat the TS through a coil. The coil stays for DHW output and job done. It'll heat fast as fook so no issue if not using the coil for input.

[PeterW]

19 minutes ago, Nickfromwales said:

My statement is for an open pipe arrangement where the boiler doesn't heat the TS through a coil. The coil stays for DHW output and job done. It'll heat fast as fook so no issue if not using the coil for input.

 

I agree my old welsh wonder, but how many standard 140 litre tanks have you seen with multiple tappings..??? If you are planning to run UFH return, boiler flow and PHE return all through the same tapping then something will starve as you will have 2 positive and one negative head pumps all running against each other when the boiler is topping up and the UFH is running and a shower kicks in...

 

You’ll spend more on cutting and fitting extra tappings than it’s worth, and the coil as heat makes the most sense as you remove the negative pressure source from the bottom tapping.... 

 

Or shall we have a lesson on fluid dynamics ...??? 

[Nickfromwales]

8 minutes ago, PeterW said:

Or shall we have a lesson on fluid dynamics ...??? 

Ah, my young apprentice...….."the force is weak with you"  

The PHE would be a hydraulically separate circuit off the coil. 

The TS would be heated by the boiler on a reversed S-plan, as in the heating would be teed off the tapping's and only drawn off when there is call for heat. When the pump in the boiler runs it will exceed the UFH manifold pump velocity and promote flow without the need for a third pump to circulate between the TS and UFH. 

Winner winner, chicken dinner.

[PeterW]

2 minutes ago, Nickfromwales said:

The PHE would be a hydraulically separate circuit off the coil

 

Now that’s an even worse idea ..!!!! Why use the PHE then..?? Just use the coil as the DHW heat ..! But it won’t be more than 6Kw so the showers will be crucified ...

 

New balls please ... 

[Tennentslager]

Och...you boys and all that pipe talk,someone  will be blushing soon?

[Nickfromwales]

1 minute ago, PeterW said:

 

Now that’s an even worse idea ..!!!! Why use the PHE then..?? Just use the coil as the DHW heat ..! But it won’t be more than 6Kw so the showers will be crucified ...

 

New balls please ... 

The coil wouldn't do DHW as it wont be efficient enough at max flow rates. Agreed. That's why its for DHW uplift. Yes, I was watching Iron Man and not concentrating and forgot its not a proper DHW coil in the top of the TS.

Damnit !

[Nelliekins]

2 hours ago, PeterW said:

54 minutes ago, Nickfromwales said:

My statement is for an open pipe arrangement where the boiler doesn't heat the TS through a coil. The coil stays for DHW output and job done. It'll heat fast as fook so no issue if not using the coil for input.

 

 

29 minutes ago, PeterW said:

 

I agree my old welsh wonder, but how many standard 140 litre tanks have you seen with multiple tappings..??? If you are planning to run UFH return, boiler flow and PHE return all through the same tapping then something will starve as you will have 2 positive and one negative head pumps all running against each other when the boiler is topping up and the UFH is running and a shower kicks in...

 

You’ll spend more on cutting and fitting extra tappings than it’s worth, and the coil as heat makes the most sense as you remove the negative pressure source from the bottom tapping.... 

 

Or shall we have a lesson on fluid dynamics ...??? 

 

Now gents, there's plenty of me and indeed my lack of understanding to go around... Can't you share? ?

 

@Nickfromwales will the coil in this cylinder be up to the production of DHW @ 24L/min (2 decent showers)? I had assumed I would need to use the 100kW PHE across the cylinder to get the DHW. 

 

@PeterW if I use this cylinder with a PHE for DHW, and a similar cylinder for UFH (a 10-15 minute burn on a roughly 3 hour duty cycle as the worst case scenario in winter doesn't sound that bad to me?), that's the cheapest (and least elegant, granted) solution I can see to solve my needs. Is that right?

 

Both cylinders connected to the boiler flow using a 3 way diverter, both with immersion heaters (priority to DHW since the required cylinder temp is higher, and thus more energy stored as heat).

 

DHW cylinder goes to 65/70C, which should still allow 50C out of the PHE to the hot water manifold, and delivered to taps at around 45C (the hot water runs are all lagged and longest is about 7m).

 

UFH cylinder goes to 35C?, with the intention of a floor temp of 21.5C, based on @JSHarris wonderful spreadsheet. 

 

There's an argument to be had that the coil in the DHW cylinder could replace or supplement the immersion in the UFH cylinder, and maybe that'd be enough to heat the UFH on its own... But I figured that might drain the heat out of the DHW cylinder too fast, and you both seem to confirm that. 

 

FWIW I have confirmed that the floor upstairs will cope with 2 cylinders, although one could hang off the ICF wall if needed anyway... (weight has become a consideration given the posi joists the cylinders are sat on!) 

 

Longer term and as budget allows, I could look to replace one or both with a Sunamp 9/12kW unit, but the cost differential at the moment (between the 2 cylinder approach and the Sunamp) is about £2k - more than the budget can take this side of the VAT reclaim unless there's RHI or similar benefit from the Sunamp? 

[Nickfromwales]

4 minutes ago, Tennentslager said:

Och...you boys and all that pipe talk,someone  will be blushing soon?

?

[Nickfromwales]

22 minutes ago, Nelliekins said:

@Nickfromwales will the coil in this cylinder be up to the production of DHW @ 24L/min (2 decent showers)? I had assumed I would need to use the 100kW PHE across the cylinder to get the DHW.

Do you have a link for the cylinder so I can see the thing in all its glory? 

[Nelliekins]

7 minutes ago, Nickfromwales said:

Do you have a link for the cylinder so I can see the thing in all its glory? 

 

Oddly I can't find it on the manufacturer's website... Here's a link to it from one of the many vendors though... RM Aquastel 140L stainless Steel indirect cylinder

[dpmiller]

only 35mm insulation?

[Nelliekins]

2 minutes ago, dpmiller said:

only 35mm insulation?

 

Yes at the moment. It'll end up in a cupboard with either EPS70 or PIR lining though, so not overly concerned about heat losses, especially if it's being used for the UFH side of things where the temp won't get much above 35C...

[Nelliekins]

OK, SWMBO has decreed that I need to get the UFH working as the priority, so it looks like the 140L cylinder will be used for that. If the system gets replaced in 6 months (once we get the VAT reclaim) it will either have worked well enough for us and will stay, or will be replaced. 

 

Presumably I just plumb the cylinder in reverse (ie boiler connected direct with a PRedV and bypass valve) and the UFH circuit across the coil? Do I use 1 pump per UFH manifold, or will 1 pump suffice? (I am thinking that 2.2km of pipe is quite a volume of water for one pump to shift) 

 

 

[PeterW]

Minimum one pump per manifold as it just won’t cut it otherwise. 

 

Why the rush to get heating on.? What stage are you at ..??

[Nelliekins]

26 minutes ago, PeterW said:

Minimum one pump per manifold as it just won’t cut it otherwise. 

 

OK. So the buffer tank affords us the luxury of no mixing valves, provided we control the cylinder temp accurately enough. Then we tee off the coil flow and return for each of the 3 manifolds, and have a pump on each circuit? Presumably we put a bypass and PRedV on the boiler flow/return, and a pump from the boiler? 

 

I will have to mount a couple of stats on the cylinder I guess, to give us tighter control over the temperature of the flow to the manifolds?

 

Then there is the UFH temp... We have temp gauges on every manifold, but since most of the house will be automated, I'd like to improve on that... Can we just put DS18B20s into brass tee fittings, and connect them over 1wire to the automation system? I guess a simple way to know if the floor is up to temp is to track the differential between flow and return across a manifold? Would that work? 

 

26 minutes ago, PeterW said:

Why the rush to get heating on.? What stage are you at ..??

 

Kitchen is installed, half of the house is plasterboarded and we need to start warming and drying the place. MVHR will be commissioned next weekend all being well... 

 

More importantly SWMBO won't "camp out" until we have something more than a rocket heater to warm it up... But we need to camp out there so we can make progress more rapidly (the mortgage payments on both houses are now more than we earn every month ?) 

 

The blog will catch up eventually, I promise! ?

Edited December 29, 2018 by Nelliekins
More to say!

[PeterW]

Ok - what are the plans for hot water ..?

[PeterW]

Just reading back - you’ll struggle to stop the boiler cycling with no mixing valve as you need to ensure the water into the floor remains below 40c at most, which means your boiler won’t be condensing. 

 

You could put a single TMV on the hot output from the tank to blend down the water to a maximum. 

 

Quickest way to control the manifolds and flow temps are the Wunda Trade automates heads that regulate flow and return delta to 7c. Not cheap at about £14 each, but worth it. 

 

A tank stat will have enough hysteresis to cope with the on/off needs on the tank. Yes, it will run at 55c or so but it will also mean you get maximum efficiency from the boiler.

 

How many manifolds are there ..? My only concern is where you have one set of circuits in screed and the other in timber as they will react very differently. I’m assuming you have multiple zones planned ..?? 

[scottishjohn]

question ?

if using gas boiler for UFH --why regulate it down as far as 40c 

I understand the bleed to make sure return is not too low

[PeterW]

2 minutes ago, scottishjohn said:

question ?

if using gas boiler for UFH --why regulate it down as far as 40c 

I understand the bleed to make sure return is not too low

 

Flow temperature has to be below 40c with a chipboard based floor and also depends on the finish above - some wood flooring and laminates have a maximum floor temperature. 

 

There is also a comfort factor, and the issue with overshooting the  target temperature as the higher the flow temperature, the easier it is to get a continuous increase of room temperature once the flow is switched off. 

[Nelliekins]

41 minutes ago, PeterW said:

Ok - what are the plans for hot water ..?

 

Either go with system as designed in OP, or find some alternative as stop-gap until we can afford a Sunamp unit (which I think should be the long term objective, given our 5kW PV system will be massively under utilised most of the time). 

 

OP solution has the benefit of mucho cheapness, but unless it'll work there isn't much point. @Nickfromwales seems convinced it'll work even if not very good from heat loss perspective... Do you concur with that assessment? If so, maybe a slightly bigger cylinder is warranted since it'll mean a longer gap between recharges...

 

5 minutes ago, PeterW said:

Just reading back - you’ll struggle to stop the boiler cycling with no mixing valve as you need to ensure the water into the floor remains below 40c at most, which means your boiler won’t be condensing. 

 

You could put a single TMV on the hot output from the tank to blend down the water to a maximum. 

 

As it happens, that TMV is already purchased, although it was originally for the DHW ?

 

Returning to the original design I was going to use the coil to limit the heat coming out of the cylinder, but TMVs are cheap enough to justify. 

 

Quote

Quickest way to control the manifolds and flow temps are the Wunda Trade automates heads that regulate flow and return delta to 7c. Not cheap at about £14 each, but worth it. 

 

Hmm, we have 28 circuits... That's 400 quid... Or 20% of a 6kW Sunamp! Eek... 

 

Quote

A tank stat will have enough hysteresis to cope with the on/off needs on the tank. Yes, it will run at 55c or so but it will also mean you get maximum efficiency from the boiler.

 

How many manifolds are there ..? My only concern is where you have one set of circuits in screed and the other in timber as they will react very differently. I’m assuming you have multiple zones planned ..?? 

 

There are 3 manifolds:

 

- 12 port upstairs, pipes in EPS under chipboard

- 12 port downstairs (10 used), pipes in concrete 

- 6 port in basement, pipes in concrete

 

Actually was looking to follow @JSHarris and have the whole house as a single zone, keeping the house at a constant 20C all the time.